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  • 8/8/2019 Skylab Experiments. Volume 1 Physical Science, Solar Astronomy

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    (NASA-EP-110) S K Y L A B E XP E RIME N TS . N 73- 3381 1V O L U M E 1: P H YS IC A L SCIENCE, S O L A RASTRONOMY (NASA) 74 p MF $1.'45; SOD HC$1.25 CSCL 22C Unclas

    G3/30 196^5UIIU7 Io.Physical Science,Solar Astronomy

    Information for Teachers, Including Suggestionson Relevance to School Curricula.

    NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

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    ExperimentsVolume IPhysical Science,Solar AstronomyProduced by the Skylab Program and NASA's Education ProgramsDivision in Cooperation with the University of Colorado

    NATIONAL A E R O N A U T I C S A N D SPACE ADMINISTRATIONWashington, D.C. 20546, May 1973

    For sale by the Superintendent of Documents, U.S. Government Printing Off i c e , Washington, D.C. 20402

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    P R E F A C ET h e m o s t i m m e d i a t e b e n e f i t s that derive f r o m a mult idiscipl ined scienti f ic program such asSkylab are a large v o lume and wide range o f sc i e n t i f i c in fo rmat io n . A secondary bene f i t i sthat this very large amount o f up-to-date in format ion can be related in a t imely manner tohigh school curr icula . The t ime la g b e t w e e n th e generat ion o f n e w i n f o r m a t i o n and itsappearance in text books i s o f ten me asured in years rather than in m o nths.It is the in tent o f t he Skylab Education Program to el iminate this characterist ically longdelay by t imely presentat ion o f sc ient i f i c in format ion generated by the Skylab program.The object ive is not to teach Skylab to the schools , but rather to use Skylab science as af o c u s fo r sc ience educat ion in the high schools . Readers are urged to use the descript ions o finvest igations and sc ient i f i c pr inc ip les , and the demonstrat ion concepts contained here in ass t imul i in ide nt i fy in g potent ial educat ional bene f i ts that th e Skylab program c an provide.

    Nat ional Aeronaut ics and Space Administrat ionWashington, B.C. 20546M ay 1973

    n

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    CONTENTSINTRODUCTION vSECTION 1THE SUN 1Importance of Solar Astronomy 1History of Solar Space Research .".. . . 2Solar Zones 3Scient i f i c Considerations 7Skylab Solar Observatory 8Experiment Schedules 9Crew Activities 9Data Availability 9SECTION 2-HYDROGEN-ALPHA TELESCOPES 13Experiment Background 13D e f i n i t i o n of Scientific Objectives 14Experiment Description 15

    Experiment Data 18Crew Activities 19Related Curriculum Topics 19Suggested Classroom Demonstration 19SECTION 3-WHITE LIGHT CORONAGRAPH (S052) 21Experiment Background 21D e f i n i t i o n o f Scientific Objectives 2 2Description of Coronagraph 23Experiment Data 25Crew Activities 26

    Related Curriculum Topics 26Suggested Classroom Demonstrations 27SECTION 4-EXTREME ULTRAVIOLET SPECTROGRAPH (S082B)E X T R E M E ULTRAVIOLET SPECTROHELIOGRAPH (S082A) 29Experiment Background 29Definition of Scientific Objectives 30Description of Experiment Hardware 31Experiment Data 34Crew Activity 35Related Curriculum Topics 35

    Suggested Classroom Demonstration 36SECTION 5-ULTRAVIOLET SCANNING POLYCHROMATOR-S P E C T R O H E L I O M E T E R (S055) 39Experiment Data 39D e f i n i t i o n o f Scientific Objectives 39Description of Experiment Hardware 39Data 41Crew Activities 42

    Related Curriculum Aspects 42Suggested Classroom Demonstration 4211 1

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

    SECTION 7-X-RAY ULTRAVIOLET SOLAR PHOTOGRAPHY (S020) 51Experiment Description 51D e f i n i t i o n of Scientific Objectives 51Description o f Equipment 52Experiment Data 53Crew Activities 54Related Curriculum Aspects 54

    SECTION 8-EXTREME ULTRAVIOLET AND X-RAY TELESCOPE (S056) 55Experiment Background 55Scientific Objectives '56Equipment 57Experiment Data 59Crew Activities 60Related Curriculum Topics 60Related Classroom Demonstration 60SECTION 9-GLOSSARY 61Suggested Reference for Further Study 63

    IV

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    I N T R O D U C T I O NThe Skylab Education ProgramThis year the United States' f i rst manned sc ient i f i c space s tat ion , Skylab , will b e l a u nc h e dinto o rbit and will be the fac i l i ty in whi c h succe ssive crew s of astronauts will p e r f o r minvestigations in a n u m b e r o f sc ient i f i c and t echnolog ical d isc ip l ines . The program o finvest igations can be d iv ided into four broad categor ies : phys ical sc iences , b iomedicalsciences, Earth applicat ions, and space applicat ions.T he Skylab sc ient i f i c program will p ro d u c e i n f o r m a t i o n t h at will increase o ur u n d e r s t a n d i n go f sc ience and wi l l extend our knowledge of subjects ranging f r o m the nature o f the un iverseto the structure of the s ingle human cel l . It is one of the object ives of the N a t i o n a lAeronaut ics and Space Adminis t rat ion that th e knowledge der ived f r o m i ts p r o g r a m s bemade available to the educational c o m m u n i t y for application to school curricula in a t imelymanner .For this reason , the Skylab Education Program was created to derive the m a x i m u meducat ional benef i ts f r o m Skylab , assist in doc um en tat io n of Skylab act iv i ti es , and enhanc ethe unders tanding of sc ient i f i c developments .T h i s d o c u m e n t i s o n e o f several volumes prepared as a part of the e d u c a t i o n p r o g r a m . It hasth e dual purpose o f i n f o r m i n g high school t eachers about th e sc ient i f i c inves t igat ionsp e r f o r m e d in orbit, and enabl ing teachers to f o r m an o p i n i o n o f t he e d u c a t i o n a l b e n e f i t s th eprogram can provide .In providing in format ion on the Skylab program, these books will def ine the object ives ofeach sc ient i f i c inves tigation and descr ibe i t s sc ie nt i f i c backgro und. The descr ip tions willinc lude discussions o f the sc ien ti f ic principles applied in the invest igations and the types o fdata generated , and the types of re lated in format ion and data available f r o m other sourcesin th e Skylab program.In the preparation of these descript ions of the Skylab activities, a c o n t i n ui n g g o a l has b e e nto build a bridge between Skylab sc ience and high school science. Discussions of thes c i ent i f i c background behind th e Skylab invest igations have been included to illustrate th es c i e n t i f i c n e e d s f o r p e r f o r m i n g those inves tigat ions in the Skylab enviro nme nt . Whe reverposs ib le , concepts fo r classroom act ivit ies have been included that use s pe c i f i c e l e m e n t s o fSkylab scienc e as fo cal points fo r the inc reased unde rstanding of selec ted subjec ts in thehigh school curricula. In some areas, these endorse current curriculum topics by providingpract ical applicat ions of relat ively familiar, but some times abstract , princ iples . In otherareas, the goal is to provide an i n t r o d u c t i o n to phenomena rarely addressed in high s c h o o lcurr icula .It is a goal of the Skylab Education Program that these volumes will stimulate the highs c h o o l t e a c h e r to the r e c o g n i t i o n that sc ient i f i c programs such as Skylab producei n f o r m a t i o n and data that nei ther are , nor w e r e p l a n n e d to be, the exclus ive domain of asmall group of scientists, but rather that these f indings are available to all who desire to uset h e m .

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    ApplicationReaders 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 f o r m o f f ilm strips, voicetapes, experiment data, etc, and can be provided to fulf ill teachers' expressed needs. Forexample, the teacher may suggest educational aids that can be made available f r o m thesei n f o r m a t i o n sources, which would be use ful in classroom situations to illustrate many of theprinciples discussed in high school curricula. These suggestions could then serve as stimulifo r development o f such aids.As information becomes available, periodical announcements will be distributed to teacherson the N A S A 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 an d Space AdministrationWashington, D.C. 20546Mail Code FE

    The basic subject o f this volume is the solar astronomy program conducted o n Skylab. Inaddition to descriptions of the individual experiments and the principles involved in theirp e r f o r m a n c e , a brief description is included of the Sun and of the energy characteristicsassociated with each zone. Whe re ve r possible, related classroom activities have beenident i f ied and discussed in some detail. It will become quite apparent to the reader that therelationships rest no t only in the f ield o f solar astronomy but also in the fo l lowing subjects:phys ics opt ics , the electromagnetic spectrum, atomic structure, etc; chemistryemissionspectra, kinetic theory, x-ray absorption, etc; biologyradiationand dependence on theSun; electronicscathode ray tubes, detectors, photomultipliers, etc; photography;astronomy; and industrial arts. The multiple educational relationships and interrelationshipsident i f i ed in this volume are shown in the table on the following page.AcknowledgementsValuable guidance w as provided in the area o f relevance to high school curricula by Dr.James R . Wailes, Professor o f Science Education, School o f Education, University o fColorado; assisted b y M r . Kenneth C . Jacknicke, Research Associate o n leave f r o m th eUniversity of Alberta, Edmonton, Alberta, Canada; M r. Russell Yeany, Jr., ResearchAssociate, o n leave f r o m th e Armstrong School District, Pennsylvania;and Dr. Harry Herzerand M r. 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 fo l lowing broad categories: physicalsciences, biomedical sciences, Earth applications, and space applications.During the eight-month operational lifetime o f Skylab, three crews, each consisting o f threemen, will live and work in orbit for periods of up to one month, two months, and twomonths, respectively.The objectives fo r each of the categories o f investigation are summarized as follows.Physical S c ie n c e T o p e r f o r m observations away f r o m th e filtering and obscuring e f f e c t s o fthe 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 f r o m which these phenomena emanate.Biomedical ScienceTo make observations under conditions d i f f e r e n t f r o m those o n Earthand thereby increase man's knowledge of the biological functions o f living organisms, and o fthe capabilities of man to live and work for prolonged periods in the orbital environment.Earth ApplicationsTo develop techniques fo r observing f r o m space and interpreting Earthphenomena in the areas of agriculture, forestry, geology, geography, oceanography, air andwater pollution, land use and meteorology, and the inf luence 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 SpacecraftThe 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 fo rgeneration of electrical power are mounted outside this module.

