A Grazing Incidence Monochromator for Satellite Studies of

the Solar He 11 Lyman-a Line at 303.8 A

P. J. Bowen, J. A. Bowles, W. M. Glencross, R. J. Speer,A. F. Timothy, J. G. Timothy, and A. P. Willmore

A small grazing incidence monochromator was flown on the NASA OSO-4 satellite to monitor variations in

the absolute flux of the solar He ii Lyman-a line at 303.8 A. The design of the instrument is described in

some detail, and the procedures for photometric calibration are discussed. Examples are shown of theresults obtained during the two-year period of operation in orbit.

Introduction

A small grazing incidence monochromator was flownon the NASA OSO-4 satellite to monitor variations inthe absolute flux of the solar He ii Lyman-a emissionline at 303.8 A. This line, one of the most intense inthe solar extreme ultraviolet spectrum, provides a majorsource of ionizing radiation for the formation of the Eand F regions of the ionosphere. The requirements forthe instrument were high sensitivity, accurate photo-metric calibration, and stability of response throughoutthe orbital lifetime coupled with low demands on thesatellite power, command, and telemetry systems.

Optical System

The monochromator was mounted in one of the fiveexperiment compartments in the rotating wheel sectionof the satellite, viewing the sun once per rotation atintervals of about 2 sec. For maximum sensitivity,wide fields of view were required both in and at rightangles to the plane of rotation to provide as long anintegration time as possible and to accommodate the 70total pitch angle variations of the wheel axis with respectto the satellite-to-sun center axis.

A spherical concave grating having a radius of curva-ture of 50 cm was employed as the dispersing element(see Fig. 1). The inlet slit (IS) was positioned onthe Rowland circle of radius 25 cm to give an angle ofincidence of 840, with the exit slit (ES) positioned toisolate the first-order 303.8-A radiation with a wave-

All authors were with the Physics Department, University Col-lege London, Mullard Space Science Laboratory, Holmbury St.Mary, Dorking, Surrey, England, when this work was done; R. J.Speer is now with Imperial College.

Received 1 May 1970.

length resolution of 10 A. The monochromator wasmounted in the satellite with the plane of dispersion inthe plane of rotation of the wheel, the 2.5-cm ruledwidth of the grating giving a field of view in this plane of309' without degrading the wavelength resolution.With a wheel rotation rate of about 0.5 revolution persee this gave an integration time for solar radiation ofabout 20 msec per rotation. An illuminated length ofthe grating rulings of 0.8 cm, coupled with inlet and exitslit lengths of 1.4 cm and 1.6 cm, respectively, gave atotal field of view at right angles to the plane of disper-sion of 90 for first-order 304-A radiation.

The diffraction grating used was an original ruled byHilger and Watts Ltd. (now Rank Precision Ltd.) ingold on a silica blank with a frequency of 1152 lines permm and blazed for first-order 304-A radiation. Anopen structure Bendix type M310 magnetic electronmultiplier with a tungsten photocathode was used as thedetector.

Two mirrors A and B (see Fig. 1) placed the inlet slit(IS) in the virtual position (I'S') complementary to theexit slit position. Radiation entering the instrumentalong the axis (marked 1216 A) at 140 to the normalentrance ray was monitored by the photomultiplier inthe zeroth order of the grating. The combination ofthe normal incidence reflection properties of the mirrorsand the cathode response characteristic of the photo-multiplier enabled virtually pure hydrogen Lyman-a(1216-A) radiation to be monitored in this mode ofoperation. Selection of mode (at the appropriate posi-tion of the wheel rotation) was controlled by visible lightphotodiodes viewing along the entrance axes. By com-bining an uncoated stainless steel mirror with an alumi-num mirror overcoated with magnesium fluoride it waspossible to obtain an over-all instrumental response at1216 A in the zeroth order relative to that for 304 A inthe first order, such that the expected output count rates

28 APPLIED OPTICS / Vol. 10, No. 1 / January 1971

N NN

' ifZ~ BackgroundN* photodiode

1216o Atphotod lode

_ -_ _ - ~ 304 Aphotodiode

Grating1200 lines/mm

( M 310Photomultiplier

Fig. 1. Optical diagram of the monochromator.

for the solar lines would be approximately equal. Itwas intended to use the 1216-A mode as a calibratemode by comparing the output of the monochromatorwith that from two nitric oxide ionization chambers,having lithium fluoride windows and fields of view of 20and 300, respectively, mounted in the same instrument.

