Adv. Space Ree. Vol.2, No.4, pp.39—47, 1983 0273—1177/83/04003909$04.50/OPrinted in Great Britain. All rights reserved Copyright © COSPAR

ADVANCED POINTING SYSTEMS FOR

SPACE ASTRONOMY

W. A. Delaniere,C. A. HendriksonandD. E. Schneible

Ball AerospaceSystemsDivisIon, Boulder, Colorado,U.S.A.

SUMMARY

Requirements on image stability are increasing, often at the same time thatinstrument external disturbances are increasing. Pointing large diameteroptics from the shuttle b*y is the prime current example of this situation.In order to achieve cost—effective advanced pointing systems in the face ofthese problems, a system approach must be taken that encompasses a realisticassessment of requirements, the best possible detector technology, and abroad look at space vehicles and pointing systems that are available. As anexample, a rocket instrument for making measurements of the interstellar gasuses a standard pointing system to achieve a spectral resolution of 2 x 10’.

POINTING SYSTEMREQUIREMENTS

A clear definition of pointing requirements in terms of stability, accuracy,and integration ,time is important. These factors are frequently confusedand time spent in defining (and negotiating~) these quantities early in a4evelopment is time well invested. Over—stated requirements can greatlyincrease costs and even jeopardize a program.

The two basic requirements for space astronomy instruments are that oneknows where one is looking (accuracy) and that the image is not blurred bymotion (stability). Both of these parameters are components of system erroras shown in Figure 1. Accuracy is affected by such things as sensorcalibration, co—alignment errors, and knowledge of the location of referenceobjects. For some instruments, such as those with very smallfields—of—view, accuracy is very important; but for others, it may not be ascritical as stability. Furthermore, pointing accuracy can be considered aseither absolute or relative. Absolute accuracy is important for extendedsource targets where measurements are to be compared with measurements madewith qther instruments. For example, absolute accuracy is essential whencomparing radio maps with an X—ray image. For the majority of instruments.relative accuracy is much more important than absolute accuracy. Ingeneral, the term ‘pointing accuracy’ means relative accuracy.

Stability requirements are the measure of how much motion is allowableduring the observation or integration time. If integration time is veryshort, the stability rate or peak slope of the motion may be the mostcritical parameter. More typically, integration times are long enough thatthe RMS value of the motion during the integration time is the significantparameter. For normally disturbed error sources, this corresponds to theone—sigma value. If integration times are extremely long (e.g., severalhours) the slower moving error sources, which are usually called driftmust also be included to obtain a total RMS value and these may be thedominant values.

The three parameters of accuracy, stability, and integration time areclosely rølated. They must be carefully evaluated and defined early in anydevelopment program.

40 W. A. Delaxoare, C. A. Hendrikson and D. E. Schneible

SLOPE STABILITY RATE

ERROR (NOT ALWAYS IMPORTANT) — ,~

~SLOPE=DRIFT

(E.G.. GYRODRIFTOR THERMAL DRIFTS)

STABILITY (JITTER)— CAN BE DEFINED IN TERMS OF:

1) PEAK—TO—PEAK2) 3o (‘½ PEAK-TO—PEAK)3) lo (=1/3 of 3o VALUES)

ACCURACY(STEADY—STATEERROR)

0 TINE~

Figure 1 Total System Error has Both Short and Long Term Components

HIGH ACCURACYPOINTING

In space astronomy, the accuracy of measurements has improved substantiallyover the years. Measurements can be divided into four categories, spatial,temporal, spectral, and radiometric. The first three have a significantimpact on the pointing requirements. The actual definition of high accuracyis dependent upon the region of the spectrum being measured, the type ofinstrumentation, and the methods of data analysis. Every space missionpresents different challenges in the accuracy requirements. For example,the Solar Optical Telescope (SOT) has a solar absolute pointing requirementof 1 arc mm. and a derived mhort—term stability requirement of 0.06 arcsec. RMS. The Space Telescope has a relative accuracy of 0.01 arc sec. withrespect to the guide stars and a short—term stability requirement of lessthan 0.01 arc sec. RMS. The raw numbers alone do not define the magnitudeof the challenge to the pointing system. In Table 1, some of the past,present, and future missions are compared. This simple tabulation shows thevariability of requirements.

