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ED 091 153 TITLE INSTITUTION REPORT NO PUB DATE NOTE AVAILABLE FROM DOCUMENT RESUME SE 016 464 Skylab Experiments, Volume I, Physical Science, Solar Astronomy. National Aeronautics and Space Administration, Washington, D.C. EP-110 May 73 81p.; For related documents, see SE 016 465, SE 016 991, SE 017 106, and SE 017 788-790 Superintendent of Documents, Government Printing Office, Washington, D.C. 20402 EDRS PRICE MF-$0.75 HC-$4.20 PLUS POSTAGE DESCRIPTORS *Aerospace Technology; *Astronomy; Demonstrations (Educational) ; *Instructional Materials; *Interdisciplinary Approach; Science Activities; Science Education; *Science Experiments; Science Materials; Secondary School Science IDENTIFIERS NASA; *Skylab Education Program ABSTRACT Up-to-date knowledge about Skylab experiments is presented for the purpose of informing high school teachers about scientific research performed in orbit and enabling then to broaden their scope of material selection. The first volume is concerned with the solar astronomy program. The related fields are physics, electronics, biology, chemistry, and photography, especially dealing with electromagnetic spectra, atomic structures, x-ray absorption, radiation, and kinetic theory. The content includes discussions of basic scientific background, a description of the sun and of the energy characteristics associated with each zone, and experiments with hydrogen-alpha telescopes, white light coronagraphy, extreme ultraviolet spectrograph and spectroheliograph, scanning polychromator, x-ray spectrographic camera, solar photography, and x-ray telescopes. Related curriculum topics and classroom demonstrations and activities are provided in detail. (CC)

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Page 1: DOCUMENT RESUME ED 091 153 Skylab Experiments, Volume I ... · ED 091 153 TITLE INSTITUTION REPORT NO PUB DATE NOTE. AVAILABLE FROM. DOCUMENT RESUME. SE 016 464. Skylab Experiments,

ED 091 153

TITLE

INSTITUTION

REPORT NOPUB DATENOTE

AVAILABLE FROM

DOCUMENT RESUME

SE 016 464

Skylab Experiments, Volume I, Physical Science, SolarAstronomy.National Aeronautics and Space Administration,Washington, D.C.EP-110May 7381p.; For related documents, see SE 016 465, SE 016991, SE 017 106, and SE 017 788-790Superintendent of Documents, Government PrintingOffice, Washington, D.C. 20402

EDRS PRICE MF-$0.75 HC-$4.20 PLUS POSTAGEDESCRIPTORS *Aerospace Technology; *Astronomy; Demonstrations

(Educational) ; *Instructional Materials;*Interdisciplinary Approach; Science Activities;Science Education; *Science Experiments; ScienceMaterials; Secondary School Science

IDENTIFIERS NASA; *Skylab Education Program

ABSTRACTUp-to-date knowledge about Skylab experiments is

presented for the purpose of informing high school teachers aboutscientific research performed in orbit and enabling then to broadentheir scope of material selection. The first volume is concerned withthe solar astronomy program. The related fields are physics,electronics, biology, chemistry, and photography, especially dealingwith electromagnetic spectra, atomic structures, x-ray absorption,radiation, and kinetic theory. The content includes discussions ofbasic scientific background, a description of the sun and of theenergy characteristics associated with each zone, and experimentswith hydrogen-alpha telescopes, white light coronagraphy, extremeultraviolet spectrograph and spectroheliograph, scanningpolychromator, x-ray spectrographic camera, solar photography, andx-ray telescopes. Related curriculum topics and classroomdemonstrations and activities are provided in detail. (CC)

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h: **sSURPLUS

DUPLICATE

U.S DEPARTMENT OF HEALTH.EDUCATION A WELFARENATIONAL INS'ITUTE OF

EOUCAT ON71-115 DOCUMENT HAS BEEN REPRODUCED EXACTLY AS RECEIVED FROMTHE PERSON OR ORGANIZATION ORIGINATING IT POINTS OF VIEW OR OPINIONSSTATED DO NOT NECESSARILY REPRESENT OFFICIAL NATIONAL INSTITUTE OFEDUCATION POSITION DR POLICY

ExperimentsVolume IPhysical Science,Solar Astronomy

Information for Teachers, Including Suggestionson Relevance to School Curricula.

)c,

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

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SiExperiments

Volume IPhysical Science,Solar Astronomy

Produced by the Skylab Program and NASA's Education ProgramsDivision in Cooperation with the University of Colorado

NATIONAL AERONAUTICS AND SPACE ADMINISTRATIONWashington, D.C. 20546, May 1973

For sale by the Superintendent of Documents, U.S. Government Printing Of lice, Washington, D.C. 20402

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PREFACE

The most immediate benefits that derive from a multidisciplined scientific program such asSkylab are a large volume and wide range of scientific information. A secondary benefit isthat this very large amount of up-to-date information can be related in a timely manner tohigh school curricula. The time lag between the generation of new information and itsappearance in text books is often measured in years rather than in months.

It is the intent of the Skylab Education Program to eliminate this characteristically longdelay by timely presentation of scientific information generated by the Skylab program.The objective is not to teach Skylab to the schools, but rather to use Skylab science as afocus for science education in the high schools. Readers are urged to use the descriptions ofinvestigations and scientific principles, and the demonstration concepts contained herein asstimuli in identifying potential educational benefits that the Skylab program can provide.

National Aeronautics and Space AdministrationWashington, D.C. 20546

May 1973

ii

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CONTENTS

INTRODUCTION

SECTION 1THE SUN 1

Importance of Solar Astronomy 1

History of Solar Space Research 2Solar Zones 3Scientific Considerations 7Skylab Solar Observatory 8Experiment Schedules 9Crew Activities 9Data Availability 9

SECTION 2HYDROGEN-ALPHA TELESCOPES 13Experiment Background 13Definition of Scientific Objectives 14Experiment Description 15Experiment Data 18Crew Activities 19Related Curriculum Topics 19Suggested Classroom Demonstration 19

SECTION 3WHITE LIGHT CORONAGRAPH (S052) 21Experiment Background 21Definition of Scientific Objectives 22Description of Coronagraph 23Experiment Data 25Crew Activities 26Related Curriculum Topics 26Suggested Classroom Demonstrations 27

SECTION 4EXTREME ULTRAVIOLET SPECTROGRAPH (S082B)EXTREME ULTRAVIOLET SPECTROHELIOGR. PH (S082A) 29Experiment Background 29Definition of Scientific Objectives 30Description of Experiment Hardware 31Experiment Data 34Crew Activity 35Related Curriculum Topics 35Suggested Classroom Demonstration 36

SECTION 5ULTRAVIOLET SCANNING POLYCHROMATORSPECTROHELIOMETER (S055) 39Experiment Data 39Definition of Scientific Objectives 39Description of Experiment Hardware 39Data 41Crew Activities 42Related Curriculum Aspects 42Suggested Classroom Demonstration 42

iii

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SECTION 6X-RAY SPECTROGRAPHIC CAMERA (S054) 45Experiment Background 45Scientific Objectives 46Equipment 47Experiment Data 48Crew Activities 49Related Curriculum Topics 49Related Classroom Demonstration 49

SECTION 7X-RAY ULTRAVIOLET SOLAR PHOTOGRAPHY (S020) 51Expoiment Description 51Definition of Scientific Objectives 51Description of Equipment 52Experiment Data 53Crew Activities 54Related Curriculum Aspects 54

SECTION 8EXTREME ULTRAVIOLET AND X-RAY TELESCOPE (S056) 55Experiment Background 55Scientific Objectives 56Equipment 57Experiment Data 59Crew Activities 60Related Curriculum Topics 60Related Classroom Demonstration 60

SECTION 9GLOSSARY 61Suggested Reference for Further Study 63

iv

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INTRODUCTION

The Skylab Education Program

This year the United States' first manned scientific space station, Skylab, will be launchedinto orbit and will be the facility in which successive crews of astronauts will performinvestigations in a number of scientific and technological disciplines. The program ofinvestigations can be divided into four broad categories: physical sciences, biomedicalsciences, Earth applications, and space applications.

The Skylab scientific program will produce information that will increase our understandingof science and will extend our knowledge of subjects ranging from the nature of the universeto the structure of the single hui an cell. It is one of the objectives of the NationalAeronautics and Space Administration that the knowledge derived from its programs bemade available to the educational community for application to school curricula in a timelymanner.

For this reason, the Skylab Education Program was created to derive the maximumeducational benefits from Skylab, assist in documentation of Skylab activities, and enhancethe understanding of scientific developments.

This document is one of several volumes prepared as a part of the education program. It hasthe dual purpose of informing high school teachers about the scientific investigationsperformed in orbit, and enabling teachers to form an opinion of the educational benefits theprogram can provide.

In providing information on the Skylab program, these books will define the objectives ofeach scientific investigation and describe its scientific background. The descriptions willinclude discussions of the scientific principles applied in the investigations and the types ofdata generated, and the types of related information and data available from other sourcesin the Skylab program.

In the preparation of these descriptions of the Skylab activities, a continuing goal has beento build a bridge between Skylab science and high school science. Discussions of thescientific background behind the Skylab investigations have been included to illustrate thescientific needs for performing those investigations in the Skylab environment. Whereverpossible, concepts for classroom activities have been included that use specific elements ofSkylab science as focal points for the increased understanding of selected subjects in thehigh school curricula. In some areas, these endorse current curriculum topics by providingpractical applications of relatively familiar, but sometimes abstract, principles. In otherareas, the goal is to provide an introduction to phenomena rarely addressed in high schoolcurricula.

It is a goal of the Skylab Education Program that these volumes will stimulate the highschool teacher to the recognition that scientific programs such as Skylab produceinformation and data that neither are, nor were planned to be, the exclusive domain of asmall group of scientists, but rather that these findings are available to all who desire to usethem.

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Application

Readers are urged to evaluate the investigations described herein, in terms of the subjectsthey teach and the text books and classroom aids they use. Teachers will be able to applythe related curriculum topics as stimuli for application of Skylab program-generatedinformation to classroom activities. This information can be in the form of film strips, voicetapes, experiment data, etc, and can be provided to fulfill teachers' expressed needs. Forexample, the teacher may suggest educational aids that can be made available from theseinformation sources, which would be useful in classroom situations to illustrate many of theprinciples discussed in high school curricula. These suggestions could then serve as stimulifor development of such aids.

As information becomes available, periodical announcements will be distributed to teacherson the NASA Educational Programs Division mailing list. Teachers wishing to receive theseannouncements should send name. title, and full school mailing address (including zip code)to:

National Aeronautics and Space Administrai,i-nWashington, D.C. 20546

Mail Code FE

The basic subject of this volume is the solar astronomy program conducted on Skylab. Inaddition to descriptions of the individual experiments and the principles involved in theirperformance, a brief description is included of the Sun and of the energy characteristicsassociated with each zone. Wherever possible, related classroom activities have beenidentified and discussed in some detail. It will become quite apparent to the reader that therelationships rest not only in the field of solar astronomy but also in the following subjects:physicsoptics, the electromagnetic spectrum, atomic structure, etc; chemistryemissionspectra, kinetic theory, x-ray absorption, etc; biologyradiation and dependence on theSun ; electronicscathode ray tubes, detectors, photomultipliers, etc; photography;astronomy; and industrial arts. The multiple educational relationships and interrelationshipsidentified in this volume are shown in the table on the following page.

Acknowledgements

Valuable guidance was provided in the area of relevance to high school curricula by Dr.James R. Wailes, Professor of Science Education, School of Education, University ofColorado; assisted by Mr. Kenneth C. Jacknicke, Research Associate on leave from theUniversity of Alberta, Edmonton, Alberta, Canada; Mr. Russell Yeany, Jr., ResearchAssociate, on leave from the Armstrong School District, Pennsylvania; and Dr. Harry Herzerand Mr. Duane Houston, Education and Research Foundation, Oklahoma State University.

vi

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The Skylab Program

The Skylab orbiting space station will serve as a workshop and living quarters for theastronauts as they perform investigations in the following broad categories: physicalsciences, biomedical sciences, Earth applications, and space applications.

During the eight-month operational lifetime of Skylab, three crews, each consisting of threemen, will live and work in orbit for periods of up to one month, two months, and twomonths, respectively.

The objectives for each of the categories of investigation are summarized as follows.

Physical ScienceTo perform observations away from the filtering and obscuring effects ofthe Earth's atmosphere in order to increase man's knowledge of the Sun and of itsimportance to Earth and mankind, to provide information in the field of stellar and galacticastronomy, and to increase man's knowledge of the radiation and particle environment innear-Earth space and of the sources from which these phenomena emanate.

Biomedical ScienceTo make observations under conditions different from those on Earthand thereby increase man's knowledge of the biological functions of living organisms, and ofthe capabilities of man to live and work for prolonged periods in the orbital environment.

Earth ApplicationsTo develop techniques for observing from space and interpreting Earthphenomena in the areas of agriculture, forestry, geology, geography, oceanography, air andwater pollution, land use and meteorology, and the influence man has on these elements.

Space ApplicationsTo develop techniques for operation in space in the areas of crewhabitability and mobility, and use the properties of weightlessness in materials research.

The Skylab Spacecraft

The five modules of the Skylab cluster are shown in the illustration.

The orbital workshop is the prime living and working quarters for the Skylab crews. Itcontains living and sleeping quarters, provision for food preparation and eating, and personalhygiene equipment. It also contains the equipment for the biomedical science experimentsand for many of the physical science and space applications experiments. Solar arrays forgeneration of electrical power are mounted outside this module.

The airlock module is the prime area in which control of the cluster internal environment,and workshop electrical power and communications sytems, is located. It also contains theairloCk through which suited astronauts emerge to perform their activities outside thecluster.

viii

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1 21...12 soe

1 Apollo telescope mount

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12 Earth resources experiments

13 Command and service module

ix

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The multiple docking adapter provides the docking port for the command/service modulesthat transport the crews, and contains the control center for the telescope mountexperiments and systems. It also houses the Earth applications experiments and spacetechnology experiments.