    The airlock module is the prime area in w h i c h control of the cluster internal environment,and workshop electrical power and communications sytems, is located. It also contains theairlock through w h i c h suited astronauts emerge to p e r f o r m their activities outside thecluster.

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    1 Apollo te lescope mount2 Solar arrays3 Sleeping quarters4 Personal hygi ene5 B i o m e d i c a l s c i e nc e e x p e r i m e n t6 Ward r o o m

    7 Orbi ta l workshop8 E x p e r i m e n t c o m p a r t m e n t9 A i r lo c k m o d u le

    10 Ai rlock external hatch11 M u l ti p l e d o c k i n g a d a p te r12 Earth resources exper i ments13 C o m m a n d an d service m o d u l e

    Orbiting Station

    IX

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    The mult ip le docking adapter provides th e d o c k i n g port fo r the c o m m a n d / se r v i c e m o d u l e sthat transport the crews, and contains the c o n t r o l c e n t e r for the telescope m o u n texper iments and systems. It also houses the Earth applicat ions experiments and spacetechnology exper iments .T he Apollo te lescope mount houses a sophist icated solar observatory havi ng eight telescopesobserv ing varying wavelengths f r o m visible, through near and far ultraviolet , to x-ray. Itco ntains the gyrosco pes and co mpute r o f the pr imary system by whic h the f l ight at t i tude o fSkylab is maintained or changed, and i t carries solar arrays by w h i c h a b o u t half of theelectrical pow er used by the c lus ter i s generated .The c o mm and and serv ice m odule i s the ve hic le in w h i c h the crew travels f r o m Earth toSkylab and back to Earth , and in whic h suppl ies are co nveye d to Skylab and exper im ents p e c i m e n s and f i lm are b r o u g h t to Earth.Skylab will f ly in a c i rcular orb i t about 4 36 ki lometers (235 naut ical m iles ) above th esurface o f Earth, and is planned to pass over any given point wi th in lat i tudes 50 n o r t h and50 south of the equator every f ive days . In its o r b i ta l c o n f i g u r a t i o n , S k yl a b will weigh o v e r44,100 k i lo g r a m s ( 2 0 0 , 0 0 0 p o u n d s ) a n d will c o n t a in n e a r ly 3 7 0 c u b i c m e t e rs (13,000 c u b i cf e e t ) f o r work and living s pa c e ( a b o u t t he s iz e o f a th r e e - b e d r o o m h o u s e ) .

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    Section 1The Sun

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    The Skylab solar astronomy program uses a careful ly chosense t o f e ight instruments in the Apollo te lescope mount ando ne other located in a n o t he r part of the laboratory . TheSkylab solar instruments are several times larger than anythat have been f l o w n b e f o r e and consequently possess greaterspectral resolution and higher angular resolving power. Thecluster o f instruments is capable o f simultaneously observinga wide range of spectral bands. These instruments arecontro l led by a trained observer w h o c a n analyze th e datapresented and alter th e observing program as required by thechanging solar condi t ions . He will be assisted by an extensivestaff o f ground based observers and analysts who will aid himin d e t e r m i n i n g th e most product ive observat ions to m a k ewith th e Skylab instruments. In addi t ion to providing ano b s e r v e r to operate th e instruments , th e presence o fastronauts also allows the use of high resolution photographicfilm to record data because the astronaut can change camerasand return th e f ilm to Earth at the end of the mission. Thesolar astronomy program will be augmented by ground basedobservat ions o f the Sun and the geo physica l phe no me naresulting f r o m i ts activity. While Skylab is in orbit, a series o fsolar observatories around th e world will keep the Sun u n d e rc o n t i n u o u s s u r v e i l l a n c e to warn o f impending so lardisturbances and acquire data to c o m p l e m e n t th e Skylabobservations.The phe no me na observable on the Sun can be de tec ted bySkylab with a reso lut ion dow n to the l imit ing reso lut ion o fground based instruments . These phenomena vary o n atimescale o f seconds an d minutes in size, shape, an dc o m p o s i t i o n .

    IMPORTANCE OFSOLAR ASTRONOMYThere are three major reasons why the study of the Sun isi m p o r t a n t :1) Solar p he n o m e n a ha v e a great i n f l u e n c e on the Earth.The Sun is the ult imate so urce o f all en ergy on the Earth,

    and all terrestrial life depends on i t . Solar activity cana d v e r s e l y a f f e c t r a d i o c o m m u n i c a t i o n b y causingionospher ic d is turbances and may have an e f f e c t o natmospher ic c i rcula t ion pat te rns that cause changes in thew e a t h e r , a l t h o u g h th e e x a c t m e c ha n i s m s a r e n o tu n d e r s t o o d . Changes in the total energy radiated by theSun over long per iods o f t ime m ay have bee n respo nsiblefo r past ice ages.2 ) Because o ur k n o w l e d g e of the Sun is greater than for anyo f the stars, many of our theories of stellar structure ande v o l u t i o n are dependent upon so lar data to provide a

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    k n o w n r e f e r e n c e point; t h e re f o r e , a n y d e f i c i e n c i e s i n o u rknowledge o f the Sun wi l l res ul t in corres ponding errorsin stellar theory. Since the Sun is the o n l y star o n w h i c hs urface fea tures can be d is t inguis hed, s tudy o f i t sstructural features wil l lead to ins ight into th e processesthat w e i n f e r may be h a p p e n i n g o n other s tars . Studies o fth e stars spectra s how magnet i c f ie lds, flares , and o t h e rp h e n o m e n a s u c h as we see on the Sun.

    3) By obs erving s o lar phenomena w e c a n s tudy a tomic ,n u c l e a r , a n d p l a sm a p h y s i c s , a e r o d y n a m i c s ,h y d r o d y n a m i c s a n d m a g n e t o h y d r o d y n a m i c p h e n o m e n athat are unobs ervable o n Earth. T he stel lar atmospherehas a range of temperatures , pressures , and magnet ic f ieldin tens i ti es ex te nding over vo lume s s everal t ime s the sizeo f th e Earth that c o u l d n e v e r be dupl i ca ted f o r study inany laboratory o n E arth. Such k no wle dge co uld he lp usto achieve co nt ro l l ed therm o nucle ar reac t ions .

    H I S T O R Y O F S O L A R S P A C E R E S E A R C HUsing s ounding rockets , bal loons , and small unmanneds pacec raf t , s o lar as trono me rs have s tudied th e Sun, th e u p p e ratmos phere , and near Earth space s ince the end of World W arII . These s tudies have revealed muc h about th e general natureo f the Sun in the x-ray and ultraviolet regions .

    Until satellites and s o u n d i n g rockets were developed i t waspossible to observe solar emiss ions only at wavelengths in theradio , infrared , and vis ible port ions of the s pec t rum thatcould penetrate the Earth's atmos phere . Thus , the ultravioletand x-ray radiat ions , which are important to the study o fhigh energy s o lar phenomena, could not be studied. Inaddi t ion , th e dayt ime a tmos pher i c s ca t t e r ing o f visible l ightcauses the sky to be so much br ighter than th e s o lar coronathat t h i s p h e n o m e n o n is only v is ible dur ing th e rare solarecl ipses and then only fo r relat ively short dis tances f r o m th es o lar s ur face . Atmo s pher i c turbulenc e caus es s himm ering o fth e obs erved image l imi t ing th e res o lut ion wi th which detailcan be observed on the s urface of the Sun.Use o f orbital s pacecraf t , s uch as O rbit ing Solar O bservatories( O S O ) , O r b i t i n g G e o p h y s i c a l O b s e r v a t o r i e s ( O G O ) , and theU.S. Navy SOLRAD satellites, has resulted in a steadilyincreas ing unde rs tanding of the explos ive and energet ic solarprocesses. H o w e v e r , the ability to obtain observat ions ofs u f f i c i e n t l y high res o lut ion in energy , t ime, and space is stilll imi ted by the size of the ins t ruments that can be carried byu n m a n n e d s p a c e c r a f t and the n e e d to c o m m u n i c a t e w i th thes pacecraf t through a t e l emetry system.

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    SOLAR ZONESThe general structure of the Sun is illustrated in Figure 1.

