photometric calibration of the euv spectroheliometer on atm

Photometric calibration of the EUV spectroheliometer on ATM

E. M. Reeves, J. G. Timothy, M. C. E. Huber, and G. L. Withbroe

This paper describes the derivation of the preflight photometric calibration of the uv spectrometer on Sky-lab. The calibration of the orbiting instrument through cross-comparison with two rocket instruments isdiscussed in assessing the observed changes in response to quiet solar regions during the mission. Formulasare presented for the determination of the instrument sensitivity, and an uncertainty of +35% is assignedover most of the 296-1340-A wavelength range.

1. Introduction

The EUV spectrometer-spectroheliometer on theApollo Telescope Mount (ATM), which has been de-scribed in detail in the preceding paper (paper 1), op-erated during the entire Skylab mission from May 1973through January 1974 and was characterized by almostperfect performance. The value of the scientific dataanalysis program which has been in progress since theend of formal mission operations depends critically ona knowledge of the photometric efficiency necessary toconvert the telemetered instrument count rate into in-tensity on the sun. The inclusion of the ATM instru-mentation in the complex Skylab program requiredcompletion of the final laboratory measurements of theHCO instrument efficiency approximately 21/2 yearsprior to launch. One final test of the instrument sen-sitivity was conducted in a limited way during the ATMthermal vacuum tests at the Johnson Spaceflight Centerin September 1972, 9 months before launch.

Although extensive efforts were devoted to assuringreliable knowledge of the photometric calibration priorto launch through the use of monitor mirrors and con-trol of contamination, experience with smaller versionsof EUV instrumentation on Orbiting Solar Observa-toriesl 2had shown that serious changes in photometriccalibration could be experienced in orbit. Since nospace qualified and reliable standard sources of inten-sity were available covering the sensitive range of theATM instrumentation, a calibration rocket program(Calroc) was used to intercompare the response of theATM instrument to a quiet region of the solar disk withthat from a smaller calibration rocket instrument,

When this work was done all the authors were with Center for As-trophysics, Harvard College Observatory and Smithsonian Astro-physical Observatory, Cambridge, Massachusetts 02138; M. C. E.Huber is now with Federal Institute of Technology, Atomic Physicsand Astrophysics Group, CH-8092 Zurich, Switzerland.

Received 16 August 1976.

viewing the same solar region. The Calroc instru-mentation was launched from White Sands, NewMexico in an evacuated nosecone, using a Black BrantV rocket, and was carefully calibrated both before andimmediately following a flight. The Calroc instrumentsobserved the same quiet solar area, several arc minutesin extent, either simultaneously or within a few hoursof astronaut observations from Skylab. The transferregion on the solar disk was chosen to be free of activeregions and filamentary structure, on the basis of pro-jected observations from ground based Ha and Ca IIspectroheliograms, and moreover was selected to be freeof coronal holes as determined from the data from theEUV video heliograph of the Naval Research Labora-tory.

Previous OSO instrumentation in this wavelengthrange relied on the assumed temporal stability of theEUV radiation from quiet solar regions to determinechanges in instrument response with time in orbit. Inthe ATM program the results of the laboratory cali-bration and the observations from two Calroc flights on9 August and 10 December 1973 were combined withdata obtained from daily monitoring of quiet solar re-gions at various wavelengths to ascertain the photo-metric calibration of the ATM instrument for the 9-month period in orbit. These same Calroc observationshave been used by Timothy3 4 to infer significantchanges in the emission of quiet solar regions over longperiods of time. This latter conclusion has simulta-neously led to a reevaluation of the concept of the pre-viously assumed stability of the solar emission in theEUV and to the justification of the need for periodicchecks on the stability of solar EUV instrumentationin space through calibration rocket instruments orsimilar means. Quiet solar regions may well serve therole of a transfer medium over short periods of time, butcurrent evidence points to the inadequacy of the quietsun as an absolute standard in the EUV spectral re-gion.

April 1977 / Vol. 16, No. 4 / APPLIED OPTICS 849

CALIBRATION ROCKET

NORTH RASTER

EAST

SOUTH

C III 977.0 A

i3T0

X 0.1. _

E Of

0.40

w O.ev)z0a:

Ld 0.;

i

-4IdOf DISTANCE (arc sec)

Fig. 1. (a) The relative positions of the calibration rocket instrumentis shown for the flight of 10 December 1973 against the backgroundof the ATM instrument spectroheliogram in C III 977 A. (b) Therelative response of the two instruments across the fields of view in-

dicated above.