Instrument Package

The optical beam of the monochromator, containingthe inlet and exit slit assemblies and the diffractiongrating, was mounted together with the open structurephotomultiplier and its associated preamplifier inside asmall vacuum chamber (see Fig. 2). This chamber wasevacuated and filled with dry nitrogen via a purge valveon the front of the package (see Fig. 3) for maximumprotection of the optical elements during the long inte-gration and launch periods of the satellite. A windowon the front of the vacuum chamber, operated by twobellows actuators for redundancy, was opened on com-mand as soon as the satellite was correctly in orbit.The counterbalance on the window ensured that themoment of inertia of the satellite was unaffected by thewindow operation.

Electronics modules, containing the logic circuitryand the low voltage and high voltage convertors weremounted on the instrument base plate back to backwith the monochromator vacuum chamber. The threevisible light photodiodes used to control the mode ofoperation of the monochromator were mounted at thefront of the package together with the two nitric oxideionization chambers. As the open structure photo-multiplier would be destroyed by operation at pressuresabove 10-4 Torr a safety connector was included in thelow voltage supply to the multiplier high voltage con-vertor. This connector, together with a safety con-nector for the window operate system and a test inputconnector for the amplifiers, was mounted on the frontof the base plate in order to be accessible from outside

the satellite. The weight ofwas 11.8 kg.

the complete instrument

Electronics

The open structure photomultiplier was operated as aphoton counter, with the output pulses fed to a chargesensitive preamplifier mounted inside the vacuumchamber and from there to an amplifier and discrimi-nator network situated in the main logic module.Values of the applied high voltage potentials for opti-mum operation of the multiplier and the resulting out-put pulse height distribution were determined in labora-tory tests prior to launch. In order to allow for possiblegain degradation in the multiplier during the orbitallifetime, the amplifier threshold was set at a level afactor of 50 below the measured mean output pulseheight from the multiplier.

Readout of the pulses into the counter selector andshift registers (see Fig. 4) was controlled by timing gatesoperated from the visible light photodiodes mounted onthe front of the package. In normal operation thereadout gate was generated by the photodiode viewing

Fig. 2. Assembly of the monochromator in the flight package.

January 1971 / Vol. 10, No. 1 / APPLIED OPTICS 29

7216 A

along the 304-A inlet axis with an acceptance anglegreater than that of the monochromator, with the pulsesproduced by the solar 304-A radiation accumulated in a10-bit counter. In the calibrate mode, the readout gatewas generated by the photodiode viewing along the 1216-A inlet axis at a different position of the wheel rotation,with the pulses again accumulated in the 10-bit counter.Selection of the operational mode was possible byground command, and a total of four commands wasavailable to the experiment, namely, experiment on,experiment off, select He ii mode, and select calibratemode.

As the instrument rotated to a position where no lightwas striking the grating a third photodiode was used togenerate the readout gate. Pulses resulting from thebackground noise in the photomultiplier were thenaccumulated in a 4-bit counter. The data in the twocounters were then shifted in two parts into the single8-bit telemetry word available to the experiment. Inthe first part, the high order bit of the word was flaggedas 1, and the 7 high order bits from the 10-bit signalcounter filled the rest of the word; in the second part,the high order bit of the word was flagged as zero, and

000100100000 �

0012160

c00v�

Fig. 3. Complete flight package mounted in the OSO wheelcompartment.

the remaining 7 bits consisted of the 3 low order bitsfrom the 10-bit signal counter plus the 4 bits from thebackground counter. This system gave a total of 1023and 15 counts per look, respectively, for the signal andbackground before overspill occurred in the counters.The current outputs from the ionization chambers were

Fig. 4. Block diagram of experiment electronics.

30 APPLIED OPTICS / Vol. 10, No. 1 / January 1971

fed to peak reading circuits and read out with twoanalog telemetry channels. A final analog telemetrychannel was used for monitoring purposes, namely,door open or closed, high voltage on or off, and 304-A or1216-A mode.

The instrument operated during the daylight part ofeach orbit with a total power consumption of 400 mW,and turn on after eclipse occurred automatically in the304-A mode.

Instrument Calibration

The main aim of the experiment was to determine theabsolute flux of the solar He ii Lyman-a line, and boththe monochromator and the two ionization chamberswere calibrated prior to launch.

The output count rate from the monochromator canbe expressed as

N(X) = A f fards.

where

N(X) = output count rate from the monochromator admittingphotons in the wavelength range XI to X2 (counts sec'-),

A = collection area of the monochromator, i.e., the area of theentrance slit (cm2),

= over-all efficiency at wavelength X (counts photon-'),= photon flux at wavelength X (photons cm-2 sec').

The over-all efficiency of the monochromator canthen be expressed as

where

E = diffraction efficiency of the grating in a given order at wave-length X (photons photon-'),

= efficiency of photomultiplier at wavelength X (counts pho-ton-').

Finally, the photomultiplier efficiency can be ex-pressed as

ax= YxP,

where

Yx = photoyield of cathode material at wavelength X (photoelec-trons photon-'),

P = probability of producing an output pulse greater than someset amplifier threshold. Generally this function will nothave a strong dependence on the wavelength of the inci-dent photon (counts photoelectron-').