    : ChromosphereU O O O to 500,000 CU S O . O O O k mI thicki FlareU p O , O O O k mS h i g h

    wtmwd C o n v e c t i o n;1 Z o n e

    I PhotosphereI 5700 to 4000 CU800 k m th i ck

    Figure 1 Solar Zones

    Corona 0 .5 to 3 mill ion C ;9. 6 mil l ion km out f r o m 5c h r o m o s p h e r e

    Center16 mill ion C i1 ,360,000 km|diameter

    With acknowledgement to 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,w h i c h is the deepest level into the Sun that we can opticallyobserve and the region f r om which energy is radiated. Thephotosphere is approximately 5 thousand kilometers (3thousand miles) thick and has temperatures as high as 6000C.Overlying the photosphere is a layer in which th e density isdecreasing but the temperature increases with increasingheight. It is in the chromosphere that the absorption thatproduces th e 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 lowdensi ty and very high temperature o f 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 o f the current problems o f solar physics. Anotherp r o b l e m that must be theoretically explained i s how energy istransmitted into the corona f r o m lower layers to cause thelarge temperature gradient. It is theorized that mechanicalenergy f r o m th e convective zone is converted into sound

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    waves i n t h e c h r o m o s p h e r e that diss ipate the energy whenthey enter the low dens i ty co rona and cause the h igh co ronalt e m p e r a t u r e . A search will be m a d e w i t h th e Skylabi n s t r u m e n t s f o r changes in the c h r o m o s p h e r e and c o r o n aassociated with an osc i l lat ion wi th a 300-second per iod thathas been observed in the p h o t o s p h e r e .S u r ro u n d i n g t h e c h r o m o s p h e r e i s t he c o r o n a w h i c h e x t e n dsvisibly outward f r o m th e c h r o m o s p h e r e f o r several mill ionmiles . The co rona is very f a i n t in comparison wi th theluminos i ty of the solar disk. T he only t ime that i ts outerreaches can be observed f r o m Earth is during a solar ecl ipse.The c o r o n a l m a t e r i a l is of very lo w d e n s i t y and is highlyi o n i z e d because o f t empe ratures up to several mi l l ion degreesthat exist in the reg ion . A s a c o n s e q u e n c e , m o s t of the l ightemitted f r o m th e c o r o n a i s caused by scattering f r o m th e freee l e c t r o n s that h a v e b e e n i o n i z e d f r o m the coronal mater ial .The ac curate me asurem en t o f the in tens i ti es o f the solarrad iat ion over the wide w avelength range made possib le bythe Skylab solar observatory instrument cluster will be usedby solar scientists to d e t e r m i n e th e variat ion o f t emperatureand other physical propert ies with depth in the solara tm o s p he r e . T h e f a c t that di f f erent types o f rad iat ion aref o r m e d at d i f f e r e n t d e p t h s e n a b l e s us to probe at d i f f e r e n tlevels into the Sun and reconstruct th e solar atmosphere . Ingeneral , th e shorter wavelength radiat ion will be e m i t t e df r o m th e hotter plasma which is at higher alt i tudes in thesolar atmosphere . F o r example , extreme ul t rav iole t rad iat ionc o m e s f r o m th e reg ion where th e u p p e r c h r o m o s p h e r e isb l e n d i n g into t h e l o w e r corona, while x-rays are formed inthe high tempe rature co rona.The most spectacular solar phenomena are the f lares thator ig inate in the chrom osphe re and lo we r co rona and extendoutward in some cases e jec t ing mater ial f r o m the Sun. Solarflares are associated with the s t rong magnet ic f ields f o u n d insunspots and transfe r the ene rgy stored in the magnetic f ieldinto elect romagnet ic rad iat ion , h igh energy part i c les , bulkphysical mo t ion of the gas , and rad io f req ue nc y emiss ion in acatastrophic e v e n t that i s no t totally u n d e r s t o o d . A solarflare begins very rapidly and is character ized by a suddenincrease in the H-alpha emission as well as a rapid bui ld-up inth e x-ray and ultraviolet emission. The flare spreads rapidlyf r o m a small origin across the surface covered by a sunspotg r o u p , r e a c h e s a m a x i m u m i n t e n s i t y in a few m i n u t e s , andthen decl ines . A medium sized f lare lasts perhaps half an h o u rwhile a large flare m ay las t four o r five hours . In addi t ion toth e elect romagnet ic rad iat ion , large c louds o f solar materialare e j e c t e d o u t w a r d f r o m th e flare and high energy cosmicra y particles are generated . The total quant i ty o f energyreleased in a large flare is m a n y t i m e s th e energy generated byall man made sources, but is only a small fract ion of the total

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    e n e r g y p r o d u c e d by the Sun. Fo r example , it has b e e nest imated that in a o ne-ho ur per iod during the massive solars torm which occurred in August 1972, the s torm producede n o u g h energy to m e e t th e elect r i cal power demands of theUnited States fo r 100 years at present consumpt ion rates.The classif icat ion of the m a n y g e o m e t r ic a l f o r m s observed inflares and the associated structures observed in the solaratmosphere around sunspots where f lares take p lace , showsth e p h e n o m e n a to be a very compl icated se t o f interact ionsi n v o l v i n g p l a s m a i n s t a b i l i t i e s , e l e c t ro d y n am i c a n dh y d ro d y n a m i c e f f e c t s.Skylab will try to observe a number of solar f lares with asm a n y of the solar expe rime nts operating simultaneo usly as isfeasible. The object ive is to obtain a diverse an d extensivec o l l e c t i o n o f data which can be used to determine th e p o i n tw h e r e th e f lare started, physical condit ions such as thet emperature , dens i ty , magnet ic f ield strength, and particleveloc i ti es in the f lare p lasma and the surroundi ng me dium,and to d e t e r m i n e ho w these condi t ions change before ,during, and af ter th e flare.O ne of the primary sc ient i f i c object ives is to obtain acomprehensive set of observations of a solar f lare from itsearliest detectable stages with th e Skylab high resolutioninstruments. It is hoped that th e ultraviolet an d x-rayobservat ions wi l l prov ide th e in format ion required todete rmine the me chanism responsible fo r tr iggering f lares andwill enable better pred ic t ions to be made o f fu ture f lares .O btaining f lare data should be faci l i tated by the f ive -mo ntht ime th e Skylab instruments c an operate in orbit. Flare dataf r o m sounding rocke t ins truments i s d i f f i c u l t to obtainbecause of the di f f i cul ty o f launching th e rocket at theproper t ime to obtain data during the eady\ phases of theflare.Large solar f lares can cause geophysical disturbances. X-raysand ultraviolet radiation greatly increase th e i on izat ion of theupper layers o f the atmo sphere on the , sunlit;side of Earth,resul t ing in d isrupted rad io communicat ions . About an hourafter the f lare, high energy cosmic rays f r o m the Sun arrive atEarth. In a day a large cloud of low energy plasma ejectedf r o m the flare arrives and causes magnetic storms. Highenergy part icles enter the atmosphere at the magnetic polesand cause auroral displays and co smic ray e f fe cts .Associated with data taken on solar flares is a longer durationstudy o f act ive reg ions . The three-dimensional structure o fth e active regions will be investigated to determine th ehorizontal and vertical variation of the temperature , dens i ty ,v e l o c i t y , a nd m a g n e t i c f i e ld . T h e s tr uc t ur e o f t h e

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    p h o t o s ph e r e , c h r o m o s p h e r e , t ra n si ti o n r e g i o n a n d c o r o n aabove sunspots will be g iven careful a t tent ion to determineho w i t relates to the p r o d u c t i o n o f flares and other t rans ientp h e n o m e n a . B o th s ho r t- te r m ( m i n ut e s t o h o u r s ) a n dlong-term (days) changes will be d o c u m e n t e d . The v e l o c i t yf ie lds in the chromosphere and corona over sunspots andact ive reg ions will be mapped by m e a n s of the D o p p l e r shiftin spectral data and the mass motion shown in thecoronagraph p ic tures .Several type s of structures are vis ible on the solar surfacew h e n v i e w e d t h r o u g h an appropriate ly f i l t ered te lescope .Large sunspot groups wi th compl icated s t ructures are qu i tenot iceable and persist fo r w e e k s u n d e r g o i n g c h a n ge s in sizeand f o rm. The spots are reg ions whe re s t ro ng magnet ic fieldsexist a n d i n h i b i t t h e c o n v e c t i v e m o t i o n o f t h e p h o t o s p h e r i cmaterial so that the sunspots are cooler than the s u r r o u n d i n gphotosphere. The magnet ic f ield f o r m s large loops up intot h e c o r o n a f r o m sunspot groups . Conde nsat ions o f c oro nalmater ial form in these loops that are visible as arch-shapedprominences when v iewed against th e l imb and as f i l a me nt swhe n pro jecte d against the d isk . Regions o f enhance demiss ion in the spectral l ines of certain elements are calledplages and of ten surround sunspot groups . M o s t of the solardisk shows a mott led appearance as the tops o f c o n v e c t i v ecells that are heated in the solar interior appear, r ise, radiateaway energy, and sink b e l o w t h e visible solar surface. Theextens ions o f these patterns into th e c h r o m o s p h e r e andc o r o n a will be invest igated with the ultraviolet and x-rayinstruments o n Skylab. The variations o f temperature anddens i ty as a f u n c t i o n o f height and the veloc i t i es of the gaswill be obtained.Because th e Skylab solar o bservato ry in strum e nt ope ratio nc o n s u m e s a cons iderable portion of the astronaut's w o r k i n gt i m e , it is essential to c o m b i n e the observat ion requirementso f th e sc ient i f i c inves t igat ions in as c o m p a c t a s e q u e n c e o fdata-taking ope rations as possible. This is acc o mplishe d byd e f i n i n g eleven Joint Observ ing Programs that investigatesolar phenomena such as prominences and f i laments , coronaltransients, and the solar wind. The observat ions onp r o m i n e n c e s and f i laments wi l l s tudy the i r evolut ion as theycross the disk as the Sun rotates and determine thethree -d imens ional s t ructure ( tempe rature and de ns i ty) o f thef i laments and the surrounding mater ial o f the chromosphereand corona. The solar wind will be invest igated by studyingthe e v o l u t i o n of the c h r o m o s p h e r i c n e t w o r k and itsextens ion into th e c o r o n a and the rate o f e x p a n s i o n o fvar ious parts o f the corona. Trans ient coronal phenomenawill be observed in the vis ible, ultraviolet , and x-ray regionsto d e t e r m i n e th e spatial and t e m p o r a l d e v e l o p m e n t o f t hef eatures , th e v e l o c i t y o f propagat ion , and the corre lat ionwith surface and inner coronal f eatures .