The Apollo telescope mount houses a sophisticated solar observatory having eight telescopesobserving varying wavelengths from visible, through near and far ultraviolet, to x-ray. Itcontains the gyroscopes and computer of the primary system by which the flight attitude ofSkylab is maintained or changed, and it carries solar arrays by which about half of theelectrical power used by the cluster is generated.

The command and service module is the vehicle in which the crew travels from Earth toSkylab and back to Earth, and in which supplies are conveyed to Skylab and experimentspecimens and film are brought to Earth.

Skylab will fly in a circular orbit about 436 kilometers (235 nautical miles) above thesurface of Earth, and is planned to pass over any given point within latitudes 500 north and50° south of the equator every five days. In its orbital configuration, Skylab will weigh over44,100 kilograms (200,000 pounds) and will contain nearly 370 cubic meters (13,000 cubicfeet) for work and living space (about the size of a three-bedroom house).

x

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Section 1The Sun

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The Skylab solar astronomy program uses a carefully. .chosenset of eight instruments in the Apollo telescope mount andone other located in another part of the laboratory. TheSkylab solar instruments are several times larger than anythat have been flown before and consequently possess greaterspectral resolution and higher angular resolving power. Thecluster of instruments is capable of simultaneously observinga wide range of spectral bands. These instruments arecontrolled by a trained observer who can analyze the datapresented and alter the observing program as required by thechanging solar conditions. I-le will be assisted by an extensivestaff of ground based observers and analysts who will aid himin determining the most productive observations to makewith the Skylab instruments. In addition to providing anobserver to operate the instruments, the presence ofastronauts also allows the use of high resolution photographicfilm to record data because the astronaut can change camerasand return the film to Earth at the end of the mission. Thesolar astronomy program will be augmented by ground basedobservations of the Sun and the geophysical phenomenaresulting from its activity. While Skylab is in orbit, a series ofsolar observatories around the world will keep the Sun widercontinuous surveillance to warn of impending solardisturbances and acquire data to complement the Skylabobservations.

The phenomena observable on the Sun can be detected bySkylab with a resolution down to the limiting resolution ofground based instruments. These phenomena vary on atimescale of seconds and minutes in size, shape, andcomposition.

IMPORTANCE OF SOLAR ASTRONOMY

There are three major reasons why the study of the Sun isimportant:

1) Solar phenomena have a great influence on the Earth.The Sun is the ultimate source of all energy on the Earth,and all terrestrial life depends on it. Solar activity canadversely affect radio communication by causingionospheric disturbances and may have an effect onatmospheric circulation patterns that cause changes in theweather, although the exact mechanisms are notunderstood. Changes in the total energy radiated by theSun over long periods of time may have been responsiblefor past ice ages.

2) Because our knowledge of the Sun is greater than for anyof the stars, many of our theories of stellar structure andevolution are dependent upon solar data to provide a

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known reference point; therefore, any deficiencies in ourknowledge of the Sun will result in corresponding errorsin stellar theory. Since the Sun is the only star on whichsurface features can be distinguished, study of itsstructural features will lead to insight into the processesthat we infer may be happening on other stars. Studies ofthe stars spectra show magnetic fields, flares, and otherphenomena such as we see on the Sun.

3) By observing solar phenomena we can study atomic,nuclear, and plasma physics, aerodynamics,hydrodynamics and magnetohydrodynamic phenomenathat are unobservable on Earth. The stellar atmospherehas a range of temperatures, pressures, and magnetic fieldintensities extending over volumes several times the sizeof the Earth that could never be duplicated for study inany laboratory on Earth. Such knowledge could help usto achieve controlled thermonuclear reactions.

HISTORY OF SOLAR SPACE RESEARCH

Using sounding rockets, balloons, and small unmannedspacecraft, solar astronomers have studied the Sun, the upperatmosphere, and near Earth space since the end of World WarIL These studies have revealed much about the general natureof the Sun in the x-ray and ultraviolet regions.

Until satellites and sounding rockets were developed it waspossible to observe solar emissions only at wavelengths in theradio, infrared, and visible portions of the spectrum thatcould penetrate the Earth's atmosphere. Thus, the ultravioletand x-ray radiations, which are important to the study ofhigh energy solar phenomena, could not be studied. Inaddition, the daytime atmospheric scattering of visible lightcauses the sky to be so much brighter than the solar coronathat this phenomenon is only visible during the rare solareclipses and then only for relatively short distances from thesolar surface. Atmospheric turbulence causes shimmering ofthe observed image limiting the resolution with which detailcan be observed on the surface of the Sun.

Use of orbital spacecraft, such as Orbiting Solar Observatories(0S0), Orbiting Geophysical Observatories (OGO), and theU.S. Navy SOLRAD satellites, has resulted in a steadilyincreasing understanding of the explosive and energetic solarprocesses. However, the ability to obtain observations ofsufficiently high resolution in energy, time, and space is stilllimited by the size of the instruments that can be carried byunmanned spacecraft and the need to communicate with thespacecraft through a telemetry system.

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SOLAR ZONES

The general structure of the Sun is illustrated in Figure 1.

Corona 0 5 to 3 million °C9 6 million km out fromchromosphere

Chromosphere1000 to 500,000-C130,000 kmthick

Flare \-1400,000 km V.high

Convection _Zone

Photosphere5700 to 4000-C4800 km thick

Figure 1 Solar Zones

z. Sun Center; 16 million C

1,360,000 kmdiameter

With acknowledgement tc theNational Geographic Society

The energy source of the Sun is at its center when hydrogennuclei are converted to helium at a temperature ofapproximately 16 million degrees. Electromagnetic energygenerated at the center requires 10 million years on theaverage to diffuse outward to the cooler surface where it isradiated into space. In the last 64 thousand kilometers (40thousand miles), the energy is transmitted by the convectivemotion of solar material that extends into the photosphere,which is the deepest level into the Sun that we can opticallyobserve and the region from which energy is radiated. Thephotosphere is approximately 5 thousand kilometers (3thousand miles) thick and has temperatures as high as 6000°C.

Overlying the photosphere is a layer in which the density isdecreasing but the temperature increases with increasingheight. It is in the chromosphere that the absorption thatproduces the famous Fraunhoffer dark lines in the solarspectrum takes place. At the top of the chromosphere is avery steep temperature gradient that marks the transitioninto the solar corona which is characterized by its lowdensity and very high temperature of several million degrees.Determination of the variation of the temperature, density,electron pressure, and other physical parameters in thetransition region between the chromosphere and corona isone of the current problems of solar physics. Anotherproblem that must be theoretically explained is how energy istransmitted into the corona from lower layers to cause thelarge temperature gradient. It is theorized that mechanicalenergy from the convective zone is converted into sound

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waves in the chromosphere that dissipate the energy whenthey enter the low density corona and cause the high coronaltemperature. A search will be made with the Skylabinstruments for changes in the chromosphere and coronaassociated with an oscillation with a 300-second period thathas been observed in the photosphere.

Surrounding the chromoSpbere is the corona which extendsvisibly outward from the chromosphere for several millionmiles, The corona is very faint in comparison with theluminosity of the solar disk. The only time that its outerreaches can be observed from Earth is during a solar eclipse.The coronal material is of very low density and is highlyionized because of temperatures up to several million degreesthat exist in the region. As a consequence, most of the lightemitted froM the corona is caused by scattering from the freeelectrons that have been ionized from the coronal material.

The accurate measurement of the intensities of the solarradiation over the wide wavelength range made possible bythe Skylab solar observatory instrument cluster will be usedby solar scientists to determine the variation of temperatureand other physical properties with depth in the solaratmosphere. The fact that different types of radiation areformed at different depths enables us to prol)e at differentlevels into the Sun and reconstruct the solar atmosphere. Ingeneral, the shorter wavelength radiation will be emittedfrom the hotter plasma which is at higher altitudes in thesolar atmosphere. For example, extreme ultraviolet radiationcomes from the region where the upper chromosphere isblending into the lower corona, while x-rays are formed inthe high temperature corona.

The most spectacular solar phenomena are the flares thatoriginate in the chromosphere and lower corona and extendoutward in some cases ejecting material from the Sun. Solarflares are associated with the strong magnetic fields found insunspots and transfer the energy stored in the magnetic fieldinto electromagnetic radiation, high energy particles, bulkphysical motion of the gas, and radio frequency emission in acatastrophic event that is not totally understood. A solarflare begins very rapidly and is characterized by a suddenincrease in the H-alpha emission as well as a rapid build-up inthe x-ray and ultraviolet emission. The flare spreads rapidlyfrom a small origin across the surface covered by a sunspotgroup, reaches a maximum intensity in a few minutes, andthen declines. A medium sized flare lasts perhaps half an hourwhile a large flare may last four or five hours. In addition tothe electromagnetic radiation, large clouds of solar materialare ejected outward from the flare and high energy cosmicray particles are generated. The total quantity of energyreleased in a large flare is many times the energy generated byall man made sources, but is only a small fraction of the total

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energy produced by the Sun. For example, it has beenestimated that in a one-hour period during the massive solarstorm which occurred in August 1972, the storm, producedenough energy to meet the electrical power demands of theUnited States for 100 years at present consumption rates.

The classification of the many geometrical forms observed inflares and the associated structures observed in the solaratmosphere around sunspots where flares take place, showsthe phenomena to be a very complicated set of interactionsinvolving plasma instabilities, electrodynamic andhydrodynamic effects.

Skylab will try to observe a number of solar flares with asmany of the solar experiments operating simultaneously as isfeasible. The objective is to obtain a diverse and extensivecollection of data which can be used to determine the pointwhere the flare started, physical conditions such as thetemperature, density, magnetic field strength, and particlevelocities in the flare plasma and the surrounding medium,and to determine how these conditions change before,during, and after the flare.

One of the primary scientific objectives is to obtain acomprehensive set of observations of a solar flare from itsearliest detectable stages with the Skylab high resolutioninstruments. It is hoped that the ultraviolet and x-rayobservations will provide the information required todetermine the mechanism responsible for triggering flares andwill enable better predictions to be made of future flares.Obtaining flare data should be facilitated by the five-monthtime the Skylab instruments can operate in orbit. Flare datafrom sounding rocket instruments is difficult to obtainbecause of the difficulty of launching the rocket at theproper time to obtain data during the early phases of theflare.

Large solar flares can cause geophysical disturbances. X-raysand ultraviolet radiation greatly increase the ionization of theupper layers of the atmosphere on the sunlit side of Earth,resulting in disrupted radio communications. About an hourafter the flare, high energy cosmic rays from the Sun arrive atEarth. In a day a large cloud of low energy plasma ejectedfrom the flare arrives and causes magnetic storms. Highenergy particles enter the atmosphere at the magnetic polesand cause auroral displays and cosmic ray effects.

Associated with data taken on solar flares is a longer durationstudy of active regions. The three-dimensional structure ofthe active regions will be investigated to determine thehorizontal and vertical variation of the temperature, density,velocity, and magnetic field. The structure of the

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photosphere, chromosphere, transition region and coronaabove sunspots will he given careful attention to determinehow it relates to the production of flares and other transientphenomena. Both short-term (minutes to hours) andlong-term (days) changes will be documented. The velocityfields in the chromosphere and corona over sunspots andactive regions will be mapped by means of the Doppler shiftin spectral data and the mass motion shown in thecoronagraph pictures.

Several types of structures are visible on the solar surfacewhen viewed through an appropriately filtered telescope.Large sunspot groups with complicated structures are quitenoticeable and persist for weeks undergoing changes in sizeand form. The spots are regions where strong magnetic fieldsexist and inhibit the convective motion of the photosphericmaterial sc that the sunspots are cooler than the surroundingphotosphere. The magnetic field forms large loops up intothe corona from sunspot groups. Condensations of coronalmaterial formjn these loops that are visible as arch-shapedprominences when viewed against the limb and as filamentswhen projected against the disk. Regions of enhancedemission in the spectral lines of certain elements are calledplages and often surround sunspot groups. Most of the solardisk shows a mottled appearance as thc. tops of convectivecells that are heated in the solar interior appear, rise, radiateaway energy, and sink below the visible solar surface. Theextensions of these patterns into the chromosphere andcorona will be investigated with the ultraviolet and x-rayinstruments on Skylab. The variations of temperature anddensity as a function of height and the velocities of the gaswill be obtained.

Because the Skylab solar observatory instrument operationconsumes a considerable portion of the astronaut's workingtime, it is essential to combine the observation requirementsof the scientific investigations in ns compact a sequence ofdata-taking operations as possible. This is accomplished bydefining eleven Joint Observing Programs that investigatesolar phenomena such as prominences and filaments, coronaltransients, and the solar wind. The observations onprominences and filaments will study their evolution as theycross the disk as the Sun rotates and determine thethree-dimensional structure (temperature and density) of thefilaments and the surrounding material of the chromosphereand corona. The solar wind will be investigated by studyingthe evolution of the chromospheric network and itsextension into the corona and the rate of expansion ofvarious parts of the corona. Transient coronal phenomenawill be observed in the visible, ultraviolet, and x-ray regionsto determine the spatial and temporal development of thefeatures, the velocity of propagation, and the correlationwith surface and inner coronal features.

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SCIENTIFIC CONSIDERATIONS

Skylab will carry two telescopes to observe the Ii -alphaspectral line at 6563 angstroms. I-I -alpha is red light emittedby the hydrogen gas from the photosphere. The intensity ofthe emission varies with structural features on the solar diskand displays many fine scale features in the solar atmosphere.Solar flares are easily observed in H-alpha light because thereis a large change in the intensity of this emission while thetotal visible radiation varies very little. By tuning the filterslightly off of the center of the line, the Doppler shiftedradiation from gas in motion towards or away from theobserver may be viewed. The H-alpha telescopes will be theprincipal aiming device the astronaut will use to point theother instruments, and will give him a common referencewith the ground-based observers during the .mission. Thetelescopes will gather data of uniform high quality takensimultaneously with data from the other instruments.

The ultraviolet spectrographs will develop high resolutionspectra of very small areas of the Sun in a wavelength rangethat cannot be observed from the ground. This is animportant region because the ultraviolet lines arise from theregions of the chromosphere, and transition region into thecorona, where analysis of the wavelength and intensities ofthe emission spectral lines permits determination of thetemperature, density, and electron pressure of the level in theSun where the radiation originated. The ultraviolet radiationis strongly enhanced in solar flares as the temperatureincreases.

