IEEE Thane,tiona on NuceaeA Science, 'oLJIS-24, lo. l, Feb.uaAy 1977

THE HEAO-A SCANNING MODULATION COLLIMATOR INSTRUMENT*

Adrian Roy, John Ballas, Nell Jagoda, Phillip McKlnnon, Alan Ramsey, Edwin WesterI

Abstract

The Scanning Modulation Collimator X-ray instru-ment for the HEAO-A satellite was designed tomeasure celestial radiation in the range between1 and 15 KeV and to resolve, and correlate, theposition of X-ray sources with visible light sourceson the celestial sphere to within 5 arc seconds. Thepositional accuracy is made possible by mechanicalcollimation of the X-ray sources viewed by theinstrument. High sensitivity is provided from twosystems each containing four gas fllled proporticnalcounters followed by preamplification, signalsumming, pulse height analysis, pulse shapediscrimination, X-ray event accumulators andtelemetry processing electronics.

Introduction

The Scanning Modulation Collimator (SMC) instrument,as shown in Figure 1, is one of four instruments tobe observing celestial radiation sources on the Amission of the High Energy Astronomy Observatory(HEAO-A). This satellite is scheduled for launchinto a near equatorial orbit from the Eastern TestRange of Kennedy Spaceflight Center in April 1977.

BRIGHTOBJECTS,ENSOR

SMC1COARSECOLLIMATOR

SAS ISTAR -ITRACKER

SMC2COARSE _COLL IMATOR

BRIGHTOBJECT-SENSOR

30 ARC SEC.COLLIMATOR

5A52- STAR

TRACKEROPTICALBtNCH

0 120 ARC SECC0COLLIMATOR

I HANDL IN'PIXTURE

Figure 1 Front View of the HEAO-A SMCInstrument

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*This work was supported by the National Aeronauticsand Space Administration under Contract NAS8-27973.

1 All authors are affiliated with American Scienceand Engineering, Inc., Cambridge, MA 02139

The HEAO-A spacecraft will provide continuousscanning of great circles on the celestial spherewith a 30 minute spin period. The spacecraft'saxis of rotation will always point toward the Sunas the Earth moves in orbit around the Sun. Aftersix months of operation, each instrument on thespacecraft will have scanned across all celestialradiation sources many times depending on thefield of view (FOV) of each instrument. The space-craft provides the necessary attitude control, thesequencing of data acquisition cycles to the variousexperiments and telemetry transmission and reception.It also provides bulk storage of data accumulatedduring each 95 minute orbit and transmits this datato one ground station during a 5 minute pass over thestation.

The SMC instrument measures celestial radiation Inthe range between 1 KeV and 15 KeV and can resolvethe spatial positions of these sources to an accuracyof 5 arc seconds.

The major elements of the SMC instrument are anoptical bench, several mechanical collimators, aproportional counter assembly and a main electronicsassembly. In addition, the SMC instrument containstwo Stellar Aspect Systems (SAS) each of whichconsists of a star tracker, a bright object sensorand associated electronics.

The optical bench Is the structure on which thecollimator, proportional counter assembly and aspectsystem are mounted, the collimators are of two types;a coarse egg-crate type to provide a 40 x 40 FWHMfield of view and a fine wire-grid type to modulatethe X-ray signal. The collimators and proportionalcounters are arranged to provide two separate datachannels whichwll permittwo dimensional spatialresolution. These data channels are referred to as SMC1and SMC2. The modulation collimator for SMCl providesa series of parallel 30 arc second FWHM transmissionbands on 4 arc minute centers. The modulation collimatorfor SMC2 provides a series of parallel 120 arc secondFWHM transmission bands on 16 arc minute centers. ByincliningtheSMCl andSMC2 transmissionbands 100from the direction of scan and 200 from each other, thecapabllity ofresolving source positions to within therequired 5 arc second accuracy has been enhanced. Theradiation source positions are correlated with visiblelight sources identified by either of the Stellar AspectSystems.

Each of the two data channels uses four argon- carbondioxide filled proportional counters. Pulse shapediscrimination Is used to separate the shorter time

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duration X-ray pulses from the longer time durationbackground cosmic events such as Gamma Rays. Eachdata channel sorts the X-ray events into three energybins by pulse height analysis (PHA) and accumulatesthis data in an 8 bit counter. Background and overscaleevents are also accumulated. The electrical configura-tion of the two SMC channels and two SAS systems canbe altered by four ten bit serial commands and twenty-six individual pulse commands. This command struc-ture makes it possible to reconfigure the instrument in-flight by activating redundant circuits, changing thresh-olds and rerouting power distribution to enhance theoperation of the instrument and circumvent certaintypes of failure.