In this paper we will primarily address the results ofthe calibration activities and the methods of estab-lishing the photometric response of the ATM instru-mentation in orbit. The procedures employed forlaboratory calibration and functional testing have beendescribed in detail elsewhere. 5 Likewise, a detaileddescription of the Calroc instrumentation is alsoavailable.6'7 Only an abbreviated summary of the in-strumentation and methods of calibration will be givenhere. We do present what we consider to be the firstintermediate photometric calibration of the HCO in-strumentation. Further refinements to the calibrationcan only be derived in the course of scientific dataanalysis, which is still in progress. Any revisions ormodifications to this proposed calibration will be madeavailable on request.

II. Instrumentation

The Calroc spectroheliometer is designed to measurethe emission of radiation in the 296-1340-A wavelengthrange with a spectral resolution of 1.6 A. The rocketinstrument was used to measure the absolute emissionfrom a 4 X 4-(arc min)2 area of the solar disk that laycompletely within the 5 X 5-(arc min)2 field of view ofthe ATM spectrometer. The upper portion of Fig. 1shows the relative positions of the fields of view for therocket and ATM instruments on a quiet region of thesolar disk during the Calroc flight of 10 December 1973.The lower portion of Fig. 1 shows the relative responseof the two instruments across the field where intensitiesof the ATM have been averaged over the same 20-secof arc by 4-min of arc areas as observed by the Calrocinstrument.

The optical schematic of the Calroc spectrohel-iometer is shown in Fig. 2, and the operating charac-teristics of the payload are summarized in Table I. Thespectroheliometer consisted of three main subsystems,namely, the telescope, the spectrometer, and thepointing reference camera system. The iridium-coatedCer-Vit telescope mirror forms an image of the sun onthe entrance slit plate of the spectrometer, a portion ofthis image being admitted through the entrance slit intothe spectrometer. The entrance slit plate is highlypolished and reflects the visible light image of the solardisk through a relay lens, a bandpass filter, and a neutraldensity filter, to the film plane of the pointing referencecamera system. The telescope mirror is an off-axisparaboloid having a focal length of 90 cm and a collec-tion area of 51 cm2 and is mounted in a cell which canbe driven about its vertical axis to provide a 1-D scanof the solar image. The entrance slit of the spectrom-eter accepts radiation from an area of the solar disk 4min of arc by 20 sec of arc in extent, a 4 X 4-(arc min) 2

area being mapped with twelve steps of the mirror scan.The mirror cell is driven by a stepper motor and cam,the step motion being initiated by a signal from a mi-croswitch on the grating drive cam. Spectral and spa-tial scans are thus synchronized with the mirror step-ping while the grating retraces at the end of each spec-tral scan. An optical reference of the grating positionis obtained by means of a light emitting diode andphototransistor which detects the long wavelength ra-diation from the emitting diode at a specific gratingangle. The optical reference signal provides a preciseindex of the grating position without recourse to anexternal light source.

The spectroheliometer was mounted in the body ofthe rocket inside a vacuum enclosure with the sun endfacing downward during launch sequence. Forward ofthis enclosure and separated by a vacuum bulkheadwere the telemetry system and the solar pointing controlunit (SPARCS). The SPARCS control jets, togetherwith the payload recovery system, were mounted underan ogive nose cone assembly. The coarse solar acqui-sition sensors were mounted in the skin of the SPARCSunit and also on the heat shield assembly at the frontof the spectroheliometer. The fine sun sensor (FSS)was mounted on the front face of the spectroheliometer

850 APPLIED OPTICS / Vol. 16, No. 4 / April 1977

He 1 584.3 A

SLIT) WIDTH

…

- ATM (21:56 GMT)___ CALIBRATION ROCKET (19:15GMT)

0I 50 0 5 2 2 30 50 100 150 200 250 3(

Fig. 2. Optical schematic of thecalibration rocket spectro-

heliometer.

TELESCOPE Il m , LENTRANCEAPERTURE ANNEL

CONCAVE DIFFRACTION ELECTRON MULTIPLIERSGRATI NG

adjacent to the telescope entrance aperture in order tominimize the effects of vibration and heating during thelaunch sequence.

The complete instrument was mounted on three padson a ring section in the rocket skin, the mounting beingdesigned to minimize the conduction of heat from theskin during the ascent stage of the flight. Electroniccircuits necessary to operate the instrument and formatthe output data, together with the low voltage powersupplies, were mounted in boxes on the center sectionof the spectroheliometer. The high voltage powersupplies for the channel electron multipliers and theelectronics associated with a cold cathode ionizationgauge, used to monitor the pressure in the instrument,were mounted on the spectrometer and remained withit during the calibration tests. The power and telem-etry interface electronics were mounted on the rocketskin adjacent to the pressure bulkhead leading to thetelemetry bay.