The procedure adopted for the calibration of themonochromator was to measure the grating and photo-multiplier efficiencies independently and then finally tomeasure the over-all instrumental efficiency as a checkon the correct assembly of the monochromator. De-termination of the grating efficiency is in principle arelatively simple task in that any linear detector (orcorrelated pair of linear detectors) may be used withoutany knowledge of their absolute response at a givenwavelength. Great care of course must be taken to en-sure that the geometry of the laboratory calibration

beam approximates as closely as possible the beam to bestudied. The measurement of the photomultiplierefficiency is a more difficult task as the response mustbe determined in absolute units of energy. This wasachieved by using a windowless double electrode ioniza-tion chamber as an absolute detectors; the absorbinggas was pure helium, with an assumed photoyield ofunity at 304 A, and nitric oxide, with an assumed photo-yield of 81% at 1216 A.2 The photomultiplier andionization chamber were mounted in tandem behindthe exit slit of a laboratory monochromator, and mea-surements were taken by the multiplier, ionizationchamber, and multiplier in sequence to ensure stabilityof the light source. Small tungsten foils were thencalibrated as secondary standards, also using the ioniza-tion chamber.

The over-all response of the assembled monochroma-tor was then compared with the saturation photocur-rent from one of these foils mounted immediately be-hind the inlet slit, with the monochromator illuminatedwith either 304 A radiation or 1216-A radiation. Theresponse was calibrated over the entire field of view ofthe instrument.

The absolute efficiencies of the two flight nitric oxideionization chambers at 1216 A were measured by E.Bruner at LASP, Boulder, Colorado, again by com-parison with windowless nitric oxide ionization cham-bers.

ResultsThe OS0-4 satellite was successfully launched on 18

October 1967. The monochromator window wasopened during the first orbit, and the instrument wasallowed to outgas for four days before turn on. Whilethe monochromator commenced satisfactory operation,it was immediately apparent that both nitric oxideionization chambers had fatigued during the four-dayperiod in orbit. In the case of the chamber with the 30°field of view, the damage was so severe that no meaning-ful results were obtained. The damage to the chamberwith the 20 field of view, however, although it increased'with time, was only slight during the first few orbits.It was thus possible to extrapolate the response back toorbit 1 (zero degradation) and to determine a value for

Fig. 5. Increase in Helium iI Lyman-a flux during an importance2 optical flare on 2nd November 1967.

January 1971 / Vol. 10, No. 1 / APPLIED OPTICS 31

Fig. 6. Long term intensity variations in the Helium ii Lyman-Cflux. A, mean value from the SL502 sounding rocket spectrom-

eters.

the flux of the hydrogen Lyman-a line at that time.The value for the hydrogen Lyman-a flux obtained inthis way for 1S October 1967 was 3.3 X 101" photonscm- 2 sec' or 5.4 erg cm- 2 sec-' (estimated photometricaccuracy 30%). By comparison, the mean valuefor the hydrogen Lyman-a flux from the monochroma-tor on 25 October 1967 was 3.6 X 10"1 photons cm-2

sec' or 5.9 erg cm-2 sec-' (estimated photometricaccuracy 30%). Thus although it was not possibleto compare the values for hydrogen Lyman-a fluxobtained from the ionization chambers with that ob-tained from the monochromator over a period of timethe initial agreement was well within the accuracies ofcalibration.

Excellent data were obtained in the He iI mode forabout six weeks until December 1967, and output countrates, depending on the value of the pitch angle, werebetween 1000 and 400 counts per look. At this timeit became evident that breakdown was occurring in thehigh voltage convertor generating large noise pulseswhich began to interfere with the operation of the satel-lite. The experiment was therefore turned off untilMarch 1968 when it was once again switched on to seewhether the problem had cured itself by extended out-gassing. Breakdown was still evident, and the experi-ment remained off until June 1968 when a further turnon gave successful results. The monochromator thenoperated correctly until December 1969 when the satel-lite was closed down for operational reasons.

The data obtained from the instrument in the He iImode over the two-year period of operation have beenof three distinct types. First, studies of short term(less than 1 h) activity; second, a monitor of the longterm day-to-day variations in the flux; and, last, amonitor on changes in the state of the earth's atmo-sphere measured by the absorption profiles obtainedwith the satellite entering and leaving eclipse.

As an example of short term activity, Fig. 5 showsthe increase in the He iI Lyman-a flux during an im-portance 2 optical flare event observed on November1967.3 A peak increase in intensity of 22% was ob-served. It should be noted that the wavelength band-pass of the instrument admits radiation from both theHe iI Lyman-ai line at 303.8 A and the Si xi resonance

line at 303.3 A (Ref. 4), and that the relative effects ofthe two lines cannot be distinguished.