Since determination of the location of X-ray sources towithin five arc seconds is the prime objective of the exper-iment, the instrument was subjected to extensive testing ina thermal vacuum environment to accurately verify thealignment stability of the modulation collimators, withrespectto each of the star tracking systems.

Collimators and Detectors

TheX-ray collimators and their associated detectors formthe heart of the SMC instrument. A modulation collimatorlis composed of four parallel wire grids. Each grid is woundwith a single layer of nickel alloy wires. The spacing be-tween the wires is equal to the wire diameter and is 0. 005inches on the 30 arc second collimator and 0 .02 0 inches onthe 120 arc second collimator. A collimator has a grid ateach end of the housing, one in the middle, and one at one-quarter the distance from the end closest to theX-raydetectors. The grids were aligned to one another whileobservingX-ray transmission patterns, and were adjustedto nearly exact registration. A lateral displacement of0. 0o0linch perpendicular to the wires was easily seen on the3 0 arc second collimator. With a two grid collimator, oneobtains an X-ray transmission which is identical to a two-picket fence Moire' pattern. The effect from adding twoother grids Is to eliminate some of the two-grid allowedtransmis sion bands, with the result that only the first,fifth, ninth, etc. bands are transmitted. The passband istriangular with the shadowed regions having four timesthe width of the base of the transmitting region. As thecollimator sweeps across a point source on the celestialsphere, ittransmits at each allowed angle as long as theobject Is in view through the egg-crate coarse collimatorin front of the wire collimators. Thus, for each collimator,the location of a point source may be anywhere on a seriesof lines drawn on the celestial sphere parallel to the wireson the collimator. The wires in the two collimators, andhence the locating lines on the sphere, are inclined to eachother at 200. The intersections of these lines are theallowed locations of the source. Identification of an X-raysource with a known celestial object is then done byoverlaying the allowed locations on a star map. The inter-sections of the transmission planes are not points, ofcourse. For a moderately strong source, the resolution isabout 5 and 12 arc seconds forthe 30 and 120 arc secondcollimators respectively. The error boxes are thenparallelograms with sides of 5 and 12 arc seconds andan acute angle of 200 between the sides.

The X-ray detectors which are associated with thecollimators are a gas-filled type of radiation detector.Each has two anode wires and the detector is biased.

in the proportional region of operation at 2300 volts.The filling gas is 90 percent argon and 10 percentcarbon dioxide. Each counter is about 5 incheswide and 16 inches long and four counters are arrangedbehind each collimator.

The X-rays enter the counters through 0.0015 inchberyllium windows which have a reasonable trans-parency for X-rays above 1 KeV. The filling gaswill also pass most X-rays below 20 KeV. This isthe energy region in which fall the emissions frommost compact celestial sources. The beryllium windowsare supported by aluminum strongbacks with circularholes in a close-packed configuration. Approkimatelytwo thirds of the counter face is active In thisarrangement. The two anodes of each counter areconnected together and the output from each counteris coupled into a preamplifier whose gain is trimmedto match its counter, so that the output of all thecounters behind each collimator can be summed.

Mechanical Design

The primary structure of the SMC instrument consistsof two beryllium modulation collimators mounted to acommon optical bench which was also made of beryllium.In addition to these assemblies, the Main ElectronicsAssembly and X-Ray Detector As sembly are mountedin separate housings which attach to the aft end ofthe optical bench. A coarse collimator is mountedto the forward end of the optical bench immediatelyin front of each collimator. These later structureswere made of 6061-T6 aluminum.

The scientific requirement for the wire grid collimatorswas as follows:

a. For each plane of wires, wire to wire centerspacing +5% of the wire diameter (L+0.00025 inchfor the 30 arc second collimator).

b. For the total collimator, any wire must be alignedwith every other wire along that collimator axisto +5% of the wire diameter over the full lengthof the wire.

c. The wire grid planes must be parallel to oneanother to +0. 002 inch.