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    S C I E N T I F I C C O N S I D E R A T I O N SS k y l a b will carry two t e les copes to obs er ve the H- alph aspectral l ine at 6563 angstroms. H-alpha is red l i g h t e m i t t e dby th e h ydr ogen gas f r o m t h e p h o t o s p h e r e . T h e i n t e n s i t y o fthe em is s ion var ies wi th s t r uc tur al f ea tur es on the solar diskand d is plays m any f ine s cale f ea tur es in th e s o lar a tm os ph er e .Solar flares are eas i ly obs er ved in H- alph a l igh t bec aus e therei s a large c h ange in th e in tens i ty o f this em is s ion w h i le th etotal vis ible r adia tio n varies ver y l i t t l e . By tuni ng th e f i l t e rsl ight ly o f f o f t he c e n t e r o f t h e l i n e , t he D o p p l e r s h if t e dr a d i at i o n f r o m g a s i n m o t i o n t o w a r d s o r aw a y f r o m th eo b s e r v e r may be v i e w e d . The H- alph a te les copes wil l be thep r i n c i p a l a i m i n g d e v i c e the as tr onaut will use to p o i n t theo t h e r i n s t r u m e n t s , an d will give h i m a c o m m o n r e f e r e n c ew i t h the gr ound- bas ed obs er ver s dur ing the m i s s i o n . Thetelescopes will gather data of u n i f o r m high qual i ty takens im ultaneous ly wi th data f r o m the other i n s t r u m e n t s .Th e ul t r avio le t s pec t r ogr aph s will d e v e l o p high r e s o l u t i o ns pectr a o f ver y s m all ar eas o f th e Sun in a wavele ngth r angethat c a n n o t be observed f r o m the ground. This is anim po r tant r eg io n becaus e th e ul t ravio le t l ines aris e f r o m th er egions of the c h r o m o s p h e r e , and t r ans i t ion r egion into theco r ona , wh e r e analys is o f th e wave length and intens i t i es o fthe em is s ion s pec t r a l l ines per m its de ter m inat ion of thet e m p e r a t u r e , d e n s i t y , and e lec t r on pr es s ur e of the l evel in theSun w h e r e the r adia t ion or ig inated . The ul t r avio le t r adia t ionis s t rongly en h ance d in so lar f lares as the tem pe r atur eincreases .Skylab s pec t r ogr aph s use both p h o t o g r a p h i c and e l e c t r o n i cm e t h o d s of data r e c o r d i n g : p h o t o g r a p h i c t e c h n i q u e s g iv ehigh spatial resolution and can gath er lar ge am ounts of datain a short p e r i o d of t i m e ; e l e c t r o n i c detectors are used toobta in h igh accur acy in m eas ur ing the in tens i ty of theem it ted l ines .High ener gy x - r ays ar e fo r m ed in th e cor ona wh er e m i l l i o ndegr ee tem per atur es s t r ip the e l e c t r o n s f r o m the a t o m s andp r o d u c e a h i g h d e g re e o f i o n i z a t i o n . L o w re s o l u ti o n s pe c t raand p h o t o g r a p h i c i m a g i n g will be o b t a i n e d by the two Skylabx-ray telescopes. Th e s pec t r a wil l e n a b l e t e m p e r a t u r e s o f t h ee m i t t i n g r e g i o n s to be d e t e r m i n e d , and the i magi ng will becorrelated with im ages taken in o t h e r w a v e l e n g t h s tod e t e r m i n e the spatial and t e m p o r a l d e v e l o p m e n t of a solarr egion that p r o d u c e s a f lare .A n o t h e r f o r m o f e l e c t r o m a g n e t i c ra d i a ti o n f r o m t h e c o r o n ai s v i e w e d in wh ite l igh t s ca t ter ed f r o m th e f r e e e l e c t r o n s . Thei n t e n s i t y o f t h e scattered l i gh t i s a m e a s u r e o f t h e e l e c t r o ndens i ty of the c o r o n a and displays arcs, rays, and s t r eam er sthat trace the m agnet ic f ie ld s t r u c t u r e . O b s e r v a t i o n of theco r ona a f te r a f lar e sh ow s th e r es pons e o f th is large , low

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    density me diu m, to a disturbance in i ts lowe r layers. Becauseth e radia t ion f r om th e c o r o n a i s a mill ion times we aker thanth e radia t ion f r o m th e solar disk, a coronagraph must bedesigned with special optics to b l o c k out the intense l ightf r o m th e solar disk. The Skylab co ronagraph will be able toobserve higher detail and greater distance s f r o m the Sun thansimilar instruments operat ing f r o m th e ground because i t willbe above the scatter ing e f f e c t s of the Ear th 's a tmosphere .S K Y L A B S O L AR O B SE R V A T O R YThe SJcylab solar observatory contains the e ight telescopesm o u n t e d o n a c o m m o n structure ( the Apollo telescopem o u n t ) 1) white l ight coronograph (S052) ,2 ) ultraviolet spectrograph (S082B),3 ) u l t r a v i o l e t s c a n n i n g sp e c t ro he l i o m e t e r -p o l y c hro m a t o r(S055),4 ) extreme ul travio le t spec trohe l iograph (S082 A),5) x-ray telescope -spec trographic c ame ra (S05 4),6) x-ray spectrograph (S056),7) two H-alpha telescopes,8) and another extreme ultraviolet spectrograph (S020)w h i c h is operated f r o m an airlock in a n o t he r part o fSkylab.The first e ight instruments are m o u n t e d in a canister so thatall of the te lescopes can p o i n t to the same area of the solarsurface . The comple te observatory is rigidly attached to thebody of Skylab the mass of wh ic h provides the stable basenecessary f o r m a i n t a i n i n g th e pointing stabil i ty required f o ra s t r o n o m y . The c o m m o n p o i n t in g ax is o f the assembly is no texpected to drift more than 1/700 of a degree dur ing a15-minute period-equivalent to a b o u t 4 0 0 0 k i l o m e t e r s (2 5 0 0miles on the Sun's surface . [The diameter of the Sun is1,390,000 kilometers (864,000 miles) an d sunspots vary insize f r o m 800 to 80,000 kilometers (500 to 50,000 m i l e s ) inw i d t h . ]With its e ight te lescopes observing the Sun in several spectralbands, Skylab provides the f i rst o p p o r t u n i ty t o p e r f o r m l o n gduratio n, high ly detailed studies of the Sun in the visibleultravio le t , ex treme ul travio le t , and x-ray spectral regionssimultaneously. (See Figures 2 and 3.) The operating l i f e t i meo f Skylab will also permit th e observations o f solar eventsthrough their l ife cycles. A sunspot m ay persist throughseveral 27-day ro ta t ions of the Sun.8

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    E X P E R I M E N T SCHEDULESThe solar telescopes are scheduled to operate more than 100h o u r s during th e first manned mission o f Skylab. T he secondand third missions will schedule additional operations so thatseveral hundred hours o f solar observation wil l be achieveddur ing th e entire Skylab f l i g ht program.The complement o f solar observing telescopes will usually beoperated in unison; however, they may also be operatedi n d i v i d u a l l y or in partial groupings. The selection o ftelescopes used for an observational period is at thediscretion of the astronauts, the f l i gh t control center at theJohnson Space Center in Houston, and the principalinvestigators associated with the investigations.Another Skylab experiment program, the Earth resourcesexperiments ( E R E P ) , 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 f a v o r of solarobservation.Since a m a j o r Skylab scientific objective is to observe andrecord the onset of solar flares which are not predictable ando f short l i f e , 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 f l ight operations toobserve the solar flare.CREW ACTIVITIESThe 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 usedfo r 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 inEV A space suits will emerge f r o m Skylab to retrieve theexposed f i lm or to install new f i l m .D A T A AVAILABILITYThe data which accrue f r o m the solar telescopes, are in threef o r m s : television, photographs, and tape-recorded digital data(similar to computer tape.)