Since the open structure channel electron multiplierscannot pump down to safe operating pressure in the 100sec between launch and high voltage turnon, the in-strument section of the payload was evacuated to apressure of 5 X 10-6 Torr prior to launch by means ofa cryogenic pumping system installed in the launchtower. An identical cyrogenic pumping system wasused to evacuate the payload during payload integrationtests in the laboratory and the vehicle assembly build-ing.

During the launch sequence the door in the rocketskin was closed, and the pump was removed about 50sec before launch. The instrument section was thusisolated under vacuum during the launch sequence untilthe time of motor separation 60 sec after launch. Atthis time the vacuum bulkhead at the rear end of theinstrument section was removed with the motor and theignitor housing, exposing the spectroheliometer to thehigh vacuum of the upper atmosphere. The instrumentwas then allowed to pump out for a further 50 sec beforehigh voltage was applied 110 sec after launch. During

Table I. Operating Characteristicsof Calibration Rocket Payload

(1) Launch vehicleRocket motor: Black Brant VC (Bristol Aerospace

Company, Winnipeg, Canada)Solar pointing control unit: SPARCS V (Lockheed

Missiles and Space Company, Sunnyvale, California)Payload length: 356.6 cm (140.4 in.)Payload diameter: 43.2 cm (17.0 in.)Payload weight: 225.5 kg (496.0 lb)Maximum altitude: 271 kmTotal observing time above 150 km: 325 sec

(2) EUV spectroheliometer(a) Telescope

Mirror:Figure: Off-axis paraboloidFocal length: 90 cmFocal ratio: f/12.5Collection area: 51 cm2

Blank material: Cer-VitCoating: Iridium

Spatial resolution: 20" X 4' (defined by spectro-meter entrance slit)

Spatial field: 4' X 4'(b) Spectrometer

Grating:Figure: SphericalRadius of curvature: 50 cmRuled area: 4 cm x 4 cm squareRuling frequency: 1800 l/mm'Coating: GoldMounting: Johnson-Onaka (modified)

Spectral range: 296-1340 ASpectral resolution: 1.6 A (FWHM)Detectors:

Detector 1: Uncoated two stage channel electronmultiplier (1340-786 A)

Detector 2: MgF-coated two stage channel electrmultiplier (850-296 A)

Detector 3: Nitric oxide ionization chamber wittMgF. window (H Ly 1215.7 A)

on

April 1977 / Vol. 16, No. 4 / APPLIED OPTICS 851

this 50-sec period the SPARCS control unit turned thepayload through approximately 180° to point thespectroheliometer at the selected region on the solardisk. Spatial and spectral scans of this region were thentaken until just before reentry. High voltage was thenturned off, and the viewing apertures of the instrumentand the FSS were covered by a shutter for protectionduring reentry and for isolation from the dust and sandof the desert ground environment prior to recovery bythe ground support team.

Ill. Photometric Laboratory Calibration

Both instruments of interest here, the ATM flightinstrument, as well as the calibration rocket instrument,were spectroheliometers, that is, they were designed todetermine the absolute intensity of a given area of thesolar disk over narrow wavelength intervals. Thus,similar procedures for laboratory calibration were em-ployed for both instruments.

The following relations and definitions were used todescribe the calibration. The output count rate, N(X)(counts sec'), arising from observations in the 1.6-Ainstrument bandpass of detector 1 of our photoelectricspectroheliometer, which is set to measure the solarintensity, 4al (in units of photons cm-2 sec 1 sr-1) in the1.6-A bandpass, from a given area on the sun inthe wavelength interval X to X2 can be expressed as

a ('\2 (X 2N(X) = A .2 R,\Ex\FxdX K J K^F\dX, (1)

where A (cm2) is the area of the telescope mirror. Theratio a/f 2 (sr) is the angular field of view, which dependson the slit area a (cm 2 ) and on the telescope focal lengthf (cm). R is the reflectance of the telescope mirror atwavelength X, and ES (counts/photon) represents theover-all spectrometer efficiency at wavelength X. Thespectrometer efficiency can further be expressed as &= A-f, where A, is the diffraction efficiency of thegrating at wavelength A, and E, (counts/photon) is thedetection efficiency of the photomultiplier at the samewavelength. Finally, the detection efficiency can beexpressed as (A = yy\-P, that is, the product of the pho-toelectric yield yx (photoelectrons/photon) and of theprobability P (counts/photoelectron) that a photo-electron produces an output pulse greater than thethreshold and is, in fact, recorded by the counter.Within the wavelength range of interest, i.e., between300 A and 1340 A, P can reasonably be expected to beindependent of wavelength. At high count rates,however, when pulse pileup may prevent the counterfrom recording each single pulse, P becomes a functionof count rate N(X); a linearity correction then has to beapplied (see paper 1). Since the instrument bandpass(1.6 A) exceeds the true spectral profile of all lines (ex-cept Ly-a) in the wavelength range, the measurementof N(X) provides the total intensity in the line, and themeasurement is then independent of the exit slit width.For continua and any residual scattered light the re-corded count rate N(X) varies directly as the instrumentbandpass and must be accounted for in the reducedvalues of the specific intensity (ergs cm-2 sec 1 sr-A-1).