The above requirements were to be maintained over atemperature range of 00C to 300C and after exposureto mechanical stresses imposed by the launchenvironment. The use of beryllium for the modulationcollimators and optical bench provides a light weightyet rigid structure with high thermal conductivityto assure minimum thermal gradients throughout thestructure.

The design of the modulation collimator required arigid assembly which would allow adjustment of eachwire grid relative to the others in order to satisfythe alignment tolerances. The resultant structure,as shown in Fiqure 2, provided a housing within whichthe wire wound grids were mounted. The internalstructure of the housing provided the re uiredrigidity with a minimum blockage of viewing areaby duplicating the construction of the wire wound grids.

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Figure 4 Collimator Mount Design

Figure 2 Collimator Housing

The wire wound grids were assembled by winding wireover one grid frame. When the full expanse of theframe has been covered, a second half of the gridframe was attached with epoxy, trapping the wires ina sandwich. After the epoxy had cured, the wireswere then cut providing the completed assembly.The grid winding machine shown in Figure 3 wasdeveloped to provide a means of feeding the wireunder constant tension and at the required spacing.

The mounting of the collimators to the optical benchrequired a rigid attachment which would provide thermalisolation of the collimators from the optical bench butin such a fashion as not to impose stresses on thecollimator due to thermal differences between themand the optical bench.

Figure 3 Grid Winding Machine in Operation

The design which was adopted is shown schematicallyin Figure 4. Mount 1 is a pin joint which providestranslational restraint in all three axes. Mount 2provides translational restraint in the X-Z plane butallows differential thermal growth along the Y axis.Mounts 3 and 4 provide lateral restraint in the Xdirection and allow for differential thermal growthin the Y-Z plane. Thermal isolation was providedby using glass/epoxy bushings between the mountingpins and the beryllium mounts.

Electrical Design

Mission success requires that the SMC instrumentoperates continuously for a six month period. This isachieved by an electrical design with redundancy ofmajor circuit components and the utilization of screenedburned in electronic component parts derated for opera-tion well below their maximum specified operatingconditions. A failure mode effects analysis was alsoperformed on the electronic circuits to insure thatthere were no single failure points in the system.

Figure 5, the Block diagram of the HEAO-A SMC instru-ment, illustrates the redundancy in design and theinterconnection of the various circuits to be described.

X-Ray Detector Electronics

Four proportional counters are used to present aneffective X-ray collecting area of 450 cm2 for eachSMC channel which will detect a source with astrenght of 10-3 Crab. Four counters per channelplus two high voltage power supplies provide thenecessary redundancy to keep at least one counteroperative even in the event of most double pointfailures. Each high voltage power supply provides2300 volts to two counters in the 30 arc second channeland to two counters in the 120 arc second channel.

Preamplifiers- Summers, and Cross Switch Amplifiers

The preamplifier is a conventional charge sensitiveamplifier, an integrating operational amplifier with again of 4. SV per picocoulomb. A 1 KeV X-ray eventwill be converted to a 200 millivolt pulse with risetimeless than 400 ns. Any preamplifier can be gated offby means of a ground command to disable the signalfrom a faulty proportional counter. Outputs from fourpreamplifiers per channel are linearly summed togetherby a pair of summingamplifiers. Redundant summersin each SMC channel allow cross switching from onechannel to the other in the event of a failure in thenormal (not cross switched) summer or the signalprocessing circuits following the normal summer in oneSMC channel. This permits the collimated outputfrom an SMC channel's detector assembly to be pre-served and processed by the other channel. By cross-switching every other orbit, the output from each detectorassembly can be time shared without loss of data thus

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X-RAY DETECTOR ASSEMBLIES MAIM ELECTRONICS ASSEMBLY

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DDCDDETECT TE DOBE-NE SUMMER__ _ _ ______ _ ____ __ _

Figure 5 HEAO-A SMC Block Diagram

preserving the effect of having two X-ray collimatingsystems to accurately pin point the location of X-raysources on the celestial sphere.

Pulse Height AnalysisEach channel has a pulse height analyzer (PHA) whichsorts the output from the summer into three spectralenergy bins of 1 to 3 KeV (PHO), 3 to 6 KeV (PHl) and6 to 15 KeV (PH2). The lowest level is thresholdadjustable between 1 and 2 KeV in four steps. Thispermits some noise discrimination without significantloss of data should the noise floor degrade up intothe lower level threshold region.

The PHA is a stacked discriminator with d.ocisioncircuitry to detect which boundaries have been crossedby the voltage pulse resulting from a cosmic event.