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    The te lev is ion p ic tures f r o m those te lescopes equipped wi thte lev is ion cameras may be t ransmit ted to g r o u n d fo r us e byground-based personnel to evaluate and direct t e lescopeperformance . Telev is ion p ic tures are also used by theastronaut at the c o n t r o l and display panel . Some o f t h ete lev is ion p ic tures may be re leased to the publ ic d u r i n gSkylab nights.Photographic data are r e t u r n e d to Earth at the c o n c l u s i o n o feach mission. These data represent pictures of the Sun in thevarious spectral wavelengths and s c i e n t i f i c data in the f o r m o fspectrographs; both types are r e q u i r e d by interested scientiststo conduct s tudies o f solar events.The tape-rec orded d ig i tal data w h i c h r e s u lt f ro m t h e e x t r e m eul t rav iole t scanning spectrohel iograph and the x-ray eventa n a l y z e r i s t ransmit ted to g r o u n d t h r o u g h the radioc o m m u n i c a t i o n s y s t e m . A s r e c e i v e d th e s e d a ta c a n n o t b eused . I t must be processed by a computer w h i c h will t h e nprovide tabular and/or graphic outputs f o r analysis .The principal invest igators and scientists retain proprietaryrights to the data f r o m the i r e xper iments fo r a per iod o f oney e a r to provide adequate t ime f o r data analysis and studyb e f o r e t h e f indings are made publ ic .Aside f r o m that i n f o r m a t i o n w h i c h NA SA m ay release duringth e Skylab fl ight, i t may be expected that results of theSkylab solar observations will be made publ ic early in 1975.

    10

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    u2S2Sg1S>aCSoOfSCi>*3cmgg&g8C5c8X,S"Si!COW38Ja-10O

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    Section 2Hydrogen-AlphaTelescopes

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    EXPERIMENT BACKGROUNDThe hydrogen-alpha (H-alpha) telescopes are designed to view

    ha and record images 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 orbit havebeen 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 and A a n g s t r o m u n i t s are thetransitions which can occur in the hydrogen atom are shown i n t e r n a t i o n a l u n i t for m e a s u r i n gin Figure 1. When the electron returns from the third to the l i ght wavelengths-second energy level, it emits H-alpha light, which is a unique 1 A = 10 cm.red light at a wavelength of 6563 angstroms.

    lo n i z a t i o n levelL y m a n seriesul t ra vi o le t

    H-alpha

    Paschenseriesin f r a r e dFigure 1 H y d r o g e n Series

    B alme r series visible

    13

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    H-alpha of theS un

    G r o u n d BasedH-alphaObservation

    Thermal and magnet ic co ndi tions o f the Sun are o f suchmagni tudes that H-alpha radiat ion is a p r o m i n e n t c o m p o n e n to f the spectrum. Observat ion of the Sun with a telescopef i t ted with a f i l ter that transmits only H-alpha red l ight (6563 1 angstrom), reveals details of the solar surface and itsactivity which are obscured by the many other wavelengthsthat comprise white l ight . Figure 2 is a typical H-alphaphotograph of the Sun.

    Figure 2 Solar Image in H-AlphaThrough a w o r l d w i d e n e t w o r k of ground based telescopeswith H-alpha f i l ters , maintaining continuous surveil lance ofthe Sun and monitoring solar act ivity (sunspots , flares, etc) ,it is possible to predict w h e n the radiation f r o m the solardisturbance will arrive at Earth to cause geomagneticdisturbances and ionospheric storms that disturb radiocommunication and navigation systems, and a f f e c t wind andweather patterns.The solar wind which causes storms o n Earth is c o m p o s e d o fi o n s an d cosmic part icles (materials of the Sun) w hose arrivalis heralded by electromagnetic radiation. The veloc i ty of thesolar wind is less than the speed of light since it is c o m p o s e do f material and is not e ne rgy.

    Solar R a d i a t i o n S p e c t r um-H-alpha6563 A

    2 0 0 0 A 15,000 A"Visible3500 to 7000 A

    H-alphaCharacterist icso f Su n

    14

    D E F I N I T I O N O F SCIENTIFIC OBJECTIVESWhe n observations are m ade with a H-alpha f i l ter , features ofthe Sun 's chromosphere are seen . The change that occurs inH-alpha radiat ion relat ive to various solar features ispresented in Figure 3. The contrast b e t w e e n these f eatureshas been used by solar astronomers for many years.

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    150I1 , . 1 0 0 fl= 3 en3 cr 50 2

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    Quiet Sun

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    Figure 3 H - A l p h a Across Solar Phenomena

    + 4O f f s e tW av el en g t h , A

    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 and scattering. TheH-alpha telescopes of Skylab will be above the atmosphereand will provide H-alpha photography of greater resolutionthan can be obtained f r o m the ground.E X P E R I M E N T DESCRIPTIONGround based telescopes fo r H-alpha observation are usuallyrefracting telescopes in which lenses are used to focus animage. These telescopes are necessarily long when a smallf i e ld of view is required. In the Skylab cluster, the telescopemust have a short physical length but still retain a narrowf i e l d 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 af o c u s behind the primary mirror. Thus, while the opticallength of the telescope is retained by the multiple reflections,the physical length is shortened.

    SecondaryMirror -

    Focus^-_- -

    Cassegrain Telescope

    15

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    H - a l p h aFilter

    Fabry-Perot

    Since H-alpha l ight is energy at a u n i q u e and discretewavelength, a f i l ter that transmits only a very narrow banda b o u t that w a v e l e n g t h i s r e q u i r e d to isolate that wavelengthf r o m t h e o t h e r c o m p o n e n t s o f w h i t e l i g h t. T h e s p e c t ra ltransmission of the H-alpha f i l t ers in Skylab telescopes is6562.8 0.35 angstroms. This f i l ter is properly called aF a b r y - P e r o t i n t e r f e r o m e t e r .The F a b r y - P e r o t i n t e r f e r o m e t e r is d e v e l o p e d f r o m th ec o n c e p t s o f construct ive an d d e s t r u c t i v e i n t e r f e r e n c e o fradia t ing waves. W h e n tw o waves of the same wavelengtharrive at the s a m e p o i n t in space and are in phase , they willr e i n f o r c e each o ther , or construct ively in terfere . I f the twowaves are out o f phase they will cancel each other, o rdestruct ively in terfere .T h e F a b ry - P e r o t i n t e r f e r o m e t e r i s c o n s tr u c t e d f r o m a disk offused s i l i ca which is 125 microns th ick . The d isk i s opt ical lypol ished and is c o a t e d w i t h a semiref lec t ive surface o n bothfaces . The faces of the disk are also maintained in parallel .Whi t e l ight i s co mposed of many wavelengths . At som ewavelengths, the th ickness o f the d isk will be equal to an oddn u m b e r o f w a v e l e n g t h s; a t other wave lengths, the disk is aneven n u m b e r o f wavelengths th ic k .Light that enters th e i n t e r f e r o m e t e r i s mult ip ly-ref lec tedb e t w e e n th e internal faces of the disk. Fo r those wavelengthsfo r whic h the d isk i s an even num be r o f wavelengths th ick ,th e in ternal re f lec t ions wi l l construct ively in terfere wi th th eincident l ight , and those wavelengths will be t ransmit tedt h r o u g h t h e i n t e r f e r o m e t e r . A t w a v e l e n g th s f o r w h i c h t hed isk i s an odd nu mbe r o f wavelengths th ick , the in ternalref lec t ions wi l l des t ruct ively in terfe re wi th the in c i de nt l ightand no transmission through th e i n t e r f e r o m e t e r will result.B e t w e e n these two extreme condi t ions , part ial des truct ivein terference takes p lace and the inc ident l ight i s attenuated.Figures 4 and 5 illustrate th e c o n s t r u c t i o n and operat ion o fthe Fabry-Perot in ter ferometer and the resultant f i l tertransmission characterist ics .R e f l e c t i v e coatings

    IX , 3X, 5X, 7X . . . (n - 1)XD e s t r u c t i v e i n t e r f e r e n c e

    2X , 4X, 6X . . . nXC o n s t r u c t i v ei n t e r f e r e n c e

    Figure 4 Interferometer Reflections16

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    0.5 -I n t e r f e r o m e t e rtransmission

    Bandpass f i l t e rt ransmission

    C o m b i n e dtransmission

    6550 A 6563 AFigure 5 Interfe ro me ter Transmiss ion 6576 A

    T h r o u g h o u t the s pec tr um of wh ite l igh t , th er e are m a n yw a v e l e n g t h s that satisfy the c o n d i t i o n s for cons tr uct ivei n t e r f e r e n c e . T h u s , in wh ite l igh t the output of thei n t e r f e r o m e t e r w i l l b e li gh t w hi c h i s c o m po se d o fwavelength s at dis cr e te in ter vals th r ough out the s p e c t r u m .To m a k e the in ter f e r om ete r wave length s e lec t ive in theH-alpha re d l igh t only , a bandpass re d glass filter i s placed inf r o n t of the i n t e r f e r o m e t e r so that o n l y red l ight in thespectral region of H-alpha is admitted to the interferometer.Th e addi t io n o f th e r ed bandpass f i l t e r i s s h ow n in F igur e 5 .