Skylab spectrographs use both photographic and electronicmethods of data recording: photographic techniques givehigh spatial resolution and can gather large amounts of datain a short period of time; electronic detectors are used toobtain high accuracy in measuring the intensity of theemitted lines.

High energy x-rays are formed in the corona where milliondegree temperatures strip the electrons from the atoms andproduce a high degree of ionization. Low resolution spectraand photographic imaging will be obtained by the two Skylabx-ray telescopes. The spectra will enable temperatures of theemitting regions to be determined, and the imaging will becorrelated with images taken in other wavelengths todetermine the spatial and temporal development of a solarregion that produces a flare.

Another form of electromagnetic radiation from the coronais viewed in white light scattered from the free electrons. Theintensity of the scattered light is a measure of the electrondensity of the corona and displays arcs, rays, and streamersthat trace the magnetic field structure. Observation of thecorona after a flare shows the response of this large, low

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density medium. to a disturbance in its lower layers. Becausethe radiation from the corona is a million times weaker thanthe radiation from the solar disk, a coronagraph must bedesigned with special optics to block out the intense lightfrom the solar disk. The Skylab coronagraph will be able toobserve higher detail and greater distances from the Sun thansimilar instruments operating from the ground because it willbe above the scattering effects of the Earth's atmosphere.

SKYLAB SOLAR OBSERVATORY

The Skylab solar observatory contains the eight telescopesmounted on a common structure (the Apollo telescopemount)1) white light coronograph (S052),

2) ultraviolet spectrograph (S082B),

3) ultraviolet scanning spectroheliometer-polychromator(S055),

4) extreme ultraviolet spectroheliograph (S082A),

5) x-ray telescope-spectrographic camera (S054),

6) x-ray spectrograph (S056),

7) two H-alpha telescopes,

8) and another extreme ultraviolet spectrograph (S020)which is operated from an airlock in another part ofSkylab.

The first eight instruments are mounted in a canister so thatall of the telescopes can point to the same area of the solarsurface. The complete observatory is rigidly attached to thebody of Skylab the mass of which provides the stable basenecessary for maintaining the pointing stability required forastronomy. The common pointing axis of the assembly is notexpected to drift more than 1/700 of a degree during a15-minute period-equivalent to about 4000 kilometers (2500miles on the Sun's surface. [The diameter of the Sun is1,390,000 kilometers (864,000 miles) and sunspots vary insize from 800 to 80,000 kilometers (500 to 50,000 miles) inwidth.]

With its eight telescopes observing the Sun in several spectralbands, Skylab provides the first opportunity to perform longduration, highly detailed studies of the Sun in the visibleultraviolet, extreme ultraviolet, and x-ray spectral regionssimultaneously. (See Figures 2 and 3.) The operating lifetimeof Skylab will also permit the observations of solar eventsthrough their life cycles. A sunspot may persist throughseveral 27-day rotations of the Sun.

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EXPERINIENT SCHEDULES

The solar telescopes are scheduled to operate more than 100hours during the first manned mission of Skylab. The secondand third missions will schedule additional operations so thatseveral hundred hours of solar observation will be achievedduring the entire Skylab flight program.

The complement of solar observing telescopes will usually beoperated in unison; however, they may also be operatedindividually or in partial groupings. The selection oftelescopes used for an observational period is at thediscretion of the astronauts, the flight control center at theJohnson Space Center in Houston, and the principalinvestigators associated with the investigations.

Another Skylab experiment program, the Earth resourcesexperiments (EREP), requires a pointing attitude in whichsolar observation is not possible. If, during one of the EREPobservation sequences, a significant solar event commences,this sequence may be interrupted in favor of solarobservation.

Since a major Skylab scientific objective is to observe andrecord the onset of solar flares which are not predictable andof short life, one x-ray telescope is equipped with an x-rayalarm. The x-ray alarm will alert the crew to an impendingsolar event and permit adjustment of flight operations toobserve the solar flare.

CREW ACTIVITIES

The solar telescopes are operated by an astronaut from acontrol and display panel equipped with the necessaryswitches, meters, and with television displays by which thecrewman may point the telescopes to the desired solarfeature.

Photographic exposure speeds, camera, and telescopeadjustments and selection of individual telescopes to be usedfor observation are also controlled by the astronaut,

The end of the first manned mission and at the beginning,middle, and end of the two later missions, two astronauts inEVA space suits will emerge from Skylab to retrieve theexposed film or to install new film.

DATA AVAILABILITY

The data which accrue from the solar telescopes, are in threeforms: television, photographs, and tape-recorded digital data(similar to computer tape.)

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The television pictures from those telescopes equipped withtelevision cameras may be transmitted to ground for use byground-based personnel to evaluate and direct telescopeperformance. Television pictures are also used by theastronaut at the control and display panel. Some of thetelevision pictures may be released to the public duringSkylab flights.

Photographic data are returned to Earth at the conclusion ofeach mission. These data represent pictures of the Sun in thevarious spectral wavelengths and scientific data in the form ofspectrographs; both types are required by interested scientiststo conduct studies of solar events.

The tape-recorded digital data which result from the extremeultraviolet scanning spectroheliograph and the x-ray eventanalyzer is transmitted to ground through the radiocommunication system. As received these data cannot beused. It must be processed by a computer which will thenprovide tabular and/or graphic outputs for analysis.

The principal investigators and scientists retain proprietaryrights to the data from their experiments for a period of oneyear to provide adequate time for data analysis and studybefore the findings are made public.

Aside from that information which NASA may release duringthe Skylab flight, it may be expected that results of theSkylab solar observations will be made public early in 1975.

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Section 2Hydrogen-AlphaTelescopes

CN

0.4=

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HydrogenAlpha

EXPERIMENT BACKGROUND

The hydrogen-alpha (H-alpha) telescopes are designed to viewand record image of the Sun in the unique red lightproduced by hydrogen.

The hydrogen atom has only one electron. In the normalstate the electron orbits the nucleus in an orbit called theground energy level. When the atom is stressed by an externalenergy, either electrical or thermal, the electron absorbsenergy and assumes a new orbital energy level. A number ofdistinct energy levels in which the electron may orbithavebeen discovered. The largest energy level that can be achievedis that at which the electron is removed completely, leavingthe nucleus (or a positive hydrogen ion).

When the external energy that drove the electron to higherorbital levels is removed, the electron gives up energy whichit had absorbed and returns to a lower level. The energywhich the electron gives up is radiated at a unique wavelengthdepending on the energy level which it leaves and to which itreturns. A series of transitions can be defined for the variousenergy levels. Those energy transitions for which the radiatedenergy is in the form of visible light are known as the Balmerseries, of which H-alpha is the transition from the third tosecond energy level. Other series are the Lyman seriesinvolving transitions to ground level and radiations ofultraviolet light, and the Paschen series involving higherenergy levels, and production of infrared radiation.

The hydrogen atom and the various energy levels andtransitions which can occur in the hydrogen atom are shownin Figure 1. When the electron returns from the third to thesecond energy level, it emits H-alpha light, which is a uniquered light at a wavelength of 6563 angstroms.

Paschenseriesinfrared

Ionization level

Lyman seriesultraviolet

Figure 1 Hydrogen Series

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Aa ngs trom units are theinternational unit for measuringlight wavelengths-

1 A = 10-8 cm.

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H-alpha of theSun

Ground BasedH-alphaObservation

H-alphaCharacteristicsof Sun

14

Thermal and magnetic conditions of the Sun are of suchmagnitudes that H-alpha radiation is a prominent componentof the spectrum. Observation of the Sun with a telescopefitted with a filter that transmits only H-alpha red light (6563± 1 angstrom), reveals details of the solar surface and itsactivity which are obscured by the many other wavelengthsthat comprise white light. Figure 2 is a typical H-alphaphotograph of the Sun.

Figure 2 Solar Image in H-Alpha

Through a worldwide network of ground based telescopeswith H-alpha filters, maintaining continuous surveillance ofthe Sun and monitoring solar activity (sunspots, flares, etc),it is possible to predict when the radiation from the solardisturbance will arrive at Earth to cause geomagneticdisturbances and ionospheric storms that disturb radiocommunication and navigation systems, and affect wind andweather patterns.

The solar wind which causes storms on Earth is composed ofions and cosmic particles (materials of the Sun) whose arrivalis heralded by electromagnetic radiation. The velocity of thesolar wind is less than the speed of light since it is composedof material and is not energy.

DEFINITION OF SCIENTIFIC OBJECTIVES

When observations are made with a H-alpha filter, features ofthe Sun's chromosphere are seen. The change that occurs inH-alpha radiation relative to various solar features ispresented in Figure 3. The contrast between these featureshas been used by solar astronomers for many years.

Solar Radiation Spectrum

H-alpha6563 A

2000Visible3500 to 7000 A

15,000 A

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H-alphaTelescopeFunction

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150

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Figure 3 H-Alpha Across Solar Phenomena

The H-alpha telescopes of the Apollo telescope mount areused to identify features on the Sun which can be examinedwith the other instruments of the cluster (see Sections 3 thru8). The camera on the H-alpha telescope will record wherethese other instruments are pointing. Significant sources ofultraviolet and x-ray emission can then be correlated withfeatures recognized in the H-alpha telescopes.

Images of the Sun in H-alpha light are provided for astronautuse on a television monitor. The crewman will then be able topoint the other instruments accurately at a feature ofinterest.

Terrestrial H-alpha telescopes are beset with a shimmer intheir images because of air turbulence .md scattering. TheH-alpha telescopes of Skylab will be above the atmosphereand will provide H-alpha photography of greater resolutionthan can be obtained from the ground.

EXPERIMENT DESCRIPTION

Ground based telescopes for H-alpha observation are usuallyrefracting telescopes in which lenses are used to focus animage. These telescopes are necessarily long when a smallfield of view is required. In the Skylab cluster, the telescopemust have a short physical length but still retain a narrowfield of view (approximately 1 degree). A Cassegraintelescope is used for this purpose.

Light entering the telescope is reflected by a sphericalconcave mirror. The reflected light is intercepted by asecondary convex spherical mirror which reflects the lightthrough an aperture in the primary mirror and is brought to afocus behind the primary mirror. Thus, while the opticallength of the telescope is retained by the multiple reflections,the physical length is shortened.

PrimaryMirror-,

Focus__

SecondaryMirror

Cassestrain Telescope

15

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H-alpha Since (-alpha light is energy at a unique and discreteFilter wavelength, a filter that transmits only a very narrow band

about that wavelength is required to isolate that wavelengthfrom the other components of white light. The spectraltransmission of the II-alpha filters in Skylab telescopes is6562.8 ± 0.35 angstroms. This filter is properly called aFabry -Perot interferometer.

Fabry-Perot The Fabry-Perot interferometer is developed from theconcepts of constructive and destructive interference ofradiating waves. When two waves of the same wavelengtharrive at. the same point in space and are in phase, they willreinforce each other, or constructively interfere. If the twowaves are out of phase they will cancel each other, ordestructively interfere.

The Fabry-Perot interferometer is constructed from a disk offused silica which is 125 microns thick. The disk is opticallypolished and is coated with a semireflective surface on bothfaces. The faces of the disk are also maintained in parallel.

White light is composed of many wavelengths. At somewavelengths, the thickness of the disk will be equal to an oddnumber of wavelengths; at other wavelengths, the disk is aneven number of wavelengths thick.

Light that enters the interferometer is multiply-reflectedbetween the internal faces of the disk. For those wavelengthsfor which the disk is an even number of wavelengths thick,the internal reflections will constructively interfere with theincident light, and those wavelengths will be transmittedthrough the interferometer. At wavelengths for which thedisk is an odd number of wavelengths thick, the internalreflections will destructively interfere with the incident lightand no transmission through the interferometer will result.Between these two extreme conditions, partial destructiveinterference takes place and the incident light is attenuated.Figures 4 and 5 illustrate the construction and operation ofthe Fabry-Perot interferometer and the resultant filtertransmission characteristics.

Reflective coatings

1X, 3X, 5X, 7X (n - 1)XDestructive interference

Figure 4 Interferometer Reflections16

2X, 4X, 6X nXConstructiveInterference

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0.5

.2 0.5

ro

c.;

CO

qrtl

0.5

4 4

Interferometertransmission

Bandpass filtertransmission

Combinedtransmission

1.41- 13 A 13 A6550 A 6563 A 6576 A

Figure 5 Interferometer Transmission

Throughout the spectrum of white light, there are manywavelengths that satisfy the conditions for constructiveinterference. Thus, in white light the output of theinterferometer will be light which is composed ofwavelengths at discrete intervals throughout the spectrum.

To make the interferometer wavelength selective in theH-alpha red light only, a bandpass red glass filter is placed infront of the interferometer so that only red light in thespectral region of H-alpha is admitted to the interferometer.The addition of the red bandpass filter is shown in Figure 5.

The complete Fabry-Perot interferometer H-alpha filterpasses 20% of the light within the very narrow bandpasswhich it provides.

Red Yellow Green Blue

Interferometer Outputin White Light

17

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Tv pe

Highlights

Analysis

The thickness of the fused silica disk (125 microns) istemperature-dependent; thus close temperature control towithin 1.00C is required to maintain the wavelengthselectivity. Temperature control can also be used to tune thefilter +1 angstrom either side of 6562.8 angstroms. Thistuning is useful in detecting motion of solar events byutilizing the Doppler wavelength shift. H-alpha radiationsources moving toward the telescope will exhibit awavelength shorter than usual. Events receding from thetelescope will exhibit longer wavelengths.

The H-alpha solar image transmitted by the Fabry-Perotinterferometer is sent to a beam splitter which divides thelight into two paths. Ninety percent of the light is directedinto the television camera and ten percent is directed to thefilm camera for photography.

EXPERIMENT DATA

Experiment data are in two forms: television andphotographs. The television circuits are used primarily by themonitoring astronauts; the television data are transmittedto Johnson Space Center for use by ground flight controlpersonnel. Some of the H-alpha television images may also besupplied to TV networks for public information during themission.

Photographic data are returned to Earth after the mission.While the data will be used primarily for correlation of solarevents with the data from other instruments, it is expected tobe of higher resolution than that obtained from ground basedtelescopes; thus, it may be of scientific merit.