For both instruments, we determined the over-allinstrument sensitivity by separate measurements of thegeometrical parameters A, a, and f; measurement of thereflectance of the telescope mirror Rx; and determina-tion of the over-all spectrometer efficiency EA. Thegeometrical measurements are straightforward, and thedetermination of the telescope mirror reflectance is, inprinciple, a simple task, since only relative intensitiesmust be measured. Nevertheless, care had to be takento minimize effects of nonuniformity of beams andphotocathodes. The most difficult part of the cali-bration was the determination of the over-all spec-trometer efficiency E,\. This parameter had' to be de-termined in absolute energy units, using a calibratedstandard detector, because standard light sources ofsufficiently high focal ratio were not available.

Figure 3 shows the experimental arrangement forcalibrating the main ATM instrument, the experimentalarrangement for the rocket instrument being verysimilar. Radiation from a stable EUV light source,usually a hollow cathode or a microwave discharge in arare gas, entered the calibration tank through a slit andilluminated the concave grating of the laboratory mo-nochromator. This monochromator grating formed areduced image of the tank slit on the spectrometer slit.The wavelength and order of the diffracted image and,consequently, of the radiation passing through thespectrometer entrance slit, could be chosen by settingthe monochromator grating at the appropriate angle ofincidence. In this way it was possible to isolate adjacentemission lines of the laboratory light sources. Thestandard detector, when placed behind the spectrom-eter entrance slit (cf. Fig. 3), intersected a monochro-matic beam and recorded the absolute flux entering thespectrometer being calibrated.

The spectrometer count rate was measured when thestandard detector was moved out of the light path.Data on the spectrometer efficiency for all detectorsand, in several grating orders, on the instrument lineprofiles, and on scattered light were all obtained duringrepeated spectrometer wavelength scans.

The spectral responses of the standard detectors wereestablished by comparison with primary standard de-tectors and sources, whose calibration in turn wastraceable to the National Bureau of Standards. Apedigree of the standards that shows the calibrationpaths to the ATM flight spectrometer was published inFig. 5 of Huber et al. 5 The main transfer standard usedin our spectrometer calibrations was an open tungstenphotodiode, but for weak lines, we also used an openBendix cone channel electron multiplier, which hadbeen calibrated against the tungsten diode. Thecathode nonuniformity of this latter detector was high(cf. Ref. 8), so that the data obtained with it were lessreliable than those obtained with the tungsten photo-diode (see Fig. 8 of Ref. 5).

The planes of dispersion of the laboratory mono-chromator grating and the flight spectrometer gratingwere mounted at right angles. This precaution wasnecessary because the diffraction efficiency of a blazedconcave grating varies significantly over its surface.Since the change in grating efficiency along the rulings

852 APPLIED OPTICS / Vol. 16, No. 4 / April 1977

LIGHT SOURCE

ENTRANCE SLIT OF TANK(600 x 300Fm2 )

FOLDING MIRROR FORZERO ORDER SETTINGS

WHITE LIGHT -SOURCE t

MONOCHROMATOR GRATING(600 i/mm)

ENTRANCE SLIT(56 X 56im 2 )

SPECTROMETER EXIT SLITS(140,360 or 750 x 1500m 2)

Fig. 3. Laboratory arrangement for spectrometer calibration.

1 I I I I 1 1 I r I - IQUIET SUN RASTERS

5 5 (arc min)2 AREA OF DISK, AVERAGE COUNT MOST PROBABLE COUNT

C 11 1335.7 A(DETECTOR I)

O VI 1031.9AI DETECTOR 3 )

Mg x 625.3A(DETECTOR 6)

200 250 300 3501973+1974

H Ly a 1215.7 A(DETECTOR 2)

600

400

200

600

400O

200

140

100

60

400

C III 977.0 A(DETECTOR 4)

OIV 554A(DETECTOR 7)

------- - - -200 250 300 350

1973-j-1974400

DECIMAL DAY

Fig. 4. Variation of the average count and the most probable count in quiet sun spectroheliograms from the ATM instrument during theSkylab mission.