The long term variations in the daily averages of theflux for nonflare periods over the first 200 days of 1969are shown in Fig. 6. The error bar represents therelative error in the day-to-day measurements and is acombination of the 3-or error on the counting statisticscoupled with the error involved in taking out the pitchangle effects. The absolute photometric accuracy isestimated at 4 30%.

The He ii Lyman-a flux was also measured by twospectrometers on a sun-stabilized Skylark soundingrocket on 3 April 1969 (day number 93). The meanvalue of the flux extrapolated to the top of atmospherefrom the peak altitude of 273 km is shown on the graph,with the relative (extrapolation) and absolute accura-cies identical to those for the satellite monochromator.From the excellent agreement between the satellite androcket data it was concluded that no measurable lossof sensitivity in the satellite monochromator in the Heii mode had occurred during the eighteen months in or-bit. This was undoubtably due to the use of grazingrather than normal incidence optics and to the safetymargin included in the photomultiplier amplifying cir-cuits. However, during 1969 a progressive loss ofsensitivity of up to 30% was observed for the mono-chromator in the 1216-A calibrate mode. It seemsmost probable that this arose from a reduction in thereflectivity of the magnesium fluoride-coated aluminummirror, although a change in yield of the photocathodeat this wavelength cannot be ruled out.

When the satellite entered and left eclipse behind theearth, the monochromator observed the atmosphericabsorption profile for the He ii Lyman-a line (see Fig.7). While it was not possible to determine directlythe atmospheric composition and temperature from theprofile of a single line, it was possible to fit the profileto a theoretical model atmosphere and to monitor varia-tions in structure as functions of local time (sunrise orsunset), position, and solar activity.

Conclusions

A small grazing incidence monochromator has op-erated successfully in orbit on the NASA OSO-4 satel-

2t

45

Fig. 7. Atmospheric absorption profile for the Helium ii Lyman-a line.

32 APPLIED OPTICS / Vol. 10, No. 1 / January 1971

o ....

C00

00

E400

00

..*- -.-- --.... ALTTUPE CF GRAZIN RAY Kso0 160 00 650 00 00 400 450 00

lite for a period of two years. Breakdown in the photo-multiplier high voltage convertor which forced the ex-periment to be turned off after six weeks of orbitaloperation cleared itself during a six-month period ofoutgassing. Recalibration by comparison with morerecently calibrated sounding rocket spectrometersshowed no loss of sensitivity in the basic monochroma-tor after eighteen months in orbit.

The mean value obtained for the He ii Lyman-a fluxwas 9.0 X 109 photons cm-2 see- (30%) with quietsolar variations of + 17% from the mean and short termflare increases of up to 25%.

The mean value for the hydrogen Lyman-a flux was3.5 X 10"I photons cm-2 sec-2 ( t30%), and the initialagreement between the monochromator and the twoionization chambers calibrated at different laboratorieswas within 10%.

This experiment has been carried out as part of the re-search program of the Mullard Space Science Labora-tory of the Department of Physics at University CollegeLondon, and the authors wish to gratefully acknowledgethe guidance given by both the head of the PhysicsDepartment, H. S. W. Massey, and by the head of theLaboratory, R. L. F. Boyd.

Thanks are due to many people in the OSO programat NASA and at the satellite contractors, Ball Brothers

Research Corporation, both for the flight opportunityand for assistance during the experiment design andmanufacture. The development of the electronics wasundertaken by Pye Laboratories, Cambridge, England,and the mechanical construction by Rank PrecisionIndustries, London, England. The authors wish toacknowledge particularly the assistance of D. Demaineof Rank Precision Industries with the optical design ofthe monochromator, J. Holmes of MSSL with the de-velopment of the window mechanism, and E. Brunerof LASP, Boulder, Colorado, with the calibration ofthe flight ionization chambers. The SL 502 soundingrocket was fired under the U.K. National Space Pro-gramme.

The experiment was funded by the Science ResearchCouncil and two of the authors (A. F. Timothy andJ. G. Timothy) gratefully acknowledge receipt of ScienceResearch Council research assistantships.

References1. J. A. R. Samson, J. Opt. Soc. Amer. 54, 6 (1964).

2. K. Watanabe, F. M. Matsunaga, and H. Sakai, Appl. Opt. 6,391 (1967).

3. ESSA, Solar-Geophysical Data, No. 285 (1968).

4. S. R. Pottasch, Space Sci. Rev. 3, 816 (1964).

Denis Gabor, professor emeritus of ImperialCollege and consultant to CBS Laboratories.

January 1971 / Vol. 10, No. 1 / APPLIED OPTICS 33