    R e d Y e llow Green BlueIn ter ferometer O u t p u tin White Light

    The c o m p l e t e F a b r y -P e r o t i n te r f e r o m e t e r H - a lp h a f i l te rpasses 20% of the l igh t wi th in the very narrow bandpasswh ich i t pr ovides .17

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    Type

    The thickness of the fused silica disk (125 microns) istemperature-dependent; thus close temperature control towithin 1.0C 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 o f solar events byutilizing th e Doppler wavelength shift. H-alpha radiationsources moving toward the telescope will exhibit awavelength shorter than usual. Events receding f r o m 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 fo r photography.

    E X P E R I M E N T DATAExperiment data a r e i n t w o forms: television a n dphotographs. The television circuits are used primarily by themonitoring astronauts; the television data are transmittedto Johnson Space Center for use by ground fl ight controlpersonnel. Some of the H-alpha television images may also besupplied t o T V networks f o r public information during th emission.

    Beam s pl i t te ra l ightly coated,p a r t i a l l y transparent mirror,w h i c h reflects part of the l ighti nc i dent on i t and transmits th eremai nder .

    Incidentlight

    2

    Ref lected/2 ^ -Transmitted

    Beam splitter

    Photographic data are returned to Earth after the mission.While the data will be used primarily fo r correlation o f solarevents with the data f r o m other instruments, it is expected tob e o f higher resolution than that obtained f r o m ground basedtelescopes; thus, it may be of scientific merit.Highlights The H-alpha photography shows details o f solar phenomena;plages, filaments, flares, an d sunspots are clearly defined.Analysis Correlation o f H-alpha photographs with data f r o m

    ultraviolet and x-ray images and corona photographs(Sections 3 thru 8) are expected to yield information on theenergy-produc ing mechanisms of the Sun and the manner inw h i c h that energy is radiated to Earth and space.Availabil i ty

    18

    Other than some TV pictures which may be released duringthe mission, data f r om the H-alpha telescopes may not bea v a i l a b l e f o r public use before 1975. Procedures fo rrequesting copies o f f l ight data will be announced at a laterdate. Section 1 describes plannned data uses andrequirements.

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    C R E W ACTIVITIESThe astronaut will use the TV display f r o m the H-alphatelevision camera to point other telescope mountinstruments. He will also evaluate the image to decide whicho f 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.R E L A T E D C U R R I C U L U M TOPICSConcepts employed in the design and application of theH-alpha telescopes may be related to a number of subjectsw h i c h are discussed in high school science programs. Subjectsin astronomy and physics are readily apparent topics:AstronomyDetails of sunspots, filaments, spicules, and plages;Relationship between sunspots and other features;Life cycle o f sunspots.PhysicsOpt ics cons t ruc t ive and destructive interference, interferom-eters, spectral analysis;Nuclear, thermal and energy considerations of the Sun.

    SUGGESTED CLASSROOM DEMONSTRATIONWhile th e isolation of the single wavelength o f H-alpharadiation at 6563 angstroms is important in the H-alphatelescope, the principle employed is also f o u n d in other f ieldswhere 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 a1) Sun screen with two pinhole apertures;2 ) 6-inch long triangular dispersing prism. T w o small prismsm ay be used but their dispersed spectra must be

    aligned;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 o p t i c a l l y flat 2-inch d iameter in ter fero me terelements (optical flats);5) su itable whi te project io n screen .C o m p o n e n t s of the demonstrat ion are arranged as s h o w n inthe accompanying sketch .

    Sunscreenwithapertures Prism

    _PathA_ \A

    PathB

    Coloredbandpassf i l ters 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 f i l ters , one at a t ime, in tol ight path B. As each of the f i l ters is inserted, only i tspart icular co lor w ill rem ain in the spectrum o n thescreen . The other co lors have been re jec ted by the f i l ters .3) Insert th e i n t e r f e r o m e t e r into light path B and c o m p a r ethe resul t ing spectrum with that f r o m l ight path A. Ther e s u l t i n g s p e c t r u m will show bright l ines at the

    wavelengths w h e r e c o n s t r u c t i v e i n t e r f e r e n c e o c c u r r e d i nthe i n t e r f e r o m e t e r .4) Insert bo th the red bandpass f i l ter and the i n t e r f e r o m e t e rin path B and c o m p a r e th e result ing spectra. Thespectrum o f l ight path B will be a few red lines. Then u m b e r o f l ines is d e p e n d e n t on the range o f wavelengthspassed by the bandpass filter.5) Repeat step 4 for the green and blue bandpass f i l ters ,respect ively . Note that there will be mo re b lue l ines thanre d lines. W h y ? Wha t is the d i f f e r e n c e b e t w e e n red andblue l ight?

    C o n s t r u c t i o n o f i n t e r f e r o m -e t e r th e i n t e r f e r o m e t e r i sconstructed f rom two opt i caflat i n t e r f e r e n c e flats and a ringof 1-mil steel shim stock. Theshim s tock r ing is of the samed i a m e t e r as the i n t e r f e r e n c eplates. The i ns i de d i ameter o fthe ring is 0.5-inch less.

    Di ameter o finter f erenceflats

    0.25 i nch 0.25 i nchTh e shim stock r i ng is nows andwiched between the op t i ca lf l a t s to f o r m a 0.001-inchspacer. Spacer

    O p t i c a l I n I O p t i c a lflat ; flat

    SUGGESTED S O U R C E SPrisms an d opt i ca l flats may beobta i ned f r o m :E d m u n d S c i e n t i f i c C o .Harrington, N e w JerseyBandpass f i l ters may be o b t a i n e df r o m :Spec ial O p t i c sCe d ar Grov e , N.J . 07009

    20

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    Section 3White LightCoronagraph

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    E X P E R I M E N T B A C K G R O U N DThe 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 f r o mthe Sun. Temperatures of the corona vary f r o mapproximately 500,000C at the chromosphere to3,000,000C at the outer fr inges .Although the corona is very luminous, it is 10~10(1/10,000,000,000) times as bright as the Sun. Norma l l y lightf r o m the brighter solar disk is scattered and d i f f u se d byEarth's atmosphere and obscures the corona. Consequentlythe only way the corona can be seen f r o m Earth is duringbrief periods of total eclipse. At these times the M o o n passesbetween Earth and Sun to occult the Sun and eliminates thesource of atmospheric scattering. It is important to note thatthe occulting object ( M o o n ) 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( f o r m s ) 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) f r o m the eye, subtends the same angle and willprovide an artificial eclipse. This concept is the basis of thedesign of the white light coronagraph.

    C o r o n as t reamers andf i l a m e n t s

    C o ro na a z o n e of hot gases andi o n i z e d atoms ra di a t i ng f r o m th eSun fo r m i l l i o ns o f m i les . V e r yl i t t le of the corona i s visiblef r o m E a r t h .

    O c c u l t , o c c u l t i n g t h edis appearance o f o n e hea venlybody b e h i n d another.

    N e ar Corona Out 1 Solar Diameter Recorded during Eclipse 21

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    CoronagraphUses V e r y fe w extended studies of the corona have been possiblebecause of the short duration of total eclipse. Seven or eightminutes is the maximum time o f total eclipse in the fewlocalities o f Earth over which totality occurs. As was notedearlier, during eclipse th e occulting disk ( M o o n ) is outside o fEarth's atmosphere and e f fe c t i ve l y eliminates atmosphericscattering. When th e occulting disk is located o n Earth an dinside 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 o n Earth has only 75% of the intensity of thelight shining on the upper atmosphere. Therefore, evenduring an eclipse, much of the faint l ight of the corona islost.

    ScatteringOcculting disc

    Atmosphere

    Limits

    2 2

    Principles of Eclipse Versus OccultingDisk for Corona ObservationThe coronagraph is useful o n Earth-based telescopes toobserve th e chromosphere and solar flares. In this usage, th eluminance o f the chromosphere i s brighter than th e attendantscattering. Also H-alpha light i s prevalent in thechromosphere so that a coronagraph equipped with anH-alpha filter permits observation o f these events. Theconcepts o f H-alpha filters with regard to H-alpha telescopesare discussed in Section 2 o f this volume.DEFINITION OF SCIENTIFIC OBJECTIVESDespite the meager data on corona observations that areavailable, it has been f o u n d that the corona varies withsunspot activity and solar magnetic fields.A coronograph situated above Earth's atmosphere will not bea f f e c t e d by the scattering and attenuation o f t he atmosphere.It will view the corona to the sensitivity limits of thephotographic fi lm that records th e image.