April 1977 / Vol. 16, No. 4 / APPLIED OPTICS 853

1401-

100 -

60

20

300

I-z0U-

200

,100

40

30

20 .~~~ ~ I I _ , I_

I I , . . . . .I . . .I .- .- ,

Z 20 , 9 AUG I0DEC _

.0 6 II 35 AUi 02 i

<r 0.2 ' '1j.jI

J .0 6' - 13 2A

-0.6

X, ,, I I ,

20

0O.6Z 2.0 _ -6 5 i_

E 1.0 Mg 55 A~0.6

152 221 344 -

200 300 135 45DAY of YEAR

Fig. 5. Comparison of the apparent relative calibration changesobserved in orbit and the results of the preflight and rocket

measurements.

is much smaller than that across the ruled width of thegrating, the effects of the nonuniformity could be re-duced by crossing the directions of the rulings. In thecase of the calibration rocket spectrometers an addi-tional precaution improved the reliability of the results:the surface area of the monochromator grating wasmasked so that efficiency variations across the surfaceof the flight grating could be mapped and then aver-aged.

In order to check the consistency of the calibrationof the ATM flight spectrometer under the conditionsof unif~orm illumination, an iridium-coated mirror wasmounted in place of the monochromator grating. Thespectral intensities of the undispersed radiation en-tering the spectrometer could then be determined withthe aid of the calibration data from a wavelength scanof the spectrometer and could be compared with thephotodiode current. Lines whose wavelengths did notfall within the range of the spectrometer could usuallybe neglected since the sensitivity of the tungsten pho-todiode which was used as standard detector droppedrapidly above the long wavelength end, and the reflec-tance of the mirror at near normal incidence becamevery small below the short wavelength limit. In the caseof a microwave discharge, the spectrum usually consistsof a very strong resonance line (e.g., the 584- line in ahelium discharge) and a few other lines of much smallerintensity. Therefore the current in the photodiode wasthat produced by the strong resonance lines-apartfrom small corrections which could be calculated. Inthis way, it was possible, at least at a few wavelengths,to check the spectrometer with a uniform beam. Theresults agree within about ±t10% with those obtainedwith the normal calibration setup, i.e., with the mono-chromator grating in place of the mirror.

In the ATM instrument, a photocell, which sensed thevisible sunlight reflected by the grating in zero order,was used to generate an optical reference signal. Thissignal was intended to stop the spectrometer grating-drive at the step number corresponding to the firstpolychromatic position. In order to facilitate themeasurement of the relative location of optical referenceand first polychromatic position, we used a movablefolding mirror mounted inside the calibration tank (cf.Fig. 3) which could be inserted or removed from theoptical axis and thus permitted us to change conve-niently the spectrometer illumination from EUV towhite light or vice versa.

Optical reference for the ATM instrument could thusbe set to coincide with the spectrometer grating stepnumber at which the C II 1335.7-A line was incident ondetector 1. The C II line could easily be generated,when CO2 gas was admitted to the hollow cathode lamp.The position of the other detectors relative to opticalreference had to be determined indirectly, because mostof the lines which they were designed to detect belongedto stages of ionization that could not easily be generatedin a steady laboratory light source. The correct locationof each of the seven flight detectors was verified fromwavelength scans of the spectrometer, with all detectorsturned on, and with the aid of combinations of rare gasesin the EUV light sources.

Following the final laboratory calibration of the ATMinstrument, all calibration checks had to be made withundispersed radiation, since the spectrometer was thenmounted in the instrument, and it became impossibleto use a monochromator to illuminate the spectrometersection. For calibration checks and for functional testsof the entire instrument which required EUV illumi-nation, a separate Cassegrainian feed system was used.The parallel light emerging from the collimated testsource could be used to illuminate the telescope mirror.A detailed description of this illumination system wasgiven by Huber et al. 5 In short, it included two lightsources (an open hollow cathode lamp and a sealedkrypton discharge tube with a MgF2 window), a filterwheel, and standard detectors which could be used bothto measure the total flux emerging from the Casse-grainian system and also to map the beam. The spec-trum of the krypton lamp consisted of the two Kr I linesat 1164.9 A and 1235.8 A. With the aid of LiF, MgF2 ,and CaF2 filters and with the standard detectors thephotometric instrument sensitivity could be determinedat these two wavelengths with an uncertainty of 20%.During the 21/2 years that elapsed between final labo-ratory calibration and launch, these measurements re-vealed no changes in the calibration.