The H-alpha photography shows details of solar phenomena;plages, filaments, flares, and sunspots are clearly defined.

Correlation of H-alpha photographs with data fromultraviolet and x-ray images and corona photographs(Sections 3 thru 8) are expected to yield information on theenergy-producing mechanisms of the Sun and the manner inwhich that energy is radiated to Earth and space.

Availability Other than some TV pictures which may be released duringthe mission, data from the H-alpha telescopes may not beavailable for public use before 1975. Procedures forrequesting copies of flight data will be announced at a laterdate. Section 1 describes plannned data uses andrequirements.

18

Beam splittera lightly coated,partially transparent mirror,which reflects part of the lightincident on it and transmits theremainder.

Incidentlight

f Reflected

Transmitted

Beam splitter

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CREW ACTIVITIES

The astronaut will use the TV display from the H-alphatelevision camera to point other telescope mountinstruments. He will. also evaluate the image to decide whichof the other instruments will provide the most useful dataand to determine those to be used for a particularobservation. Other related crew activities are discussed inSection 1.

RELATED CURRICULUM TOPICS

Concepts employed in the design and application of theH-alpha telescopes may be related to a number of subjectswhich are discussed in high school science programs. Subjectsin astronomy and physics are readily apparent topics:

Astronomy

Details of sunspots, filaments, spicules, and plages;

Relationship between sunspots and other features;

Life cycle of sunspots.

Physics

Opticsconstructive and destructive interference, interferom-eters, spectral analysis;

Nuclear, thermal and energy considerations of the Sun.

SUGGESTED CLASSROOM DEMONSTRATION

Interferometer While the isolation of the single wavelength of H-alpharadiation at 6563 angstroms is important in the H-alphatelescope, the principle employed is also found in other fieldswhere spectral isolation or analysis is performed. Lasertechnology also employs wavelength interferometers. Thisdemonstration will show how the interferometer is used toseparate a single wavelength in comparison with a fullspectrum.

Materials required for the demonstration are a-

1) Sun screen with two pinhole apertures;2) 6-inch long triangular disperSing prism. Two small prisms

may be used but their dispersed spectra must bealigned;

3) at least three colored (red, green, blue) bandpass filters.These are 2-inch squares of colored glass;

19

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4) two optically flat 2-inch diameterelements (optical flats);

5) suitable white projection screen.

interferometer

Components of the demonstration are arranged as shown inthe accompanying sketch.

N

Sunscreenwithapertures

#/1 Path A

Path B

Prism

7

Coloredbandpassfilters

Interferometer

1) Pass the sunlight through the apertures of the Sun screento the prism to obtain two identical spectra of the Sun.

2) Insert the colored bandpass filters, one at a time, intolight path B. As each of the filters is inserted, only itsparticular color will remain in the spectrum on thescreen. The other colors have been rejected by the filters.

3) Insert the interferometer into light path B and comparethe resulting spectrum with that from light path A. Theresulting spectrum will show bright lines at thewavelengths where constructive interference occurred inthe interferometer.

4) Insert both the red bandpass filter and the interferometerin path B and compare the resulting spectra. Thespectrum of light path B will be a few red lines. Thenumber of lines is dependent on the range of wavelengthspassed by the bandpass filter.

5) Repeat step 4 for the green and blue bandpass filters,respectively. Note that there will be more blue lines thanred lines. Why? What is the difference between red andblue light?

20

Construction of interferom-eterthe interferometer isconstructed from two opticalflat interference flats and a ringof 1-mil steel shim stock. Theshim stock ring is of the samediameter as the interferenceplates. The inside diameter ofthe ring is 0.5-inch less.

Diameter ofinterferenceflats

The shim stock ring is nowsandwiched between the opticalflats to form a 0.001-inchspacer.

Opticalflat

Spacer

Opticalflat

SUGGESTED SOURCES

Prisms and optical flats may beobtained from:

Edmund Scientific Co.Barrington, New Jersey

Bandpass filters may be obtainedfrom:

Special OpticsCedar Grove, N.J. 07009

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Section 3White LightCoronagraph

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SolarPhysics

EXPERIMENT BACKGROUND

The white light coronagraph has been designed to gatherextensive photographs of the Sun's corona during the7-month lifetime of Skylab.

The Sun is surrounded by a great cloud of hot gases andionized atoms of solar materials. This cloud, called thecorona, extends visibly for millions of miles outward fromthe Sun. Temperatures of the corona vary fromapproximately 500,000°C at the chromosphere to3,000,000° C at the outer fringes.

Although the corona is very luminous, it is 10-'times as bright as the Sun. Normally light

from the brighter solar disk is scattered and diffused byEarth's atmosphere and obscures the corona. Consequentlythe only way the corona can be seen from Earth is duringbrief periods of total eclipse. At these times the Moon passesbetween' Earth and Sun to occult the Sun and eliminates thesource of atmospheric scattering. it is important to note thatthe occulting object (Moon) is outside of the atmosphere.

Above Earth's atmosphere where scattering is no longerpresent, an artificial eclipse can be created with an occultingdisk. A disk with a diameter slightly larger than the apparentSun is used to occult the Sun. The disk of the Sun subtends(forms) an angle of 32 minutes (approximately 0.5 degree). Asmall disk of 15 mm (0.6-inch) diameter at a distance of 152cm (5 feet) from the eye, subtends the same angle and willprovide an artificial eclipse. This concept is the basis of thedesign of the white light coronagraph.

Coronastreamers andfilaments

Near Corona Out 1 Solar Diameter Recorded during Eclipse

Coronaa zone of hot gases andionized atoms radiating from theSun for millions of miles. Verylittle of the corona is visiblefrom Earth.

Occult, occultingthedisappearance of one heavenlybody behind another.

21

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CoronagraphUses

Limits

22

Very few extended studies of the corona have been possiblebecause of the short duration of total eclipse. Seven or eightminutes is the maximum time of total eclipse in the fewlocalities of Earth over which totality occurs. As was notedearlier, during eclipse the occulting disk (Moon) is outside ofEarth's atmosphere and effectively eliminates atmosphericscattering. When the occulting disk is located on Earth andinside of the atmosphere, scattering still occurs and preventsobservation of the corona. Also coronal light is severelyattenuated (reduced) as it passes through the atmosphere.The sunlight on Earth has only 75% of the intensity of thelight shining on the upper atmosphere. Therefore, evenduring an eclipse, much of the faint light of the corona islost.

Region oftotal eclipse

ICoronaisal.M.4vyr-

Earth ..,..

a...,....,6,3_ Scattering

Occulting discAtmosphere

Principles of Eclipse Versus OccultingDisk for Corona Observation

The coronagraph is useful on Earth-based telescopes toobserve the chromosphere and solar flares. In this usage, theluminance of the chromosphere is brighter than the attendantscattering. Also H-alpha light is prevalent in thechromosphere so that a coronagraph equipped with anH-alpha filter permits observation of these events. Theconcepts of H-alpha filters with regard to H-alpha telescopesare discussed in Section 2 of this volume.

DEFINITION OF SCIENTIFIC OBJECTIVES

Despite the meager data on corona observations that areavailable, it has been found that the corona varies withsunspot activity and solar magnetic fields.

A coronograph situated above Earth's atmosphere will not beaffected by the scattering and attenuation of the atmosphere.It will view the corona to the sensitivity limits of thephotographic film that records the image.

rf Flare

Chromosphere

EglagEMINIft4 Edge of occulting disk

Flare observation using acoronagraph.

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RecordingIntervals

PolarizationFactors

OccultingDisks

Over the extended time available during Skylab, thecoronagraph will record photographs .at varying intervals.Intervals as short as 13 seconds will permit analysis of rapidmotion of material in the corona.

Since the Sun rotates on an average of 28 days, observationsof the corona with relation to solar rotation will also beprovided. The coronal features associated with surfacefeatures observed by H-alpha telescopes (Section 2) can becorrelated as the solar features appear at one solar limb or theother.

The light radiating from the corona is polarized; that is to saythe waves are highly oriented in a given plane. Thepolarization of the corona results from interaction of theionized atoms emanating from the Sun and the magneticfields of the Sun and solar storms. Polarizing filters in thecoronagraph will permit the polarization of the corona to hedetermined and its relationship to the sunspot activity andmagnetic fields that affect the corona.

DESCRIPTION OF CORONAGRAPH

The white light coronagraph is a long tube having severalocculting disks coaxially mounted in its length. A singleocculting disk will have light diffracted about itscircumference. The diffraction of a single disk would create adiffused light ring and poorly defined image at the imageplane of the coronagraph. Consequently, additional disks,which are slightly larger than the first disk, are used tointercept the undesirable diffracted light.

Light diffraction over an opaque edge is shown in theillustration. When a beam of white light impinges on anopaque material, light passing over the edge of the material isdiffracted. The light of the longer wavelengths (red) is bentto a greater extent than the shorter blue wavelengths.Diffraction is discussed more fully in Section 4.

White light - An = X2 Xn1,0

31.

Opaqueedge

blue2 gree n

X3red

Diffraction of White Light Passing Over an Opaque Edge

Diffusedshadowarea

Solar rotation is not uniform;rotation is 25 days at theequator and 30 days at thepoles.

Solar limbthe extreme edges ofthe solar disk.

Solarlimb

Polarizationa distinctorientation of the wave planeand travel of electromagneticradiation.

Diffractionwhen a ray of whitelight passes over the edge of anopaque surface, the componentsor the light are bent inproportion to their wavelength.White light is diffracted into aspectrum.

23

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Field ofView

PolarizingFilters

Camera

24

One of the occulting disks is adjustable in its position alongthe length of the tube and laterally in the tube to providefine adjustment of the occulting. This disk has electronicphotosensors positioned about it so as to detect the edge ofthe shadow of the other disks and provide pointing signals formanual or automatic operation of the instrument.

The space or annulus formed by the occulting disks and theinside wall of the tube defines the field of view of thecoronagraph. The field of view pennii,s observation of thecorona from 1.5 to 6 solar radii (4 million kilometersplus,the solar radius is almost 700,000 kilometers). Lightfrom the corona travels down the tube to the field lenses andfolding optics which focus the image of the corona at theimage plane of the camera.

Adjustable disk andpointing sensors

Main occultingdisks

Imaging lensesBeam splitter

Film

Foldingmirrors 1.001011

Polarizingfilter

Cameras

Optical Scheme of White Light Coronagraph

By using a series of polarizing filters in front of the imageplane, the orientation or polarization of various areas of thecorona can be determined. The coronagraph uses three filtershaving different polarizing orientations. High intensity imagesof parts of the corona indicate polarization of the coronallight in the orientation direction of the specific filter used. Assunspots and solar magnetic fields vary, the polarization willalso vary.

After the light has passed through the polarizing filter, it istransmitted through a beam splitter (Section 2) which dividesthe light into two paths: one path presents an image of thecorona on a TV camera which displays the corona to theastronaut; the other path provides an image of the corona onthe film camera which records the images on 35 millimeterfilm. The film magazine has a capacity for 8025 exposures.

Annulus

Coronagraphtube

Occultingdisk

Folding opticsa system ofmirrors that reduces physicallength of a telescope whilemaintaining optical length.

Polarizing filter

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Type

EXPERIMENT DATA

Data obtained by the coronagraph will be in the form ofphotographs of the Sun out to 6 solar radii. Thecharacteristics of the expected photographs are shown in theillustrations; picture A was taken from Earth during the solareclipse of 1966; picture B shows the solar eclipse of 1970.The variation in shape and filament and streamer structureillustrates the changes that occur in the corona. The whitespot in picture A is the planet Venus.

A 1966 Eclipse

B 1970 Eclipse

7

Highlights The photographs from the coronagraph will have muchgreater clarity and definition because of the absence ofatmospheric scattering and attenuation. Details of the coronastructure, with its tenuous streamers and filaments, will beevident through the use of the polarizing filters.

25

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Analysis When corolla photographs are correlated with the I-I-alphaimages of the solar surface (Section 2) and the ultravioletspectrographs (Section 4), much of the mechanisms andenergy characteristics of solar phenomona will be revealed.The II-alpha photos will show the location of solar surfacefeatures and the intensity of solar activity. Correspondingly,coronagraph photos will show the magnetic field activitythrough the polarization of the streamers of ionized materialsin the corona. Spectrographs of corona streamers willidentify the ions comprising the corona and the energy levelsnecessary to cause the ionization. These combined factors arethe source of the solar wind that arrives on Earth and affectsour atmosphere, weather, and environment,

Availability The data from the coronagraph will not be available forpublic use before 1975. Procedures for requests for copies offlight data will be announced at a later date. Section 1describes initial data uses and requirements.

CREW ACTIVITIES

The coronagraph will be operated for several hours dailyduring Skylab missions. Section 1 describes crew activitieswith regard to tilt complement of telescopes for solarobservation.

RELATED CURRICULUM TOPICS

The concepts employed in the design and application of thecoronagraph may be related to a number of subjectsdiscussed in high school science programs includingastronomy, physics, and photography.

Astronomy

Solar coronaenergies, motion of materials, solar wind,variations with solar storms;

Eclipsescause, frequency, regions of total eclipse.

Physics

Opticsdiffraction, lenses, reflection, mirrors, polarization,light scattering;

Gas lawspressure and temperatures, thermal velocity,kinetics;

Photography

Filmreciprocity and sensitivity, photographic resolution;

Mathematics

Trigonometryangles and sizes of apparent Moon, Sun, andocculting disks.

26

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SUGGESTED CLASSROOM DEMONSTRATIONS

Occulting A demonstration of the use of occulting disks may beprovided in the following manner:

1) Prepare a slide for 35mm or overhead projector. The slidewill have a circle in its center and may have faint areas Projectedaround the circle, to simulate the Sun and corona. circle

Occulting

2) In a darkened room, project the slide so that a 6-inchII

disk

diameter circle is shown on the screen.

3) Using a small disk of 5/8-inch diameter locatedapproximately 3 feet from the eye, demonstrate that thesmall occulting disk will hide the illuminated circle, butstill permit viewing of the areas surrounding it.