Pulse Shaipe Discrimination

Each SMC channel has a pulse shape discriminator(PSD) which permits tagging all events within thethree energy bins with risetime less than 400 nsto be counted as X-rays. All other events of slowerrisetime are considered not to be X-rays. The 400 nstime is an arbitrary threshold. The actual PSDthreshold setting is determined by separate measure-ments utilizing calibrated Al K a , Fe55, and Sr85X-ray sources and a Co60 Gamma ray source andadjusting the risetime threshold until the background

corrected ratios of X-ray acceptance and Gamma rayrejection are on the order of 80%. A second settingof the PSD threshold is provided with a higher X-rayacceptance and a lower Gamma rayrejectionratio. ThePSD may be disabled and in this mode all events in thethree PHA channels are counted.

Data Processing Electronics

Three eight bit accumulators are provided for each SMCchannel to count events in the PHO, PH1 and PH2 energyregions. The SMC1 channel accumulates events for40 ms periods and the SMC 2 channel for 160 msperiods. Because of telemetry limitations, one 16-bitcounter in each channel is time shared and counts PSDrejects for one half the time lower level crossings fora quarter of the time and overscale events (above 15 KeV)for a quarter of the time. Another 8-bit accumulatorwith a three bit prescale counts the sum of the lowerlevel crossings from SMC1 and SMC2 and this is referredto as the total rate or high frequency counter.

Digital Cross Switch

Analog data which has been cross switched after theredundant summer amplifiers is processed into digitaldata by the other channel's PHA and PSD processingelectronics. In order that the data be presented totelemetry processed at consistent accumulation intervals,a digital cross switch is provided at the telemetry outputports to cross switch only the PHO, PHI and PH2

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primary science data into their assigned telemetryports at the assigned telemetry output rates.

Star Tracker and Support Electronics

star- aspect systems and the Electronics ProcessingAssembly (EPA). The EPA contains 19 electronicscircuit modules, two of which provide Interface betweenthe spacecraftand star aspect electronics.

Two separate star aspect systems SAS1 and SAS2 areprovided as part of the SMC instrument for the HEAO-Aspacecraft to be used for stellar aspect determination.Star trackers which are part of these systems scanan 80 x 80 field of view and track visible light sourceswithin that FOV. A 13 bit A/D converter digitizes twoanalog star position coordinates output from thetrackers into 3 . 5 arc second increments. Visible starintensity is resolved into eight levels of brightness bya 3 bit A/D converter. Calibration lights mounted onthe 30 arc second collimator project simulated starfield light patterns into the star trackers thus providinga fixed reference between this collimator and the twostar trackers. Knowing the initial alignment of thesimulated star field within the aspect sensor's fieldof view and having calibrated that position relativeto the X-ray axis during alignment testing, anyfurther relative displacement of the X-ray and opticalaxes can be detected and accounted for.

Electronic PackaginaA modular design concept from major assembliesdownto the small electronic circuit modules permittedpieces of the system to be built and tested Individuallybefore being finally assembled and functionally testedand calibrated as a complete system.

The X-ray Detector Assembly (XDA) houses eightproportional counters, their associated preamplifiersand two high voltage power supplies.

The Main Electronics Assembly, shown in Figure 6,contains two low voltage power supplies, spacecraftinterface connectors, and coinnectors to the XDA, the

Figure 6 Main Electronics Assembly Mounted

Most of the logic decision circuitry is provided byCMOS logic devices and iln some isolated cases wlErecritical timing is of the essence, low power TTL Isused. The power consumption for the SMC instrumentexcluding the two aspect systems, but including lossesdue to power supply Inefficiency was measured as4.6 Watts. Each of the star aspect systems separatelydissipate 5. 2 Watts.

Alignment Stability

The precise measurement of the position of celestialX-ray sources by the SMC instrument requires calibra-tion of the relationship between the star tracker andmodulation collimator. This calibration will beperformed in orbit using bright, identified celestialX-ray sources. Since there are relatively few sourceswhich can be used for this calibration, it Is essentialthat the tracker-collimator be stable over the rangeof expected environment. The most critical environ-ment affecting the alignment stability once in orbit istemperature. To determine the tracker-collimatorstability as a function of temperature, an extensivealignment stability test was performed during thethermal vacuum portion of the qualification/acceptanceverification program.