IV. Instrument Calibration and its Behavior in Flight

For use with solar observations, it is desirable tomodify the definition of instrument sensitivity slightlyfrom that given in the context of the laboratory cali-bration. In particular, to indicate solar intensities weuse units of energy rather than photons and describe theinstrument response as count accumulated during a

854 APPLIED OPTICS / Vol. 16, No. 4 / April 1977

Table II. Instrument Efficienciesfor Detectors 2 to 7, Relative to Detector 1

Detector rXD

2 0.0443 1.144 1.035 1.086 0.987 1.02

400 600 800 1000 1200 1400WAVELENGTH ()

Fig. 6. Prelaunch instrument efficiency k xl, for detector 1.

Table Ill. Over-all Instrument

data gate time n(X) rather than as count rate N(X).Accordingly, for the individual detectors we rewrite Eq.(1) as

IXD Sx (t)F(t) D()kxD () (la)

IXD = solar intensity measured by detector D withan appropriate instrument bandpass AX(ergs cm-2 sec-1 sr-' AX-)

nD(X) = count accumulated during one data gatetime (counts At-1) from detector D,

At = gate time, 0.041 sec,k D = over-all instrument efficiency for detector D

[(counts At-) (erg-' cm2 sr sec AX)],S\(t) = a function of time and wavelength relating

the calibration to 977 A near the midpoint ofthe wavelength range,

F(t) = a function relating the instrument responseat 977 A as a function of time.

Since the over-all instrument sensitivity is similar forthe seven EUV detectors, it is convenient to give theefficiency for any given detector k D as a product

k\ ) = hx\ r ), (lb)where rD is the over-all instrument efficiency forspectral intensities measured by detector D relative tothat measured by detector 1 (see Table II). Thewavelength bandpass is different for the various de-tectors (see Paper 1) and must be taken into consider-ation in measuring continua or in removing residualcontinua from isolated emission lines. The values ofk\ for first and second grating orders are plotted in Fig.6, and the corresponding numerical data are given inTable III.

The relative or interdetector efficiencies for the in-strument in orbit were determined in a special series oftests where the grating was scanned with all detectorsturned on (cf. paper 1). These scans were usuallyconducted under astronaut control with the instrumentpointed at fairly intense regions in order to minimizestatistical inaccuracy in the comparison. This orbitalinterdetector calibration was particularly importantbecause detectors 2, 3, 4, and 7 were exchanged in theflight instrument after final prelaunch spectrometer

Efficiency for Detector 1, k1,given in (counts erg- cm2 sr sec t-') as Measured During Final Laboratory Calibrationa

First grating order Second grating orderx(A ) kX1 X(A ) kX1 X( ) kX1

300 5.04 (-5)a 850 6.30 (-1) 300 2.06 (-4)350 1.26 (-3) 900 6.36 (-1) 350 3.54 (-3)400 1.07 (-2) 950 5.94 (-1) 400 1.98 (-2)450 5.65 (-2) 1000 5.16 (-1) 450 6.05 (-2)500 2.23 (-1) 1050 4.13 (-1) 500 1.43 (-1)550 5.07 (-1) 1100 3.22 (-1) 550 2.33 (-1)600 6.58 (-1) 1150 2.50 (-1) 600 1.64 (-1)650 5.38 (-1) 1200 1.93 (-1) 650 7.78 (-1)700 4.02 (-1) 1250 1.42 (-1) 700 2.95 (-2)750 4.36 (-1) 1300 7.93 (-2)800 5.56 (-1) 1350 4.22 (-2)

a Numbers in parentheses indicate power of ten.

April 1977 / Vol. 16, No. 4 / APPLIED OPTICS 855

calibration had been completed. Hence the values ofrx\D had to be determined in orbit. The data fromtwenty-eight scans through the mission showed that therelative interdetector efficiency rD was independentof wavelength and time within an uncertainty of ±10%.Table II indicates the efficiencies for the various de-tectors relative to detector 1 averaged over the mission.The relative efficiency (rx2 ) for detector 2 was deter-mined to be 0.044, i.e., to the specified transmission ofthe attenuator.

In contrast to earlier EUV satellite instruments thatemployed normal incidence optics,",2 the ATM instru-ment did not show large and fluctuating efficiencyvariations with time in orbit. However, we did observea decrease in signal level with time when monitoring theintensity of the quiet sun. Figure 4 shows the averagecount level in 5 X 5-(arc min) 2 spectroheliogram for theseven detectors at the principal polychromatic positionas a function of time during the mission and also themost probable count in the quiet sun spectroheliograms(as derived from histograms). The latter correspondsquite closely to the intensity of the centers of super-granulation cells9 for lines formed in the chromosphereand transition region. Rasters in quiet regions neardisk center were obtained routinely throughout themission for the purpose of establishing trends in thephotometric efficiency of the instrument. The datashown have been selected to exclude active regions,filaments, and coronal holes. The curves are third orderpolynomial fits to the data points. The scatter of datapoints around the curves was less than ±10% (except forthe coronal line, Mg x 625 A, where it was ±15%).