4) A nighttime demonstration using the full Moon may beperformed. Locating the occulting disk so that the fullMoon is occulted (a) measure the distance from the eyeto the disk; (b) calculate the angle subtended by the diskand the eye; and (c) using the calculated angle and adistance of 250,000 miles, calculate the approximatediameter of the Moon.

WARNING: DO NOT ATTEMPT OCCULTINGDEMONSTRATIONS USING THE SUN. EYE DAMAGEMAY RESULT.

Diffraction This demonstration illustrates the diffraction of white lightpassing over a sharp opaque edge.

1) Use a laboratory grade lens to focus sunlight on a whitesurface. The Sun should be focused to a small tittle.

2) Insert the edge of a knife or razor blade into the focusedbeam of sunlight so that some sunlight falls on the knifeand some passes beyond.

3) Inspection of the shadow of the knife will show that theshadow is diffused and is not as sharp as the blade edge.

4) The diffused shadow will result from diffraction of lightover the knife edge.

5) This demonstration may be extended to a discussion ofits application in determining the quality of parabolic orspherical concave mirrors for use in telescopes. The test isknown as the Foucault knife edge test.

feet6 inches

Sunlight

Shadow

Focused light

27

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Magnetic FieldEffects onIon Trajectories

28

The direction in which an ion is moving will be affected bythe, presence of a magnetic field. The trajectory of the ionwill be altered in a direction perpendicular to the magneticfield. This demonstration shows how the variations of thesolar magnetic fields affect the trajectory of ions in thecorona to eau' the streamers and filaments that areobserved. The iagnetic fields act to either concentrate theions or to disperse them, depending on the relationshipbetween the ion motion and magnetic field orientation. Aconcentration of the ions produces an increase of light. Adispersion of ions reduces the light.

1) Obtain a neon- or argon-filled tube, similar to a neon signtube. A suggested size would be 1/2 -inch diameter x 6inches long. A neon sign transformer is also required.

2) Obtain a strong bar magnet; any shape is adequate.

3) Connect the tube and transformer; ignite the tube.

4) Locate the magnet in any position along the tube; notethe change in light intensity near the magnet as the iontrajectories are altered.

5) Alter the position and orientation of the magnet; observethat the light changes with changes in the magnetic field.

Magneticfield

Transformer

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Section 4Extreme UltravioletSpectrograph

Extreme UltravioletSpectroheliograph

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9'IBV1IVAV Ad03 1S38EXPERIMENT BACKGROUND

The extreme ultraviolet spectrograph and the extremeultraviolet spectroheliograph are designed to recordphotographs and spectra in a region of the solar spectrumthat is not available on Earth because of atmosphericabsorption.

Basic Optics The extreme ultraviolet spectrograph and the extremeultraviolet spectroheliograph are similar in their basicoperation; consequently, they will be considered together intheir basic optical considerations. Differences in theiroperation are discussed separately.

White light with wavelengths between 3000 and 10,000angstroms can be resolved into its spectrum by passing a raythrough a prism. However, at wavelengths shorter than 3000angstroms, the energy in the light ray is absorbed by theprism. Wavelengths shorter than 3000 angstroms are knownas ultraviolet (UV), extreme ultraviolet (XUV), and x-ray.

In the discussion of the white light coronagraph (Section 3)the diffraction of white light passing over the edge of anopaque surface was illustrated. This concept is extended inthe diffraction grating. By using an opaque plate in which anumber of parallel slits are cut, diffraction of light will occurat the edge of each slit as white light is passed through thegrating. It may further be shown that because of the spacingof the slits, the wavefront emerging from the grating willconstructively interfere in some directions and destructivelyinterfere in other directions as a function of wavelength.Because of the wave interference, white light passing throughthe grating is resolved into its spectrum. Transmissiongratings are formed by ruling parallel opaque lines on glass. Atransmission grating also requires a lens to focus thediffracted light to a well defined spectrum.

To circumvent the difficulty of absorbtion of ultraviolet andshorter wavelengths in the grating material, the grating ismade reflective. Several thousand parallel lines per inch arescribed on a reflective base material. The grooves are speciallyshaped in a sawtooth form. Due to the shape of the groves,energy reflected from the many facets is reflected at variousangles. The shape of the groove is called the blaze. Variationsof the blaze angles permit the reflective grating to bedesigned for greatest efficiendrat specific spectral regions;thus the blaze for ultravio10 is different from the blaze forred 'visible light. Even flat reflecting grating a lens is

-still required to focus th red light into a spectrum.

A concave sphericarini cus radiation falling on it tofocal, point. If a diffraction grating is ruled on a sphericalmirror, the difftoeied light will also arrive at a focal point.

a 4BEST COPY AVAILAbLf.

Extreme ultraviolet is generallyconsidered to be the region ofthe electromagentic spectrumbetween 150 and 700 angstroms.

SpectrographA system forresolving electromagnetic energyinto its component wavelengths.

SpectroheliographA picture ofthe Sun in a particularwavelength of the spectrum or adevice for producingspectroheliograph

White Light

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However, the focal point for each wavelength is differentbecause of the blaze of the grating. The spherical concavegrating produced eliminates the requirement for a focusinglens.

Experience Spectrographs have been in use in astronomy for many years.They are commonly used to analyze the visible light from theSun and stars. Most of our current knowledge of thecomposition of heavenly bodies has been derived fromspectrographic analysis of their light. However, radiations atshorter wavelengths than visible light are absorbed in Earth'satmosphere and are not available to ground based telescopes.

The solar chromosphere and lower corona are much hotterthan the surface of the photosphere, which is characterizedby the white light it emits. To observe these hotter regions ofthe solar atmosphere, one must observe in the ultraviolet oreven x-ray spectral regions. Because these radiations areabsorbed in Earth's atmosphere, spacecraft or satellites arenecessary to transport the instruments away from Earth, tothe regions of space where this radiation is available.

Short term observations in these spectral regions have beenperformed from balloons and sounding rockets and orbitingsolar observatory (OSO) satellites. However, these data arelimited to small spectral sections.

Figure 1 shows the various layers of Earth's atmosphere andthe types of solar radiation that are absorbed in therespective layers.

D region:72 to 93 kmisessissontstsvaStratosphere16 to 72 kmsmossuasser.Troposphere0 to 16 km

F region60 to 960 km

E region93 to 160 km

With acknowledgement to the NFigure 1 Atmospheric Absorption of Radiation

DEFINITION OF SCIENTIFIC OBJECTIV.

I.Geo ciety

Spectrograph The extreme ultraviolet spectrograph will linespectra of small selected areas of the Sun an oss' bsin two wavelength bands: 970 to 1970 angstroinS or 13940 angstroms.

30

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Spectroheliograph

The extreme ultraviolet spectroheliograph will photographthe solar chromosphere in extreme ultraviolet wavelengthsbetween 150 and 625 angstroms.

Because the extreme ultraviolet region of the spectra doesnot penetrate Earth's atmosphere, observations have beenquite limited in this region. Data obtained by these Skylabtelescopes will provide information on the materialcompositon of the chromosphere and the energycharacteristics of the Sun. These pieces of the solar puzzle, inturn, will provide clues to the mechanisms by which solarflares occur.

DESCRIPTION OF EXPERIMENT HARDWARE

Spectrograph In a spectrograph, light is admitted to the diffraction grating Micron = 0.000001 meterthrough a narrow entrance slit. The entrance slit of theextreme ultraviolet spectrograph is 10 microns wide and 300microns long (0.0004 in. wide x 0.012 in. high).

Optical Path

The light admitted to the grating is an image of the slit. Thediffraction grating then reflects the diffracted image to thefilm plane. Since the diffraction grating is a sperical reflector,the diffracted slit image is focused at the film plane.

= 0.00004 inches

EntranceSlit

SpectraImage

4.0

Figure 2 depicts the complete optical system of the extreme Filmultraviolet spectrograph. Light enters the spectrograph Plane

through an aperture and falls on the primary spherical mirror.

Primaryspherical mirror

Main grating

EntranceInstrumentaperture

Predispersergrating

Waveband (1 of 2)selectingApt

Ar.

Fig Ure,s2c.

Film planespectrum

stem of the Extreme Ultraviolet Spectrograph

a,11ft2:7,1;.,

CorY

DiffractionGrating

31

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Spectro-heliograph

32

The primary mirror focuses the full image of the Sun(approximately 10 millimeters diameter) on the spectrographentrance slit. Because of the small size of the slit, only a smallsection of the solar image is passed through. The sectionpassed by the slit corresponds roughly to 1000 miles wide x30,000 miles long of the solar surface. This segment ofsunlight is passed on to a predisperser grating.

If only one diffraction grating were used in the spectrograph,the entire solar spectrum would be focused at the film plane,with the result that the spectrum would be very crowded anddetails could not be detected or resolved. However, by usinga predisperser grating and waveband selecting slit, thecrowding of the desired spectrum is eliminated. Thepredisperser diffracts the entire solar spectrum and thewaveband selecting slit is located so that only the desiredsection of the predispersed spectrum is passed to the maingrating. Two different predisperser gratings are used whichhave a different blaze and number of lines. Thus, either oftwo sections of the solar spectrum may be sent to the maingrating. One predisperser grating is ruled with 150 groovesper millimeter and resolves the spectrum from 1940 to 3940angstroms. The other is ruled with 300 grooves per millimeterand resolves from 970 to 1970 angstroms.

The waveband selecting slit passes the predispersed portion ofthe spectrum to the main grating. The main grating diffractsthe predispersed spectrum and focuses it on the film plane.The final spectral images are diffracted over 240 millimeterslength on the film. For the long wavelength spectrum thisyields a scale of 8.3 angstroms per millimeter; the shortwavelength spectrum has a scale of 4.2 angstroms permillimeter.

The resulting spectrum is a band of irregularly spaced linesand bands spread out over the 240 mm of film. Each lineindicates a unique state of ionization of a particular kind ofatom. In the visible spectrum ionized sodium vapor radiatestwo distinct lines about 6000 angstroms. Mercury vaporradiates 10 different wavelengths depending on the energystate of the mercury atom. In the discussion of H-alpha inrelation to H-alpha telescopes, it was noted that hydrogenemits radiation peculiar to the particular energy level towhich it was excited. Every element when energized toparticular states emits a unique radiatpn. Thus, when thespectrum is analyzed, the constituent atomic omissions whichcomprise the spectrum become evident. ft

The extreme ultraviolet spectroheliographrecords images of the solar chromosphere invarious spectral lines throughout the spectiii,625 angstroms. Figure 3 depicts the optical stspectroheliograph.

icallythe

PredisperserSelects Part

of Spectrum

.1 I 1PredisperserA means ofselecting small sections of aspectrum for further diffraction.

240 mm

970° - 4.3°/mm 19701940°-8.3°/mm 3940

IIIH FE

1

Cl

Typical spectrum

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3" Grating rotation

--Internal rejectionmirror

Aluminum filter

Film strip

Instrumentaperture

Externalrejectionmirror

Figure 3 Optical System of the Spectroheliograph

In this instrument the entrance slit of the spectrograph isdeleted to permit the spherical concave diffraction grating toview the entire Sun. The grating diffracts the full solar imageinto its spectral components. However, as a spherical mirrorit also focuses the diffracted images onto the film strip. Theresulting photograph is a series of pictures of the solarchromosphere. Where the spectrum is continuous the solarimages run together and are blurred. Where a distinctradiation line occurs in the spectrum, a distinct picture of theSun is recorded in the light of that particular radiation. Theappearance of the diffracted solar images on the film isshown in Figure 4.

o coopI I I I I I I I I I I I I I I I I I 1 I I I I I I I I I I I I

300 350 400 450 500 550 600Long wavelength

Short wavelength150 200 250 300 350

Figure 4 Diffracted Solar Images

Since the full image of the Sun falls on the grating, the fullydiffracted spectrum is wi r than the length of the film strip.To accommodate the' y ed spectrum width on the filmstrip, the diffraction gird g is rotated through an angle of 3degrees. This rotation - focusestocuses either the short wavelength,150 to 350 angstroms, or long wavelength, 300 to 650angstroms, bands on the film strip.

33

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Cameras

Type

Highlights

34

In addition to the desired portions of the solar spectrum, thewhite light spectrum also falls on the grating and isdiffracted. This portion of the spectrum coulti causeunwanted heating of the instrument. For this reason a mirroris located in a position to intercept the diffracted visiblespectrum and reflect it back to the entrance aperture and outof the instrument.

The cameras of the spectrograph and the spectroheliographare similar in design, each camera containing 200 film strips.The film strips are 258 millimeters long and 35 millimeterswide and are mounted in holders. A film changingmechanism, removes one holder at a time to locate the filmin the image plane. After exposure the mechanism places theholder in the exposed stack and places a new one in the filmplane.

The shutter of the camera is a rectangular blade which ismoved down by sliding in guides to uncover the cameraaperture. After exposure the shutter slides upward to coverthe aperture.

Because the spectroheliograph may have a considerableamount of scattered white light in the film plane, a 1000angstrom thick aluminum foil filter is placed over the cameraaperture. The aluminum filter rejects long wavelengths buttransmits wavelengths of 150 to 625 angstroms.

EXPERIMENT DATA

The data from the extreme ultraviolet spectrograph and theextreme ultraviolet spectroheliograph are recorded on 35-mmfilm strips in the form of a strip of varying intensity havingnumerous dark or light lines across it. The appearance of thespectrograph is illustrated in Figure 5.

Figure 5 Spectrograph Data

A bright line in the spectrograph indicates that a particularchemical element is radiating at that wavelength. A dark lineindicates that the solar atmosphere is absorbing radiation atthat wavelength.

The format of the data of the extreme travioletspectroheliograph is shown in Figure 4. On SepteMber 22,1968 a spectroheliograph was flown on a high altitude rocketto obtain Figure 6. This is a photograph of the Sun in the

ShutterFilmHolder

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Analysis

wavelength of ionized helium (304 angstroms). Note thatimages of the Sun are present also at wavelengths of iron (FeXV and Fe XVI) 284 and 335 angstroms. The light of Fe XVis strong enough to slightly overlap the helium image. Notethat the recorded Fe XV and Fe XVI emissions occur in thesame area of the solar disc as the high energy He II emissions.

Figure 6 Spectroheliograph Data (from High Altitude Rocket Flight9-22-68)

These spectrograph and spectroheliograph data will identifythe constituents of the solar atmosphere and show therelative abundance and energy state of each constituent.