SKYLAB SPACEOSO ATM SOT OAO TELESCOPE HEAO SIRTF AXAF

TYPE SOLAR SOLAR SOLAR STELLAR STELLAR~ STELLAR TELLAR STELLAR

STATUS FLOWN FLOWN PLANNED FLOWN IN FLOIdN ONCEPTUAL CONCEPTUAL

DEVELOPMENT

POINTING SYSTEM — COARSE SPIN SKYLAB SHUTTLE N/A N/A N/A SHUTTLE N/A

VECTOR withCHGS

- INTERMEDIATE N/A N/A AGS OR N/A N/A N/A ~GS N/AIPS

-FINE 2—AXIS 2.AxIS INTERNAl SPACECRAFT SPACECRAFT SPACECRAFT INTERNAL SPACECRAFTCG. C.G. IMCGIMBAL GIMBAL

ACCURACY — ABSOLUTE 60 TBD 1

-RELATIVE 3 2.5 60 0.04 0.01 0.15 0.5

STABILITY 1 ‘0.5 0.06 0.04 TBD (1 0.1 TBD

(‘0.01) (0.5)

All numbers in arc seconds

Table 1 Pointing System Performance of some Astronomy Missions

Advanced Pointing Systems 41

POINTING SYSTEMTYPES

Here, the science instrument and a star tracker are fastened to a spacecraftand the entire assembly is pointed in the desired direction. This approachhas worked tell for low resolution systems such as the High EnergyAstronomical Observatories. It will be used for the Infrared AstronomicalSatellite (IRAS) and for ROSAT, the German/British X—ray Satellite.

Here, the science instrument is mounted on a pointed assembly that ismounted on a spacecraft. Two or three independent pointing systems may beused. The planned Solar Optical Telescope (SOT) will be attached to theshuttle via a pointing system. The shuttle has its own independentorientation system, and the SOT has an internal image motion compensation(IMC) system. This is illustrated in Table 1 with coarse, intermediate andfine pointing systems for some of the missions..

In this case, the science instrument is the satellite, the sensor isinternal to the instrument, and the pointing system moves the entiresatellite. Copernicus (OAO) used this method and achieved relative accuracyof 0.04 arc sec. Space Telescope will also use this technique and, presentestimates predict a reistive accuracy of 0.01 arc seconds.

ERROR SOURCES

A typical control system is illustrated in Figure 2. The target and thereference (guide stars) maybe different. The first error source is thatthe absolute positAon of the reference is only known to the limit of currentobservational techniques. The best that can be done is to butte the targetwith respect to the guide stars. In the case of Space Telescope, if thetarget is outside the instrument entrance aperture, the entire telescopewill scan the error zone until the target is detected by the instrument.Hence, the relative accuracy between the target and references will beestablisned.

REFERENCE ATTITUDESTARS

CONTROL

TARGET SYSTEM

Figure 2 The Elements of a Pointing Control System

Some targets have absolute position errors that are temporally varying. Inthese cases, the error could be inconsequential if the target is within thefield—of—view of the instrument or disastrous if outside the field—of—view.

The star/sun tracker errors are usually the dominate errors in the systemdesign. The magnitude of these errors depends upon the field—of—view of thetracker, the magnitudes and number of stars being tracked, the update rate,and the collecting efficiency of the optics. To minimize these errors,different field—of—view trackers are often used on a single mission forcoarse, intermediate, and fine guidance sensors. The errors within a given

42 W. A. Delamare, C. A. }Iendrikson and D. E. Schneible

tracker are caused by optical and electron optic distortion, stability ofoptical and electron optical paths and signal—to—noise ratio performance.The characteristics of some star trackers and sun sensors are shown inTable 2.

Alignment of the components in the complete system are one of the mostcommon error sources. Co—alignment between the tracker and the instrumentchanges as a function of launch and temperatures. Internal structureswithin the instrument cannot be considered as rigid for very high accuracyinstruments as minor temperature differentials can cause significantmis—alignment errors.