The major elements of support equipment for the align-ment stability test consisted of a thermal Vacuum testfixture, a 150 foot helium filled duct and X-ray andoptical sources. The thermal vacuum test fixtureIncluded a thermal shroud which closely approximatedthe thermal characteristics of the spacecraft cavitywhich simulated the deep space temperature at theviewing aperature of the SMC instrument, a vacuumfacility door containing X-rayand optical ports and adolly on which the aforementioned equipment was mountedThis fixture containing the instrument was mated withan ion pumped thermal vacuum facility. The X-ray andlight sources were placed approximately 160 feet fromthe instrument. The sources were mounted in a fixedgeometry on a translation tablehaving+18 inches ofhorizontal travel which permitted the angle of the sourcebeams to be varied in relation to the instrument.A laser also mounted on the translation table was used toestablish a fixed reference between the sources and theinstrument. The laser optical reference, established byauto-reflection of its laser beams off the front surface ofan alignment cube fixed on the front of the instrumentpermitted repeatability of the test set up.

The angular divergence of the X-ray beam incident onthe instrument was sufficiently collimated by locatingthe source at a distance of 160 feet and by usingaperature slit plates mounted in front of each collimator.

Relative alignment data of the tracker to collimator wasobtained following stabilization of operating instrumenttemperatures at -10 C, +60C and +270C. After

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adjusting the position of the source translation tableusing the laser optical reference, the X-ray source wastranslated in increments of 3 arc seconds (0. 030 inchesin translation) for SMC1 and 12 arc seconds (0.120inch translation) for SMC2. X-ray data obtained fromthe instrument PHA channels was observed and plottadto determine the peak of a transmission band corres-ponding to a plane of maximum transmission. Withthe X-ray source repositioned at the plane of maximumtransmis sion, "star"position measurements of thelight sources in each aspect sensor were made.

This test was repeated for three elevations of thesource translation table at each of two transmissionbands for each collimator. Data at the threetemperatures were then compared to determine theamount of thermally induced motion between thetracker and the collimator. The results indicateda temperature sensitivity of about 0.4 arc secondsper CO over a temperature range of 37 -CO. Thepredicted on orbit short term variations of the instru-ment are about 1CO per orbit and long term variations

will be less than 100C. Hence the tracker collimatorsystem will easily meet a stability requirement of 5 arcseconds.

Acknowledgements

The authors wish to thank Herbert Gursky1 of theSmithsonian Astrophysical Observatory, the PrincipalInvestigator who conceived the HEAO-A SMC Experi-ment; Hale Bradt1 of the Laboratory for Plasma Physicsand Space Science at the Massachusetts Institute ofTechnology, the Co-Principal Investigator; DanSchwartz and Rodger Doxsey, Sr. Project Scientists forthe Smithsonian Astrophysical Observatory and

Massachusetts Institute of Technology, respectively,who assisted in the alignment and testing of theinstrument; Dave Haerison, Jim Sanders, and JohnD'Angelo who designed most of the electroniccircuits; Franz Keller, our outstanding packagingengineer; Dave Boyd and Marty Pitasi, who providedthermal designs; Joe Grigas and Mary Veronelli, whoturned schematics into hardware designs, WalterAnzalone, Frannie Maurice, Barbara Fitzgerald andDick Tofuri whose skills were used to wire andassemble most of the instrument; Mike Sawczuk, whofaithfully assisted In the interconnection designand testing of all modules; Phil Gray, the ProgramManager, who guided the program through some verydifficult final stages, and many others includingVaman Bawdekar, Sal Briguglio, John Clark, DaveCipolle, Dan Dennis, Arthur Ead, Lou Fiore, ShelbyHildebrand, Bruce Holmes, Shirly Kilkelly, Mark Levine,Roy Miller, Terry Pintal, Jim Silva, Guy Venuti, BernieWasserman and Wil Wilson, all of AS&E, and RickDower of MIT.

Particular thanks go to our good natured secretary,Barbara Johnson, who not only helped us generatethe reams of paper that accompany a program of thistype, but then cheerfully continued on and assembledthis paper into its final form.

References

1. H. Bradt, G. Garmire, M. Oda, G. Spada,B. V. Sreekantan, P. Gorenstein and H.Gursky, the Modulation Collimator in X-RayAstronomy, Space Science Reviews, Vol. 8,pp. 471-506, 1968.

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