When these data are correlated with data from H-alpha(Section 2) and x-ray (Sections 6, 7, 8) data, information onthe complete energetic state of the Sun will be obtained.

Availability The return of data, data analysis procedures, and availabilityof data for public use is discussed in Section 1.

CREW ACTIVITY

The astronauts will select spectral ranges to be used in therespective instruments and will also control the pointing ofthe instrument to select the most advantageous subject forobservation at a given time.

General crew activities are discussed in Section 1.

RELATED CURRICULUM TOPICS

The identification of chemical compounds by the applicationof spectrography is employed in many industries. Theextreme ultraviolet spectrograph and spectroheliograph areapplications of similar instruments in solar research. Relatedcurriculum topics on spectrography are listed.

Astronomy-

1) Chromospherematerial constituents of the solaratmosphere;

35

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2) Energy considerations of identified constituents.

Physics-

1) Opticsdiffraction gratings.

Chemistry-

1) Ionization states and energy levels of chemical elements.

SUGGESTED CLASSROOM DEMONSTRATION

This demonstration will show how chemical identification isaccomplished through spectrography. It will also show thatspectral components of the Sun may be identified bycomparison with spectra from known chemicals.

Materials for this demonstration are-

1) Concave diffraction grating blazed for visible light;

2) Lenslaboratory type;

3) Spectrograph entrance slit (2-in. square aluminum platewith a slit 0.5 mm x 10 mm);

4) Laboratory Bunsen burner;

5) Holder to burn chemicals in Bunsen burner flame;

6) Small quantities of table salt, sugar, soda bicarbonate,and powdered pencil lead;

7) White cardboard, 18x6 in., for spectrum plane.

Procedure

1) From the radius of curvature of the grating, draw on alarge flat surface, the Rowland circle, the radius of whichis R = 1/2 r where r is radius of curvature of the grating.

2) Locate the diffraction grating, entrance slit, and spectralplane about the Rowland Circle as shown in Figure 7.The angle, 0, should be between 20 and 40 degrees.

3) Use the lens to focus a beam of sunlight onto theentrance slit, and observe the resulting solar spectrum inthe spectral plane; especially note the location of brightlines and dark bands in the spectrum.

4) Replace the beam of sunlight, with light from the Bunsenburner and focus the flame on the entrance slit. Note the

36

Suggested sourcesConcave Diffraction Grating:

Bausch & LombAnalytical Systems Division820 Linden AvenueRochester, New York 14625

Catalog NR 35 -52 -11 -040.

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differences between the solar spectrum and the flamespectrum. What differences now are evident in the brightlines?

5) Burn a sample of each of the material specimens in theflame of the burner. For each of the chemicalcompositions analyzed, make a note of locations ofbright lines in the spectrum, for example, the sodium insalt will exhibit two bright closely spaced yellow lines.Sugar will exhibit bright lines characteristic of carbon andhydrogen.

6) Compare the spectra obtained from the samples with thespectra from the flame and sunlight. What lines in thesunlight and flame can be identified with lines from thesamples? Make a list of the elements which can beidentified in sunlight.

7) Remove the entrance slit and position the Bunsen burnerand lens so that an image of the flame is formed on thegrating.

8) Observe the diffracted spectra in the spectral plane. Inwhat colors are images of the flame seen?

9) Burn some of each of the sample materials in the flame.Note the location of each of the bright images in thespectrum. What differences are apparent between imagesin green light and red light? What do the differencesindicate?

Radius ofgrating

Spectralplane(white cardboard)

Grating

! Entrance slit

Figure 7 Experiment Layout

4c:=> Lens

Bunsen Burner

37/3y

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Section 5Ultraviolet ScanningPolychromator-Spectroheliometer

U)C0

Ql

U)

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Physics

History

ScanningModes

EXPERIMENT BACKGROUND

The optical concepts of the ultraviolet scanningpolychromator-spectroheliometer are similar to the extremeultraviolet spectrograph discussed in Section 4. A concavespherical mirror is used to focus the solar image on anentrance slit that passes a segment of the image to a concavediffraction grating in order to resolve a spectrum of the Sun.However, this instrument employs a system of moving eitherthe mirror or the diffraction grating to effect scanning of thesolar image, and electronic measurement of spectral radianceintensity instead of photographic records.

Similar experiments have been flown successfully on theorbiting solar observatory (OSO) family of spacecraft.However, these instruments have only afforded a field ofview corresponding to the full solar disc. The instrument inthe Skylab solar observatory has a somewhat longer focallength which will permit detailed analysis of smaller segmentsof the solar surfaces. Spectral details of solar phenomenasuch as sunspots, flares, and plages will be available.

DEFINITION OF SCIENTIFIC OBJECTIVES

The ultraviolet spectroheliometer will obtain data from thesolar atmosphere in the spectral region of 300 to 1350angstroms. Small segments of the solar surface having activephenomena will be analyzed at discrete spectral intervalswithin the overall range of the instrument.

One operating mode of the spectroheliometer will providespectral analysis, at seven different wavelengths, of a squarearea of the solar surface subtended by an angle of 5 secondsat Skylab (about 6000 kilometers on the Sun's surface). Thealternative operating mode of the spectroheliometer willprovide a spectrograph from 300 to 1350 angstroms of asingle selected point within the above square area.

These two operating modes will permit individual detailedanalysis of spectral energies of sunspots, plages, filaments,and spicules.

DESCRIPTION OF EXPERIMENT HARDWARE

The ultraviolet scanning polychromator-spectroheliometerhas the basic form of a spectrograph; however, the sphericalmirror and the diffraction grating are both mounted in amovable fashion, to permit raster scanning of the solarsurface.

The spectroheliograph operates in either of two modes. Theseare the mirror scan mode and the grating scan mode. Mirrorscan is used to scan an area of the solar surface; grating scanis used for detailed analyses of a selected line of the mirrorscan.

SpectrobeliometerAninstrument to measure theintensity of spectral wavelengthsof the solar spectrum

Po I y c h r o m a t orA device toproduce colors from a singlelight source; similar to aspectrograph

1920 arc sec1,390,000 km

5 arc sec3600 km

39

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ImageMovement

DiffractionGrating

40

Mirror ScanThe optical system of the ultraviolet scanningpolychromator-spectroheliometer is depicted in theillustration. The primary mirror is mounted in a 2-axis gimbalwhich permits the mirror to be driven or positioned to viewany point in a given area. To scan in a raster fashion, themirror is rotated about the horizontal axis to scan one line ofthe raster. It is then returned to its, original position androtated about the vertical axis which positions the mirror toscan the next line. The mirror then rotates again on thehorizontal axis to scan a second line. This process is repeated60 times to create a 60-line raster image of the scanned area.

Instrumentaperture

Grating

Gimbaledprimarymirror

Entranceslit 7 photo

detectors

Optical Scheme of Ultraviolet ScanningPolychromatorSpectroheliometer

The image of the Sun formed by the spherical mirror isfocused on the entrance slit of the diffraction grating.However, the image on the entrance slit moves in accordancewith the movement of the mirror. Thus, the light admitted tothe grating by the slit changes as the image on the slit moves,reflecting variations in the characteristics of the light emittedalong a specific traverse of the Sun's disc.

The diffraction grating is a concave grating with 1800 linesper millimeter, i.e., it is blazed to resolve the spectrum from300. to 1350 angstroms. The diffracted spectrum is focusedon an array of seven photodetectors that generate an electricsignal in proportion to the intensity of radiation falling onthem. The seven photodetectors are located at the followingwavelength positions of the projectei spectrum representingthe following emissions:

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Angstroms

1 1335 Carbon 11

2 1219 Lyman Alpha

3 1031. OxygenNl

4 977 Carbon III

5 912 Lyman Continuum

6 625 Magnesium X

7 554 Oxygen IV

As the spectrum changes with mirror movement, each of thephotodetectors will generate a signal proportional to thevariations of intensity of solar emission at its assignedspectral wavelength. The signals from the photodetectors arerecorded on a tape recorder and later transmitted to Earth bythe Skylab telemetry system.

Grating Scan ModeA detailed examination of any selectedline of the mirror raster scan may be performed whendesired. To accomplish this mode, the spherical mirror ispositioned and stopped to focus the particular point of thesolar image on the entrance slit. The diffraction grating isthen driven back and forth in a horizontal direction, whichcauses the diffracted spectrum to sweep over thephotodetectors. In this mode, only the third photodetector isused and the others are turned off. Since the diffractedspectrum is being swept across this detector, its output nowrepresents the intensity of all wavelengths of the spectrumand is a detailed spectral analysis of a unique solar area.

DATA

Type The data from the mirror scan mode are composed of sevenMirror channels of digital data and recorded on magnetic tape. TheScan Data tape will be analyzed by computer. The computer output

may be in a tabular form or it may be graphic, in which caseraster scans will be reconstructed to produce maps of spectralradiation in the square area scanned, for each of the sevenwavelengths.

GratingScan Data

The data from the grating scan mode are provided by onechannel of recorded digital presentation of the output of onephotodetector, and represents the spectrum, between 300and 1340 angstroms of a single point in the raster scannedarea. Computer output of this data may be in tabular orgraphic form,

TelemetryA system fortransmitting data by radio

300 1350angstroms angstromsGrating scan spectrum

Reconstructedultraviolet- scanningimage

41

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Highlights The resolution provided by the ultraviolet spectroheliographwill permit evaluation of individual parts of solar phenomenaand provide information with regard to energy variationswithin parts of a solar event.

Availability The basic raw data require computer analysis to be useful.Reproduced computer outputs will not be available until1975. (Section 1 describes data distribution and use.)Procedures for requesting copies of reduced data will beannounced at a later date.

42

CREW ACTIVITIES

The astronaut will determine the area to be observed andperform instrument pointing, and selection of the scanningmode to be used.

Other crew activities are described in Section 1.

RELATED CURRICULUM ASPECTS

The ultraviolet spectroheliometer applications and designhave relevance to several aspects of a science curriculum.Related subjects for study are

AstronomySpectra, energy, and temperatures in the chromosphere.

PhysicsConcave and convex mirrors; light meters and luminance.

ElectronicsPhotodetectorsAnalog-to-digital conversionTelemetry

SUGGESTED CLASSROOM DEMONSTRATION

The application of photodetectors and electronics to analyzeoptical data is shown in this demonstration. The materialswhich were required for the spectrograph demonstration inSection 4 are also used for this demonstration. Additionalmaterials required are tabulated.

1) Photodetectors and associated circuitry;

2) Vacuum tube type sensitive voltmeter.

NOTE: Numerous combinations of photodetectors, circuits,and meters are possible. No recommendation for specificparts is given.

Suggested Source:

Allied Radio Corp2400 W. Washington BlvdChicago, Illinois 60612

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Pr oc elure Set up the spectrograph as described in Section 4.

1) Cut a 3mm (1/8 in.) wide slit in the white cardboardspectral plane. The length of the slit should be.as long asthe spectrum obtained in the demonstration in Section4. The slit should be positioned so that the sunlightspectrum appears above and below the slit.

2) Move a photodetector along the spectrum and recordthe average meter reading for each color.

3) Position the photodeteetbr at each bright line in thespectrum and record the meter reading on each line.NOTE: Meter readings will indicate relative intensitiesonly, not luminance values.

4) Burn some of each of the materials suggested in Section4 and record the color and meter reading for each brightline for each material, respectively.

5) Compare the relative meter readings from the spectrallines with the meter readings of the spectral lines ofsunlight. What do the differences in the readings mean?

6) Using only the Bunsen burner flame, position thephotodetector on a bright spectral line in the flamespectrum. Slowly slide the Bunsen burner about so thatthe flame is scanned from side to side by the entranceslit. Record the meter reading for several positions ofthe flame. How do the meter readings compare with theflame position? How might this compare with spectrallines in a sunspot?

43/11

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Section 6X-RaySpectrographicCamera

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Physics EXPERIMENT BACKGROUND

EntranceAperture

The ability of x-rays to penetrate or be absorbed in solidmaterials is well known. Thus it will be appreciated thatx-rays cannot be reflected in the normal fashion from amirror; x-rays can only he reflected at grazing incidence,which means that the energy is traveling on a path that is lessthan 10 degrees from the plane of the mirror. The energygrazes the reflecting surface.

The first section of the optical imaging section, shown in theillustration, is a cylinder in which the inner wall is shaped toform a paraboloid of revolution. The second section issimilarly formed from the figure of revolution of ahyperboloid. Radiation that arrives at the telescope andimpinges on the paraboloid section is reflected at grazingincidence to the hyperboloid section. It is again reflectedfrom this section to the image plane. Because of thecurvature of the walls of the sections, it may be shown thatradiation impinging at any place in the pariboloidal sectionwill be brought to focus at the image plane.

Paraboloid

Hyperboloid

Focalsurface

Incidentparaxialradiation

Grazing Incidence Telescope Geometry

Since the reflective surface of the telescope optic system ison the inner wall of a tube, the central area of the aperture isnot used; rather, the true aperture is a narrow ring orannulus. Consequently the central area is stopped off for heatand light rejection, or it may be used for other purposes.

Incident Reflectedray ray

45

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TransmissionGratings

The use of reflecting diffraction gratings has been discussedfor other instruments earlier in this volume. It will be readilyappreciated, that a reflection grating cannot be used forx-rays because x-rays would either pass through the grating orbe absorbed by it. If, on the other hand, a grating is ruled ona thin membrane which is transparent to x-rays but possiblyopaque to visible light, diffraction will still occur but theenergy will be transmitted through the grating.

Experiment X-ray images of the Sun have been obtained from telescopesHistory flown in sounding rockets. However, the field of view of

these images has been the full solar disk and the resolutionhas been limited. Long focal length telescopes to obtaindetailed x-ray images have not been possible. The data thatare available, such as the x-ray photograph, indicates thatx-ray radiation results from solar activity such as sunspots,flares, plages, prominences, and from the corona.