Figure 5 shows the combined results of the laboratorycalibration, the two calibration rocket flights, and thedaily monitoring of the intensity of the quiet sun withthe HCO instrument. The data points drawn as opencircles represent the absolute intensity of the quiet sunin five spectral lines as measured on three dates: 1 June(Day 152); 9 August (Day 221); and 10 December (Day344) 1973. The first date marks the beginning of themission, when the absolute intensity was determinedby use of the preflight calibration, the latter two datesare those of the two successful calibration rocket flights.The solid curves of Fig. 5 represent the average signallevel that was obtained from daily monitoring of thequiet solar atmosphere near disk center from Fig. 4, butnormalized to unity at day of year (DOY) 221. (Notethat Fig. 5 is on a logarithmic scale while Fig. 4 is on alinear scale.)

The data in Fig. 5 described so far are thus of twoquite different types: absolute intensities of the quietsun from the calibration rocket flights (open circles) andthe average change in the count rate measured fromquiet solar regions, normalized to DOY 221. The fac-tors normalizing the three absolute calibrations (opencircles) for each wavelength were chosen for a best fitbetween data points and curves. In addition, minorcorrections had to be applied to the measured absoluteintensities so that they referred to the same quiet solaratmosphere as the count-level curves. Such correctionswere necessary, because the average count in the indi-vidual sample fields observed (where absolute intensi-

04

03 \Bx

02

o B

0 - B,

-02

-0 3

-04

1400 1200

Fig. 7. Graph of the

n

z

.0z

0

;lL

1000 800 600 400WAVELENGTH (A)

wavelength dependent calibration constantsB,\' and Be.

ties had actually been measured by the calibrationrocket spectrometers) differed somewhat from the av-erage trend of the quiet field represented by the smoothcurves of Fig. 4 for the particular days of the compari-son. Consequently, the absolute intensities on Fig. 5were adjusted so that they corresponded to the averagedata.

In order to derive a more descriptive and useful for-mulation of any wavelength or time-dependent sensi-tivity changes we undertook an analysis of the trend ofthe instrument response to the quiet sun from the manywavelength scans which were performed in quiet areasduring the mission. This analysis produces systematicagreement with comparable trends in measured re-sponse to the quiet sun from the raster data shown inFig. 4 within an average uncertainty of 10% for allwavelengths and over the duration of the mission.Following the definitions of Eqs. (la) and (lb) the ob-served values of SA(t) are

Sx(t) = J0TB(DOY-150/250)

for 150 DOY < 400, for X 977 A, (2a)S\(t) = 1 0 O83(DOY-150/106)

for 150 DOY < 250, for > 977 A, (2b)

SX(t) = 10Bx +A(DOY-250/150)

for DOY > 250, for X > 977 A. (2c)

DOY refers to the year 1973 and is continued into 1974without reset. The values of B\(t) and BA'(t) are shownin Fig. 7 for the wavelength range of the instrument.

The agreement between the relative response of theinstrument to the quiet sun and the independent mea-surement of the quiet solar flux from the Calroc flightswhich are compared in Fig. 5 suggests the conclusionthat the instrumental sensitivity remained constantduring the mission and that therefore the intensity ofthe quiet sun changed appropriately. While there isevidence for some change in the quiet solar intensitywith time during the solar cycle,' 4 we do not feel thatthe change in response represented by, for example, thecurve for 977 A in Figs. 4 and 5 amounting to a factor of2.8 over the mission, can be considered as demonstrablyreal. However, there is evidence for a change in the

856 APPLIED OPTICS / Vol. 16, No. 4 / April 1977

solar intensity from the quiet sun approaching ±15%during the mission, and this effect will be discussed inmore detail in a later paper. For the purpose of thisdiscussion we assume that within our currently esti-mated uncertainty the intensity of the quiet sun at 977A is constant in time over the Skylab mission. Thetemporal sensitivity at the reference wavelength of 977A is given by the function F(t) in Eq. (la) and has thevalue

F(t) = 0.87 . 10 0.45(DOY-150/250) (3)

This value of F(t) was derived from the relative re-sponse of the instrument at 977 A, corrected for themean intensity of the quiet sun as derived from the twocalibration rocket flights at both 977 A and 1032 A.Equations (1) through (3) provide the necessary for-mulation for correcting the telemetered signal (count/0.041 sec) into units of intensity (ergs cm- 2 sec- 1 sr-1)within the specified detector bandpass.