    Edge o f o c c u l t i n g diskF l a r e observationcoronagraph . u s i n g a

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    O v e r the e x t e n d e d t i m e a v a i l a b le d u r i n g S k y l a b , thec o r o n a g r a p h w i l l r e c o r d p h o t o g r a p h s a t v a r y i n g i n t e r v a l s .I nt e r val s as s h o r t as 13 s e c o n d s w i l l p e r m i t a n a l y s i s of rapidm o t i o n o f m a t e r i a l in the c o r o n a .S i n c e the Sun rotates on an ave r age of 28 d a y s , o b s e r v a t i o n so f the c o r o n a w i t h r e l at i o n to solar rotation w i l l also bep r o v i d e d . The c o r o n a l f e a t u r e s associated w i t h s u r f a c ef e a t u r e s o b s e r v e d by H - a l p h a telescopes ( S e c t i o n 2) can bec o r r e l a t e d as the s o l a r f e a t u r e s a p p e a r at one s o l a r l i m b or theother.T h e l i g h t r a d i a t i n g f r o m t he c o r o n a i s p o l a r i z e d ; t h a t i s t o s a yt h e wave s are h i g h l y o r i e nt e d i n a g i ve n pl ane . T h ep o l a r i z a t i o n of the c o r o n a r e s u l t s f r o m i n t e r a c t i o n of thei o n i z e d a t o m s e m a n a t i n g f r o m the Sun and the m a g n e t i cf i e l d s of t h e Sun and so l ar storms. P o l a r i z i n g f i l t e r s i n t h ec o r o n a g r a p h will p e r m i t the p o l a r i z a t i o n of the c o r o n a to bed e t e r m i n e d and its r e l a t i o n s h i p to the s u n s p o t a c t i v i t y andm a g n e t i c f i e l d s that a f f e c t the corona.D E S C R I P T I O N OF C O R O N A G R A P HT h e w h i t e l ig h t c o r o n a g r a p h i s a lo n g tube h a v i n g s e v e r alo c c u l t i n g d i s k s c o a x i a l l y m o u n t e d in i ts l e n g t h . A singleo c c u l t i n g d i s k w i l l h a v e l ig h t d i f f r a c t e d a b o u t itsc i r c u m f e r e n c e . T h e d i f f r a c t i o n o f a si n g le d i sk w o u l d c r e a te ad i f f u s e d l i gh t r i ng a n d p o o r ly d e f i n e d i m a g e at the i magepl ane of the c o r o n a g r a p h . C o n s e q u e n t l y , a d d i t i o n a l d i s k s ,w h i c h are sl ight ly larger than the f i r s t d i sk , are used toi n t e r c e p t the u n d e s i r a b l e d i f f r a c t e d l i g h t .L i g h t d i f f r a c t i o n o v e r a n opaque e dge i s sh o wn i n t h ei l l u s t r a t i o n . W h e n a be am of w h i t e l i g h t i m p i n g e s on anopaque mat e r i a l , l i gh t pass i ng ove r the edge of the mat e r i a l isd i f f r a c t e d . The l ight of the l o n g e r w a v e l e n g t h s ( r e d ) is bentto a greater extent t h an the shorter b l u e w a v e l e n g t h s .D i f f r a c t i o n i s d i sc usse d mo r e f u l l y i n S e c t i o n 4 .W h i t e l i g h t - \ n = A j , X 2 X n

    O p a q u ee d g e

    D i f f u s e ds h a d o wa r e a

    S o l ar r o t a t i o n i s n o t u n i f o r m ;r o t a t i o n is 25 d a y s at thee q u a t o r and 30 d ays at thepoles.S ol ar l i m b t h e e x t r e m e e d ge s o fth e s o l a r d i s k .S u n s p o t

    Po la r i za t i o n a d i s t i n c to r i e n t a t i o n o f t h e w a v e p l a n ea n d t r a v e l o f e l e c t r o m a g n e t i cr a d i a t i o n .

    D i f f r a c t i o n w h e n a r ay o f w h i t el i g h t passes o v e r the ed g e of ano p a q u e s u r f a c e , the c o m p o n e n t sof the l i g h t are bent inp r o p o r t i o n to t h e i r w a v e l e n g t h .W h i t e l i g h t i s d i f f r a c t e d i n t o as p e c t r u m .

    D i f f r a c t i o n of White Light Pass ing Over an Opaque Edge

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    O n e o f t h e occulting disks is adjustable in i ts position alongthe length of the tube and laterally in the tube to providef i n e adjustment of t he occulting. This disk has electronicphotosensors positioned about it so as to detect the edge ofthe shadow of the other disks and provide pointing signals form a n u a l o r automatic operation o f t h e instrument.

    Field o f T h e space o r a n n u l u s f o r m e d by the occulting disks and theV i e w inside wall of t he tube d e f i n e s th e f ie ld o f view of t hecoronagraph. The f ield o f view permits observation of thecorona f r o m 1.5 to 6 solar radii (4 million kilometersplus,the solar radius is almost 700,000 kilometers). Lightf r o m the corona travels down the tube to the f ield lenses andf o l d i n g optics which focus the image of the corona at thei ma ge plane o f t he camera.

    Adjustable disk andp o i n t i n g sensors

    Ma in o c c u l t i n gdisks

    C o r o n aImaging lenses- Bea m sp l i t t e r

    Came ras

    - Polar i z i n gfilter

    Annulus

    Coron ag rap htubeO c c u l t i n gdisk

    F o l d i n g o pt i cs a system o fm i r r o r s that reduces physicall e n g t h o f a telescope wh i lem a i n t a i n i n g optical l e n g t h .

    Optical Scheme o f White Light Coronagraph

    P o la r i z i ngF il te rs

    Came ra

    24

    By using a series o f 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 polar iz ing orientations. High intensity imageso f parts of the corona indicate polarization of the coronallight in the orientation direction of the specific filter used. A ssunspots and solar magnetic f i e lds vary, th e polarization willalso vary.A f t e r the light has passed through the polarizing f i l t e r , 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 f i lm camera which records the images on 35 m i l l i m e t e rf i l m . T he f i lm magazine has a capacity f o r 8025 exposures. Polar i z i n g f i l t e r

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    EXPERIMENTDATAData obtained by the coronagraph will be in the f o r m ofphotographs of the Sun out to 6 solar radii. Thecharacteristics of the expected photographs are shown in theillustrations; picture A was taken f r o m 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.

    B 1970 Eclipse

    The photographs f r o m 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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    An al ys is

    A va i la b i l i ty

    W h e n c o r o n a p h o t o g r a p hs a r e c o r r e l at e d w i t h t h e H-alphaim ages o f th e s o lar s ur face ( Sec t io n 2 ) and th e ul t r avio le ts p e c t r o g r a p hs ( S e c t i o n 4 ) , m u c h o f t he m e c h a n i s m s a n dener gy ch ar acter is t i cs o f s o lar ph enom ona will b e r e v e a l e d .T h e H - a l p h a p h o t o s will s h o w t h e l o c a t i o n o f s o l ar s u r f a c ef e a t u r e s a n d t h e i n t e n s i t y o f s o la r a c t i v i t y . C o r r e s p o n d i n g l y ,c o r o n a g r a p h photos will s h o w t h e m a g n e t i c f i e l d a c t i v i t yt h r o u g h t h e p o l a ri z a t i o n o f t h e s t re a m e r s o f i o n i z e d m a t e r i a lsin th e co r ona . Spe ctr ogr aph s o f co r ona s tr eam er s willi d e n t i f y t h e i o n s c o m p r i s i n g t h e c o r o n a a n d t he e n e r g y l e v e lsnecessary to cause the i o n i z a t i o n . T he s e c o m b i n e d f ac t o r s areth e s our ce o f th e s o lar wi nd th at arr ives o n Ear th and a f f ec tsour a t m o s p h e r e , w e a t h e r , and e n v i r o n m e n t .T h e data f r o m the c o r o n a g r a p h w i l l n o t b e avai lable f o rp u b l i c us e befo r e 1975. Pr oc edur e s fo r r eque s ts fo r co pies o ff l ight data will be a n n o u n c e d at a later date. Section 1des cr ibes in i t ia l data uses and r e q u i r e m e n t s .C R E W A C T I V I T I E ST h e c o r o n a g r a p h will be oper ated fo r s ever a l h our s da i lyd u r i n g S k y l a b m i s s i o n s . S e c t i o n 1 des cr ibes c r ew ac t iv i t i eswith regard to the c o m p l e m e n t of telescopes for solarobs er vat ion .R E L A T E D C U R R I C U L U M T O P IC SThe concepts e m p l o y e d in the design and a p p l i c a t i o n of thec o r o n a g r a p h may be related to a n u m b e r of s ubjec tsdiscussed i n h i g h s c h o o l s c i e n c e p r o g r am s i n c l u d i n gastronomy, ph ys ics , and p h o t o g r a p h y .AstronomyS o la r c o r o n a e n e r g i e s , m o t i o n o f m a t e ri a ls , s o la r w i n d ,var iat ions with solar storms;E c l ip se sc a u se , f r e q u e n c y , re g i o n s o f total ec l ips e .PhysicsO p t i c s d i f f r a c t i o n , l e n s e s , r e f l e c t i o n , m i r r o r s , p o l a r i z a t i o n ,l ig ht s cat ter ing;Gas lawspressure and t e m p e r a t u r e s , t h e r m a l v e l o c i t y ,k ine t ics ;PhotographyF i l m r e c i p r o c i t y and s e n s i t i vi t y , p h o t o g r a p hi c r e s o l u t i o n ;M a t h e m a t i c sT r i g o n o m e t r y a n g l e s a n d s iz e s o f a p p ar e n t M o o n , S u n , a ndo c c u l t i n g d i s k s .