46

'kr

The Sun, Viewed Only as a Source of X-Rays

SCIENTIFIC OBJECTIVES

The study of x-ray radiation from the corona and varioussolar active regions will increase our understanding of thestructure of the corona and the processes that cause solarflares. By photographing the Sun at intervals of small anglesof rotation, it is expected that effective stereo pairphotographs will be obtained to permit three-dimensionalanalysis of solar x-ray activity. This data will be correlatedwith the ultraviolet spectra so that further understanding ofthermonuclear energy mechanisms can be derived.

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TransmissionGrating

Filters

EQUIPMENT

The x-ray spectrographic camera photographs x-ray images ofthe Sun in the spectral region from 3 to 60 angstroms. Theillustration shows the experiment configuration and will beused to describe salient features.

Camera.

X-raysLarge x-ray optics

JFilter wheel Image Gratings

dissector

Plan of X-ray Telescope

Small x-ray optics

Grazing incidenceoptics

The grazing incidence optics section of the x-rayspectrographic camera consists of two concentric grazingincidence mirrors. The outer mirror has a diameter of 34.5cm (12 in.); the inner mirror a 22.9 cm (9 in.) diameter. Bothmirrors have a focal length of 233.4 cm (84 in.). This nestedconfiguration provides an increased mirror aperture forcollecting x-ray energy.

As noted in the basic physics discussion, the central area ofthe grazing incidence optics is not effective for x-ray imagingand may be stopped off. This central section is used for asecond 7.6 cm (3 in.) diameter grazing incidence mirror. Thismirror, used for x-ray alarm and astronaut TV display, will bediscussed later, The central core also permits the inclusion ofa visible light telescope for data reference purposes.

Directly behind the grazing incidence optics, there is atransmission grating that serves two functions: (1) it diffractsx-ray energy to produce x-ray spectroheliograms; (2) itfunctions as an iris to control the amount of x-ray energy atthe camera. The grating may be moved out of the opticalpath when desired.,

A six-position filter is used to select the exact spectral regionto be photographed. The various materials, thicknesses, andfilter responses are listed:

Filter Material Transmitted Wavelengths (angstrom)0.013 mm (0.0005 in.) beryllium 3 to l80.003 mm (0.000125 in.) Teflon 3 to 14, 18 to 30Blank (i.e., no filter) 3 to long wavelength

optic limit0.0057 mm (0.00023 in.) Parylene 3 to 18, 44 to 700.05 mm (0.002 in.) beryllium 3 to 11

0.025 mm (0.001 in.) beryllium 3 to 14

p

47

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Camera

TYPe

A 3-angstrom short wavelength is the response limit of thegrazing incidence optic system.

The telescope is equipped with a camera that uses 70-mm,Panatonic-X aerial film. The film magazine has a capacity of7200 photo frames. Exposure time of the camera is variablefrom 1/64 to 256 seconds. Data recorded on the photographsare: the x-ray image, visible light image, fiducial marks, andtime. The camera x-ray aperture is a window of 0.0003 cmaluminized polypropylene. This window prevents anyradiation (ultraviolet and visible) from entering the camera.

As discussed earlier, a 7.62 cm (3 in.) diameter grazingincidence mirror is located in the center of the main system.This small mirror has a 81.3 cm (32 in.) focal length. At itsfocal plane the x-ray image is focused on a 0.45 cm (3/8 in.)thick crystal of sodium iodide. This crystal scintillates in thepresence of x-rays; that is to say, it produces visible light inthe image of the x-rays. This scintillating crystal is in directcontact with the photocathode of an image dissector which isone form of a TV camera tube. This system provides andx-ray image of the Sun, which is displayed on the astronaut'scontrol and display panel, and serves as an x-ray viewfinderfor the astronaut. It is used as an x-ray flare alarm when thecontrol and display panel is unattended.

A visible light telescope is mounted in the central section ofthe main grazing incidence optics. The visible light image istransmitted through a very dense neutral density (no color)filter. The filter reduces the visible light to a level suitable forphotography in the camera.

EXPERIMENT DATA

Experiment data are provided in two formstelevision andphotographs. The television circuits are used primarily forastronaut monitoring purposes; photographic data arereturned to Earth following the mission.

Highlights The x-ray spectrographic camera will provide low resolutionspectra that will measure the intensity of emission of x-raylines from elements such as iron, silicon, oxygen, andmagnesium. The time-related development of the lineintensities during a solar flare is of importance in determiningthe mechanism responsible for initiating a solar flare.

Analysis

48

The density of the spectral line images will be measured todetermine the intensity of the radiation emitted by eachelement. This information will be correlated with other datataken in ultraviolet and visible light. The data will beanalyzed to determine the time-related development of thet .nperatures and densities of the region of the solaratmosphere emitting the radiation, and their relation to thesolar magnetic field.

Fiducial marksmarks placed onfilm images to define scalefactors.'

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The quiescent corona will be studied to determine themechanism of coronal heating. Such problems as the locationof the energy transfer across the chromosphere-coronainterface, the influence of the magnetic field on coronalheating, and the geometrical structure of thechromosphere-corona interface will be investigated.

Availability Data from the x-ray spectrographic camera will not beavailable for public use before 1975. Procedures forrequesting copies of flight data will be announced at a laterdate. Section 1 describes planned data uses and requirements.

CREW ACTIVITIES

The astronaut will use the TV display from the x-rayspectrographic camera to locate the regions on the solar diskthat are emitting x-rays. He will decide what x-ray and otherwavelength observations are required in the area. The TVdisplay will then be used to point the other Apollo telescopemount instruments to the proper area.

Other related crew activities are discussed in Section 1.

RELATED CURRICULUM TOPICS

Several topics of a science curriculum are related to the studyof solar x-rays and equipment.

Astronomy

Energy levels of sunspots, filaments and plages, and flares ofthe chromosphere.

Physics

Energy levels associated with ions that exhibit x-rays

Ionization states of elements to produce x-rays.

Mathematics

Foci of paraboloid and hyperboloid functions.

RELATED CLASSROOM DEMONSTRATION

Xray Filters X-ray telescopes are equipped with various materials that actas spectral filters for x-rays. This demonstration will comparethe x-ray filtering qualities of these materials and thicknesses.

Materials required for this demonstration are

A - Polyethylene film - 6x10 in.

B - Aluminum Foil 6x10 in.49

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C - 0.001 in. thick steel 6x10 in.

D - Copper foil 6x10 in.

E - Balsa wood 10x12 in.

F - X-ray film - 10x12 in. (4 required)

AluminumPolyethylene Copper

Steel

,rifirl° in.

Balsa wood sheet

12 in.

Procedure 1) Cut a 3x10 in. strip from materials A, B, C, and D;

50

2) Cut 2x10 in. and lx10 in. strips from the respectivematerials;

3) Assemble the various strips of material on the balsa woodsheet as shown in the illustration. Drops of householdepoxy cement at the ends of the strips will bond theassembly;

4) Arrange with a local hospital or clinic to x-ray theassembly on a sheet of x-ray film. Use four differentexposure times (It is advisable to discuss plans with thehospital administration early in the project.);

5) Compare the x-ray absorption properties of the materialsfor the different thicknesses and exposure times.

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CO

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Cf)

Section 7X-Ray UltravioletSolar Photography

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Physics

History

EXPERIMENT DESCRIPTION

The x-ray/ultraviolet solar photography experiment willphotographically record the x-ray and extreme ultravioletsolar spectra between 10 and 200 angstroms.

A diffraction grating is suitable for resolving very shortwavelength spectra such as extreme ultraviolet and x-ray;however, to avoid penetration of the grating by x-rays, itmust be used in a grazing incidence configuration. In thisconfiguration, the radiation impinges on the grating at anangle of incidence less than 10 degrees from the plane of thegrating. Grazing incidence also provides greater dispersionand thus better resolution of the short wavelengths.

Consideration of the optical geometry of a concave sphericaldiffraction grating shows that optimum performance isobtained when the grating, the spectrograph slit, and the filmplane or spectrum surface are all on the circumference of acircle, known as the Rowland Circle, which has the samediameter as the radius of curvature of the diffraction grating.The Rowland Circle is also tangent to the face of the grating.The illustration shows the optical schematic of the grazingincidence spectrograph.

Film

Grating

Slit

Filter

Optical Scheme of Extreme Ultraviolet Spectrograph

Solar spectra in the region of 10 to 200 angstroms have beenobtained from many sounding rocket and balloon flights.These data have provided valuable information with regard tobasic solar composition and energy levels. When solar flaresand phenomena occur, the composition of the flare ischaracterized by a change in the predominance of certaintypes of ions and by the attendant emission energy levels.

DEFINITION OF SCIENTIFIC OBJECTIVES

The objective of the x-ray/UV solar photography experimentis to photograph the extreme ultraviolet and x-ray spectra ofquiet and active Sun conditions in the wavelength regionranging from 10 to 200 angstroms.

Spectra of the full disc will be obtained during periods ofminimal solar activity (quiet Sun). Spectral data of a solarflare will be obtained by centering the experiment on thatregion of the disc. The x-ray and extreme ultraviolet

Film Slit

Normal incidence

Rowland Circle

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52

radiation originates in normally quiet regions and increasesrapidly in intensity above active regions. The radiationconsists of emission lines of highly ionized ions produced bya combination of thermal and nonthermal processes.

The development of active regions and flares on the solar discresults in a large increase in the total x-ray and extremeultraviolet emission. in general, the increase in radiation isgreater for the shorter wavelengths; the emission levels mayincrease by a factor of 10 for the flux below 10 angstroms.X-ray bursts are thought to be produced by nonthermal andquasithermal processes associated with flares. These burstssignificantly contribute to the overall x-ray flux from anactive region.

The x-ray and extreme ultraviolet data acquired by thisexperiment will contribute to understanding of thecharacteristics of the solar atmosphere and its distortion byactive regions and flares. The data will expand knowledge ofthe solar spectrum, contribute to solar flare predictionstudies, and aid in predicting the quality of radiocommunications at various frequencies during solar storms.

DESCRIPTION OF EQUIPMENT

The experiment equipment consists of two sections, thespectrograph and a boresighted viewfinder. The spectrographresolves the solar spectrum between 10 and 200 angstroms.The boresighted viewfinder enables the astronaut to centerthe entrance slit of the spectrograph on a given area of theSun.

The elements of the spectrograph are filters, entrance slit,diffraction grating, and film magazine.

The diffraction grating, a spherical concave mirror that hastwo grating surfaces, is positioned to lie on the RowlandCircle. One half of the mirror is ruled at 2400 lines per inch;the other half is ruled at 1200 lines per inch. The half that isruled at 2400 lines per inch provides a spectrum of 10 to 100angstroms with a resolution of 0.05 angstrom. The other halfof the mirror yields a spectrum of 20 to 200 angstroms witha resolution of 0.08 angstrom. (Resolution may be defined asthe ability to distinguish individual spectral lines.) Theresultant spectra are spread over a 15 cm (6 in.) film plane.

The diffracted spectrum is focused on a plane tangent to theRowland Circle. To achieve satisfactory focus on the film, itmust fall on the curve of the Rowland Circle. The illustrationshows how 10 strips of film are mounted on the filmmagazine, and an example of the two spectra imaged on thefilm. The optical schematic of the x-ray/ultraviolet solarphotography experiment is presented in the illustration.

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Film strip

Film drum

10 to 100angstroms

Film Magazine and Data Format

Two filters are used with the spectrograph. Visually opaquemetal films that transmit only the x-ray and ultravioletenergy are used, thereby preventing visible light and nearultraviolet from entering the instrument. One filter is madefrom indium and beryllium and passes from 0 to 110angstroms in the indium section and 110 to 200 angstroms inthe beryllium portion. The other filter is half indium, halfboron. The spectral response of the boron is 66 to 200angstroms. The sections are mated to the spectral response ofthe halves of the diffraction grating.

20 to 200angstroms

EXPERIMENT DATA

Type Data from the x-ray ultraviolet solar photography experimentis in the form of film strips of the solar spectra. Itsappearance is similar to the ultraviolet spectrographs whichare discussed in Section 4, the only difference being thespectral range.

Highlights X-ray and extreme ultraviolet radiation of the solar spectrumare portrayed. The radiation lines will show which ions arepresent in the chromosphere and the ionization state inwhich they exist.

Analysis These spectra, when correlated with the x-ray images of theSun recorded by the x-ray telescopes and the x-ray eventanalyzer (Sections 6 and 8), will show the sources of thex-ray/ultraviolet energy, the distribution of the energy in theSun, and the spectral composition of the solar materials actedon by the x-ray/ultraviolet energy.

Availability The x-ray/ultraviolet solar photography data are of greatscientific value and will be studied intensely. Information onthe results of the experiment will not be available for public

Filter

Indium0 to 110angstroms

Beryllium110 to 200angstroms

orBoron66 to 200angstroms

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consumption until 1975, as discussed in Section 1.Procedures for requesting copies of data will be announced ata later date.

CREW ACTIVITIES

The x-ray/ultraviolet solar photography experiment isperformed in a scientific airlock of the Skylab orbitalworkshop module. During operation of the experiment, thecrewman will be in communication with the astronautstationed at the Apollo telescope mount control and displaypanel. The crewman, using the boresighted viewfinder in theequipment as an aiming device, will be able to advise thesecond crewman how to maneuver the spacecraft intoposition to center the solar flare in the entrance slit of thespectrograph.

This experiment is performed within the spacecraft andtherefore will not require extravehicular activity for dataretrieval.

RELATED CURRICULUM ASPECTS

The x-ray/ultraviolet solar photography experiment is relatedto several aspects of a science curriculum. Topics inastronomy, chemistry, and physics are subjects for additionaldiscussion.

Astronomy

1) Solar sources of x-ray and ultraviolet radiation;

2) Energy levels of solar x-ray/ultraviolet radiation;

3) Solar x-ray/ultraviolet spectra compared with stellarsources.

Chemistry

1) Absorption and transmission of x-rays;

2) Ionization states of elements.

Physics

1) Energy of x-rays;

2) Spectrographyidentification of materials.

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Section 8Extreme Ultravioletand X-RayTelescope

CO

0

cc

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Physics

EXPERIMENT BACKGROUND

This experiment is comprised of two different instruments:(1) a grazing incidence x-ray telescope very similar to the onediscussed in Section 6 and used in the x-ray spectrograph tophotograph x-ray images of the Sun in the spectral region of5 to 35 angstrons; (2) an electronic x-ray event analyzer thatanalyzes the spectrum between 2.5 and 20 angstroms.