V. Estimated Uncertainty of the Calibration

We estimated that the following uncertainties werepresent when we determined k x1 during the final labo-ratory calibration and subsequently applied the cali-bration to the first day of solar observations.

(1) Geometrical measurements (area of slita, focal length f, area of telescope mirror A) ±5%

(2) reflectance of telescope mirror R ±15%(3) secondary standard (uncertainties in

yield and in the measurement of the photo-current) ±15%

(4) over-all spectrometer efficiency Ex (in-cluding uncertainties owing to imperfectgrating illumination and to drift in laboratorylight source output, as well as uncertaintiesfrom counting statistics during calibration andin gate time) ±25%

(5) allowance for change in calibration be-tween final laboratory calibration and high-voltage turnon in orbit ±10%square root of sum of squares ±35%

Accordingly, in Fig. 5 the data points for DOY 152(prelaunch calibration) have been assigned an errorbracket corresponding to a factor of 1.35. The pre-ceding uncertainty estimate, however, is valid only forthe wavelength range indicated by solid lines in Fig. 6;at wavelengths, where the curves are dashed, the un-certainty of the laboratory calibration may reach afactor of 2.

The uncertainty of the Calroc intensities is smaller,because additional precautions had been taken thereespecially in the spectrometer calibration, where sep-arate measurements were made for partial grating areas.Timothy et al. 8 give a 20%b uncertainty for the labora-tory calibration. If this is combined with an estimated10% uncertainty allowing for change sensitivity as wellas for residual atmospheric absorption, one obtains atotal uncertainty of approximately ±25%. Accordingly,

in Fig. 5 the data points for DOY 221 and 344 (Calrocflights) are shown with an error bracket correspondingto ±25%. However, since the calibration rockets weremeasured before and after each flight and used the sameset of standards, the relative calibrations between thetwo flights should be appreciably less than 20%. Thereis, therefore, conflicting evidence between the imputedchange in instrument calibration with wavelength andtime during the mission and the absolute measurementsof solar intensity.

Combining the approximate errors discussed abovewe estimate that the formulation given by the equa-tion

IxD = S() . nD

with S\(t), F(t) given by Eqs. (2a), (2b), (2c), and (3)represents the best calibration currently available, withan assigned uncertainty of ±35% over the range of datashown by the solid curve of Fig. 6 and possibly ap-proaching a factor of 2 outside that range. The cali-bration assigned here may be modified as the ATM finaldata analysis continues. These refinements will beavailable either through the National Space ScienceData Center or from the ATM scientific staff at HarvardCollege Observatory.

The Calroc spectroheliometer was designed and builtby the technical staff of the Solar Satellite Project atHarvard College Observatory with qualification andintegration support from Ball Brothers Research Cor-poration (BBRC) in Boulder, Colorado. We expressour thanks to the many engineers and technicians ofHCO, BBRC, and NASA who contributed to the even-tual success of the calibration program. We express ourthanks particularly to W. Harby, R. M. Chambers, andD. A. Roalstad who, respectively, directed the technicalactivities of the ATM, the Calroc, and BBRC support.This work was supported by the National Aeronauticsand Space Administration under contract NAS 5-3949.

References1. E. M. Reeves and W. H. Parkinson, Appl. Opt. 9, 1201 (1970).2. M. C. E. Huber, A. K. Dupree, L. Goldberg, R. W. Noyes, W. H.

Parkinson, E. M. Reeves, and G. L. Withbroe, Astrophys. J. 183,291 (1973).

3. J. G. Timothy, Bull. Am. Astron. Soc. 7, 407 (1975).4. J. G. Timothy, Astrophys. J., to be submitted (1977).5. M. C. E. Huber, E. M. Reeves, and J. G. Timothy, "Space Optics,"

Proc. IXth International Congress, International Commissionfor Optics, Santa Monica, 9-13 Oct. 1972, B. J. Thompson andR. R. Shannon, Eds. (U.S. National Academy of Sciences,Washington D.C., 1974), p. 33.

6. J. G. Timothy, R. M. Chambers, A. M. d'Entremont, N. W.Lanham, and E. M. Reeves, Space Sci. Instrum. 1, 23 (1975).

7. J. G. Timothy and E. M. Reeves, Prog. Astronaut. Aeronaut. 48,123 (1976).

8. J. G. Timothy and L. B. Lapson, Appl. Opt. 13, 1417 (1974).9. E. M. Reeves, Sol. Phys. 46, 53 (1976).

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