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    S UG G E ST ED C LA SS RO O M D E M O N S T R A T IO N S

    A d e m o n s t r a t i o n of the use of o c c u l t i n g d i s k s may beprovided in the fo l l owi ng m a n n e r :1) Prepare a slide fo r 35mm o r o v e r h e a d p r o j e c t o r . The slide

    will have a circle in its c e n t e r and may have fa i n t areasaround th e c i rc le , to simulate the Sun and c o r o n a .2 ) In a d a r k e n e d r o o m , p r o j e c t th e slide so that a 6-inchdiameter c i rc le i s shown o n the scree n .3 ) U s i n g a small d isk o f 5/8-inch d iameter locateda p p r o x i m a t e l y 3 f e e t f r o m th e e y e , d e m o n s t r a t e that th esmall o cc ult ing disk w ill hide the i l lu mi nate d circ le , butst i l l permit viewing of the areas surrounding i t.4) A n ight time dem onstration us ing the full M o o n m ay b ep e r f o r m e d . L o c a t i n g th e occul t ing d isk so that th e fullM o o n is o c c u l t e d (a ) measure th e distance f r o m the eyeto the disk; (b) calculate th e angle subtende d by the diskand the e y e ; and (c) using th e calculated angle and adistance o f 250,000 miles, calculate th e approximated i a m e t e r o f t h e M o o n .

    P r o j e c t e dc i r c l eO c c u l t i n g"d i sk

    6 i n c h e s

    W A R N I N G : D O N O T A T T E M P T O C C U L T I N GDEMONSTRATIONS USING TH E SUN. EYE D A M A G EM A Y RESULT.This demonstrat ion i l lustrates th e d i f f r a c t i o n o f white l ightpassing ove r a sharp opaque edge.1) Use a laboratory grade lens to focus sunl ight o n a whites urface . The Sun should be f o c u s e d to a small circle .2 ) Insert the edge of a kni fe or razor b lade in to the focusedbeam of sunl ight so that some sunl ight falls on the kni feand some passes beyond.3) Inspect ion of the shadow of the k n i f e will s h o w that th eshadow is di f fus ed and is not as sharp as the blade edge.4) The di f fus ed shadow will result f r o m di f f ra c t i on o f l ightover the k n i f e edge .5) This demonstrat ion may be e x t e n d e d to a discussion o fi t s appl icat ion in determ in ing the qual i ty o f parabol ic orspher ical co ncave mirrors for use in te lescopes . The test isk n o w n as the F o u c a u l t k n i f e edge test.

    S u n l i g h t

    Focused l i g h tShadow'

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    Magnetic Field T he direction in which an i on i s moving will be affected byEffects on the presence o f a magnetic f i e l d . T he trajectory o f t h e i o nIon Trajectories wjjj be aiterecj jn a direction perpendicular to the magneticf i e l d . This demonstration shows how the variations of thesolar magnetic f i e lds a f f e c t the trajectory of ions in thecorona to cause the streamers and f i l a m e n t s that areobserved. The magnetic f ie lds act to either concentrate theions or to disperse them, depending on the relationshipbetween the ion motion and magnetic f i e ld orientation. Aconcentration of th e ions produces an increase o f 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 6i n c h e s 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; note

    the change in light intensity near the magnet as the iontrajectories are altered.5) Alter the position and orientation of the magnet; observe

    that the light changes with changes in the magnetic f i e l d .

    T r a n s f o r m e r

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    Section 4Extreme UltravioletSpectrographExtreme UltravioletSpectroheliograph

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    E X P E R I M E N T B A C K G R O U N DThe 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.

    O p t i c s T h e 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.W h i t e 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 knowna s ultraviolet (UV), extreme ultraviolet ( X U V ) , a n d 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 thegr at i ng . It may further be shown that because of the spacingo f 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 thed i f f r a c t e d light to a well defined spectrum.To circumvent the d i f f i c u l t y 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. Variationso f the blaze angles permit the reflective grating to bedesigned for greatest e f f i c i e n c y at specific spectral regions;thus the blaze for ultraviolet is different f r o m the blaze forred visible light. Even with a f la t reflecting grating a lens isstill required to focus the diffracted light into a spectrum.A concave spherical mirror will focus radiation falling on it tof o c a l point. If a diffraction grating is ruled on a sphericalmirror, the diffracted light will also arrive at a focal point.

    Extreme ultraviolet i s generallyconsidered to be the region ofthe electromagentic spectrumbetween 150 and 700 angstroms.S p e c t r o g r a p h A system f o rr es o l v in g electromagnetic energyinto its component wavelengths.S p e c t r o h e l i o g r a p h A picture o fthe Sun in a par t icu la rw a ve l e ngt h of the spectrum or ad e v i c e f o r p r o d u c i n gspectroheliograph

    W h i t e L i g h t

    Red

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    E x p e r i e n c e

    Spectrograph

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    H o w e v e r , th e foc a l p o i n t f o r each wavelength is d i f f e r e n tbecause of the blaze of the grating. The spher ica l concavegrat ing produced e l iminates th e r e q u i r e m e n t f o r a f o c u s i n glens.Spec trographs have been in use in a s t r o n o m y fo r many years.T he y are c o m m o n l y u se d to analyze th e visible l ight f r o m th eSun and stars. M o s t o f o u r current knowledge of thec o m p o s i t i o n o f heavenly bodies has been der ived f r o mspectrographic analysis o f the ir l ight . H ow eve r , radia tio ns atshorter wavelengths than visible l ight are absorbed in Earth'sa tmosphere and are not available to grou nd based telescopes.The so lar chromosphere and l o w e r c o r o n a are m u c h hotterthan the sur face o f the photosphere , w h i c h is character izedby the white l ight i t em its. To observe these ho t te r regions o fthe so lar a tmosphere , one must observe in the ul travio le t o reven x-ray spectral regions. Because these radiations areabsorbed in Earth's atmosphere , spacecraft or satell i tes arenecessary to transport the instruments away f r o m Earth, toth e regions o f space where this radiation is available .Short term observations in these spectral regions have b e e np e r f o r m e d f r o m balloons and s o u n d i n g r o c k e t s and orbi t ingsolar obse rvatory (O SO ) satell ites. H ow eve r, these data arel i m i t e d to small spectral sections.Figure 1 s ho w s the various layers of Earth 's a tmosphere andthe types of solar radiation that are absorbed in therespective layers.

    Stratosphere16 to 72 kmM N W M M H M B & 'Troposphe0 to 16 km

    W i t h a c k n o w l e d g e m e n t t o t h e N a t i o n a l G e o g r a p hi c So c i e t yFigure 1 Atmospheric Absorption of Radiation

    DEFINITION OF SCIENTIFIC OBJECTIVEST he extreme ul travio le t spec trograph will obtain detailed l inespectra of small selected areas of the Sun and across the l imbsin two wavelength bands: 970 to 1970 angstroms or 1940 to3940 angstroms.

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    T he e x t re m e u l t ra v i o l e t s p e c t ro h e l io g r a p h will p h o t o g r a p ht he sol ar c hro mo sphe re i n e x t re m e u lt ra v i ol e t wa ve l e ngt hsb e t w e e n 150 and 625 a ngst roms.Be c a use the e xt re me u l t ra v i ol e t r e g i on of the spe c t ra doe snot pe ne t rat e Ea r t h ' s a t mo sphe re , obse rva t i ons ha ve be e nqui t e l i mi t e d i n t h i s r e g i o n . Dat a obt a i ne d by t he se S kyl a bt e l e s c o p e s w i l l p r o v i d e i n f o r m a t i o n o n t h e m a t e r i a lc o m p o s i t o n of the c h r o m o s p h e r e and the e n e r g ycharacter ist ics of t he S un. T he se p i e c e s of t he solar p u z z l e , int u r n , will p r o v i d e c l u e s to the m e c h a n i s m s by w h i c h solarflares occur.D E S C R I P T I O N O F E X P E R I M E N T H A R D W A R EIn a spectrograph, l ight is admit ted to the d i f f r a c t i o n grat ingt h r o u g h a na rrow e nt ra nc e s l i t . The entrance sl i t of thee xt re me u l t ra v i ol e t spe c t rogra ph is 10 m i c r o n s w i d e and 300mi c rons l ong (0 . 00 04 i n . wi de x 0 .012 i n . h i gh ) .The l ight admit ted to the grating is an i ma ge of the slit . T hed i f f r a c t i o n gra t i ng t he n re f l e c t s the di f f ra c t e d i ma ge to thef i lm pl a ne . S i nc e th e d i f f r a c t i o n grating is a spe r i c a l r e f l e c t or ,t he d i f f ra c t e d s li t i mage i s fo c use d a t t he f i lm pl a ne .

    Path Fi gure 2 de p i c t s t he c o mpl e t e opt i c a l syst e m of t he e xt re meul t ra v i ol e t spe c t rogra ph. Li ght enters the spectrographt hrough a n a pe r t ure a nd falls on t he pr i ma ry sphe r i c a l mi r ror .

    P r i m a r ys p h e r i c a l m i r r o r

    P r e d i s p e r s e rgr a t i ng( I o f 2 )

    F i l m p l a n es p e c t r u m

    F igur e 2 O ptical System of the E x t r e m e U l t r a v io l e t Spe c t r o gr a ph

    M i c r o n = 0.000001 m e t e r= 0.00004 i n c h e s

    E n t r a n c eSli t Spe c t r aI m ag e

    D i f f r a c t i o nG r a t i n g

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