At very short wavelengths of the x-ray spectrum a diffractiongrating can no longer provide sufficient dispersion to yieldgood resolution of the spectrum. Wavelengths shorter than 20angstroms require the application of electronic sensors andtechniques.

A variation of the Geiger-Mueller tube, used for determiningradio activity levels, is the basis of an x-ray spectrometer. Theillustration shows the basic configuration of a proportionalcounter tube.

Metalwindow

. Cathode

Photon

a e 0 Anode0 43.Cb

6)GasMolecules

Proportional counter tube

Ionizing gas

An x-ray photon of energy enters the tube through a windowmade of a very thin metal sheet. On entering the ionizing gasvolume, the photon's energy is dissipated by ionizing one ormore gas molecules. The gas-ions are then attracted by theelectric field to the cathode which produces an electric pulsein the associated circuits.

The energy of the entering photon determines the extent ofionization that results. The shorter wavelength photons havegreater energy than longer wavelength photons.Consequently, short wavelength photons generate largeramplitude electric pulses at the cathode.

By electronically sorting the amplitudes, the pulses can beclassified to represent the spectra of the x-ray energy enteringthe tube. Also by using different window materials anddifferent ionizing gas mixtures, the wavelength or spectralresponse of the tube may be controlled. The illustrationshows the different spectral responses of aluminum andberyllium.

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History

56

60

50

40

30

20

10

90% xenon + 10% methane counter with 0.0006 mmaluminum window

06 8 10 12 14 16 18 20

50

40

30

20[xl

10

90% xenon + methane counterwith 0.025 mm berylliumwindow

5 6 7

Wavelength, angstrom

Spectral Response of Aluminum and Beryllium

10

Since 1960 satellite, rocket, and high altitude balloon flightshave provided data on the x-ray radiation from the Sun.However, instruments carried on these flights are short livedand have limited fields of view and spatial resolution. Modelsof the solar flare mechanisms based on available data havebeen postulated, but detailed analyses of the x-ray spectra areneeded to verify these models.

SCIENTIFIC OBJECTIVES

Correlation between x-ray spectroheliograms and the data inH-alpha and ultraviolet wavelengths are expected to reveal

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X-rayTelescope

X-ray EventAnalyzer

the spatial and temporal relationships of solar events. Becausethe shorter wavelengths are indicative of higher temperaturesin the emitting gas, additional' pictures will permit analysis ofthe development of an active region as it proceeds from thesolar surface to the chromosphere and to the corona. Ofprimary interest is the determination of the location of theinitiation of a flare in the solar atmosphere, and of themechanisms involved.

EQUIPMENT

The extreme ultraviolet and x-ray telescope comprise twoindependent instruments: a grazing incidence telescope andcamera that photographs x-ray events in the 5 to 33 angstromspectral region; (2) an x-ray event analyzer or x-rayspectrograph that analyzes the x-ray spectrum from theentire solar disk between 2.5 and 20 angstroms.

The x-ray telescope is a grazing incidence telescope ofapproximately 25 cm (10 in.) diameter aperture and 190 cm(75 in.) focal length. The x-ray image is focused on the focalplane of the camera. A six-position filter provides for spectralselectivity in the camera. Filter characteristics are listed:

Filter Material Spectral Response, angstrom

Beryllium 5 to 11

Aluminum 5 to 8 and 8 to 20

Titanium 5 to 12 and 27 to 33

Beryllium 5 to 12

Aluminum 5 to 8 and 8 to 18

Neutral density Visible light.

The neutral density filter is used to photograph the Sun inwhite light for data correlation purposes.

A variable speed rotating blade shutter provides variation offilm exposures between 1/3 and 288 seconds. The shutter isof sufficient thickness to absorb and stop x-rays that impingeon it in the closed position.

The x-ray event analyzer operates independently of the x-raytelescope, to provide a 10 wavelength analysis of spectralintensities between 2.5 and 20 angstroms.

Two proportional counter tubes are used to divide thespectrum into two parts. The counter tube characteristics aretabulated.

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Pulse HeightAnalyzer

58

WavelengthIonizing Response,

Window Material Gas Mixture angstrom

Beryllium 0.025 mm Xenon/Methane 2 to 8thick, 1.27 cmdiameter

Aluminum 0.0006 mm Argon/Methane 8 to 20thick, .32 cmdiameter

The simplified flow diagram of the x-ray event analyzershows that the outputs from the two proportional countertubes are fed to two pulse height analyzers, one having fourchannels, the other having six channels.

Aperturedisk

0ktf_FO 0

Amp

- Ch 1 20 A-Ch 2 16 A-Ch 3 12 A-Ch 4 8 A

-Ch 1 7.25 A-Ch 2 6.00 A'Ch 3 5.50 A-Ch 4 5.00 A-Ch 5 4.50 A-Ch 6 3.75 A

4-channelpulseheightanalyzer

Proportioncounter No. 1

Aluminum(8 to 20 A range)

&channelpulseheightanalyzer

Proportioncounter No. 2

Beryllium(2 to 8Arange )

Amp

X-ray Event Analyzer

Pulsecounterandaperturecontrol

Each channel of a pulse height analyzer is biased to accept alarger voltage pulse level than the preceding channel. Aninput pulse of small amplitude may be only large enough toexceed the bias of the first channel and would register acount there. A larger pulse might be able to appear in two orthree channels and a large enough pulse will drive allchannels. When pulses appear in two or more channelssimultaneously, redundancy circuits reject the pulse from thechannels that have the smaller bias voltages and allow it toenter only that with the largest bias. In this manner, the pulseamplitudes are sorted into their proper channels.

Because short wavelength photons entering the counter tubeare more energetic than long wavelength photons, they willproduce larger pulses in the counter tubes. Thus, pulsesentering the respective channels of the pulse bright analyzerare classified as to the wavelength ranges of the photons thatproduced them.

H 4 VH0

H3 V I"- 00 CD

C.) C.)CI)H2 V I-- cu

-5 qla. E

11

3 V pulsein

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Type

Counter circuits in the pulse height analyzer, count thenumber of pulses appearing in each channel over an intervalof time. The count rate !,,-,nerated indicates the intensity ofthe radiation that produced the pulses.

One of the limitations of proportional counter tubes is theirsensitivity to counting rate. In normal range, pulse amplitudeis proportional to photon energy. However, if the number ofphotons entering the tube becomes too great, theproportionality no longer exists. To eliminate this factor, thecount rate circuits of the pulse height analyzer are used tocontrol a motor driven disc having four different sizedapertures. The aperture disc is located in front of theproportional counter tube window to limit the radiationentering the tube. The scale factors of the instrument areadjusted for each of the apertures.

The outputs of the pulse height analyzer, pulse rate counter,and aperture disc position are transmitted to ground over thetelemetry system. In addition, the data are displayed on adigital count display and a strip chart recorder located on thecontrol and display panel in Skylab so that the astronaut maydetermine the level of solar activity.

EXPERIMENT DATA

Experiment data is in two forms--photographs and electronicx-ray spectra. The electronic data is processed and displayedin real time to warn the astronauts of an impending solarflare, and is also transmitted to ground for later analysis.Photographic data is returned to Earth following the mission.

Highlights The photographic images will act as a filter photometer todetermine the location and spectral region of the x-rayemission. The x-ray event analyzer measures the intensity and"hardness" of the spectrum, or ratio of high energy to lowenergy x-rays. In the early stages of a solar flare, the intensitywill show a rapid increase and the high energy x-rays willbecome more abundant.

Analysis Several rocket- and balloon-carried instruments haveindicated that flares consist of very energetic shortwavelength processes, but because of the limited observationtime available and insufficient spectral and spatial resolution,it has not been possible to develop and verify detailedmodels. The spectral data obtained by the experiment will beanalyzed to give flare temperatures, densities, and chemicalabundances. The filtergrams will indicate both the temporaland spatial variations of these quantities in flare regions. Ofparticular interest will be the processes occurring during theinitial stages of flare development and the influence of themagnetic field in sunspots on flare development.

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Availability The x-ray event analyzer information is transmitted to theJohnson Space Center for use by ground flight controlpersonnel. It is also displayed to the astronauts on countersand on a history plot (strip chart recorder). Photographicdata from the extreme ultraviolet and x-ray telescope will notbe available for public use until 1975. Procedures forreques+ing copies of flight data will be announced at a laterdate. Section 1 describes planned data uses and requirements.

CREW ACTIVITIES

The astronaut will use the x-ray event analyzer informationto anticipate solar flare occurrence and begin an appropriateobservational sequence. Other related crew activities arediscussed in Section 1.

RELATED CURRICULUM TOPICS

The x-ray event analyzer can be related to the followingtopics in electronics: (1) data sensing tubes, (2) pulse andgate circuits, (3) biasing circuits, (4) general computercircuits, (5) data processing, and (6) telemetry.

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Section 9Glossary

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Arc-Minute Minute/ /60 of a degree of angle

Arc-Second Second-1/3600 of a degree of angle

Balmer Series The series that results from radiation versus energy levelswhich progressively pull electrons from other shells of thehydrogen atom; all radiations of the Balmer series are atvisible light wavelengths.

Beam Splitter

Blaze

A semitransparent mirror which transmits part of the lightand reflects the remainder, to divide the incident light intotwo beams.

The shape and angles of the sawtooth-shaped grooves of areflecting diffraction grating. The blaze and fineness (numberof lines per millimeter) determine spectral response andresolution of the grating.

Chromosphere The region between the apparent solar surface and the baseof the corona. It is the source of solar prominences.

Corona The envelope of hot gases and ionized materials whichsurrounds the Sun. The corona extends from thechromosphere (approximately 6000 miles from the solarsurface) outward for several millions of miles.

Coronagraph An instrument employing occulting discs to form an artificialeclipse of the Sun to permit study of the solar corona.

Diffraction When a ray of white light passes over a sharp opaque edge, itis broken up into its spectrum. The ray of light is bent fromits previous path and the degree of bending is proportional toits wavelength.

Extra Vehicular Activity Activities in which an astronaut, wearing a space suit,performs work in space outside of his spacecraft.

Extreme Ultraviolet The region of the spectrum between wavelengths of 100 and700 angstroms.

Fabray-Perot Interferometer

Faculae

Fiducial Marks

Folding Optics

A form of filter that operates by destructive interference onall wavelengths except that which it is designed to pass.

Filamentary streamers of hot brighter material associatedwith sunspots.

Reference marks superimposed on photographic images fordata correlation and scale factors.

A mirror system used to reduce the physical length of atelescope or optical instrument while maintaining opticallength.

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Gimbals

Granules - Rice Grain

Grazing Incidence

Hydrogen-Alpha, H-alpha

Lyman and Paschen Series

A device consisting of two frames pivoted on axes at rightangles to each other, so that one frame may move within theother.

Elements comprising the mottled visible surface of thephotosphere.

Light arriving at a reflecting surface from an extreme obliqueangle.

Hydrogen-alpha is the particular radiation at 6563 angstromswhich results when the electron returns from energy levelthree to level two after having the orbit distorted by anexternal force, usually electrical.

These series are similar to the Balmer series but involveenergy levels that differ from the Balmer series. The Lymanseries provides ultraviolet radiation from higher energy levels;the Paschen series produces infrared radiation from lesserenergy levels.

Monochromator A device to isolate a single color or spectral wavelength froma spectrum.

Nautical Mile

Neutral Density Filter

Occult, Occulting

Photocathode

Polarization

Polychromator

Predisperser

Prominences

Proportional Counter Tube

Pulse Height Analyzer

62

1.15 statute miles, 6076 feet.

A filter that attenuates or reduces all regions equally; it is notspectrally selective (no color).

Occulting is the disappearance of ore heavenly body behindanother. An eclipse is natural occuiting of the Sun by theMoon. Artificial occulting is used in the coronograph.

A cathode that releases electrons in proportion to light fluxshining on it.

A distinct orientation of the wave motion and travel ofelectromagnetic radiation.

A device to produce colors from a source of white light;synonymous with spectrograph.

A secondary diffraction grating used to preselect a portion ofa spectrum to be dispersed by the main diffraction grating.

Jets of luminous matter ejecting from the chromosphere forthousands of miles.

An electron tube that produces an electric pulse which isproportional to the energy of a photon entering the tube.

An electronic device that sorts and classified electric pulsesapplied at its input.

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Rowland Circle A circle whose radius equals the radius of curvature of aspherical diffraction grating and is tangent to the grating.

Solar Limb The extreme edges of the apparent solar disk.

South Atlantic Anomaly

Spectrograph

Spectroheliograph

Spectroheliometer

Spectrum

Strip Chart Recorder

Sunspots

Telemetry

Transmission Grating

Ultraviolet

Video Display

X-ray

A region over the South Atlantic Ocean in which the VanAllen Radiation Belts are nearest Earth.

A system for resolving light of electromagnetic radiation intoits component wavelengths.

A picture of the Sun in a particular spectral region or a devicefor producing spectroheliographs.

A device for producing and measuring the spectrum of theSun.

Distribution of electromagnetic energy as a function ofwavelength; for example, visible light has a spectrum of 3500to 8000 angstroms (bluered).

An instrument that records events of data represented byelectrical signals on a moving paper scroll.

Darker areas of photosphere, thought to give rise to solarstorms.

A system for transmitting data and measurements overextended distancestransmission is often by radio means.

A diffraction grating in which energy is resolved to spectralcomponents on transmission through the grating.

The spectrum of wavelengths of electromagnetic radiationbetween 700 angstroms and visible light of 3500 angstroms.

A television-like display of data (pictures).

The region of the electromagnetic spectrum of approximately3 to 100 angstrom.

SUGGESTED REFERENCES FOR FURTHER STUDY

AstronomySolar Research

Larousse Encyclopedia of Astronomy, Prometheus Press,New York, New York 1968

The Solar Corona, John W. Evans; Academic Press, NewYork, New York 1963

Optics

Geometrical and Physical Optics, R. S. Longhurst; JohnWiley & Sons, Ltd, New York, New York, 1968

Fundamentals of Optics, F. A. Jenkins and H. E. White,McGraw-Hill Book Company, New York, New York, 1957.

63