NASA SHUTTLE ATM ESASTD STAR STAR DIGISTAR FSS TPD HASS

TRACKER TRACKER

FIELD OF VIEW DEG 8 10 3 2.5 2

RELATIVE ACCURACY

ARC—SEC. 6 ‘40 3,Li 2 1

Z FOV 0,021 0.1 3.00’4 0.022 0.01’4

SHORT TERMSTABILITY

ARC.SEC. R,M.S. 15 18 0.1 0.1 0.02

UPDATE TIME SECONDS 0.1 0.OD 1 0.1 0.2

SENSOR IDT lOT CTD SI CTD

Table 2 Star Trackers and Sun SensorsIllustrating the difficulty of comparing performances

IMAGE STABILITY

The short term stability of the system determines the optical quality of thespatial or spectral image. This stability is best expressed as the rootmean square of the deviation of the telescope axis during the integrationtime of the instrument. The pointing system must provide this stabilityand, in some cases, this can require a two or even three—stage arrangement.This is illustrated in the logic chart, Figure 3.

SINGLE STAGE SYSTEMQUIET SPACICRAF~

• 1RKE-FLY~~AO,b1)s’rABn~REQUIREMENTS

NOISY SPACRCRâPT• MANNED SPAC~RA7r~Y~B,SBUTI1~• DUAL STAGE SYSTEM

GOOD ISOL&TING POINTER• C.G. ~OU1~flOSOs,SrIIAB/Arg)

POOR ISOLLTING POINTER• END PR~TSR(1PS,~GS) VER~ER

THREESTAGESYSTEMS

IIiAGE NOTIONGO~PENS&TION(SOT,SIRTF)

INCREASING COMPLEXITY AND COST

Figure 3 Fine Pointing May be Achieved with One,Two, or Three Stage Pointing Systems

Advanced Pointing Systems 43

In a free—flying spacecraft, disturbances can usually be kept low enoughthat only one stage is required to achieve fine pointing. That is, thespacecraft itself is kept stable enough to satisfy all requirements. Ifdisturbances from internal moving parts can be kept low and ifhigh—resolution, proportional torquing is used, the requirements of evenlarge diameter optics can be met. Space Telescope and OAO are examples.

If the spacecraft is ‘noisy’ such as a manned spacecraft (crew motions), orhas an impulsive reaction control system, or is a spinner with less thanperfect balance, then a second—stage gimbal or isolation system is requiredto achieve fine pointing. The first stage then performs only intermediatepointing. On spacecraft such as the Orbiting Solar Observatories (OSO) andSkylab, the instruments were clustered in well—balanced center of gravitymounts and thus very well isolated from disturbances. The instruments onSkylab/ATM were sensitive to about 0.5 arc—sec and. showed no degradation atall. The OSO instrument package showed stability of about 1 arc—sec in thepresence of 0.5 Hz inputs of many arc—minutes.

If the demands for a ‘universal’ system dictate the use of an end—pointingsystem such as IPS or AGS, stability will not be good enough for most largeoptical systems. In these cases, the designer must add a thirdsupplementary system. This can be a vernier gimbal such as the AnnularSuspension Pointing System (ASPS) that is being developed or can be aninternal image motion compensation (IMC) system. Examples of the latter canbe found in the conceptual designs of SOT and SIRTF. The feedforwardsystems that are being added to AGS and IPS in which external disturbancesare sensed by accelerometers and fed forward into the servo loops can alsobe thought of as a third stage of pointing. In these examples, the firststage does coarse pointing, the second stage does intermediate pointing, andthe third stage achieves fine pointing.

Admittedly this is a simplistic view of pointing system selection logic.Many other factors come into play in any realistic situation. However,examples exist of all these levels of complexity and, ineyitably, costsincrease with complexity. The total system must be considered at the outsetof a design.

REDUCING POINTING INSTRUMENT ERRORS

Many methods can be and have been used to reduce pointing errors. The firstgeneral technique is to improve the performance of the sub—systems. One cantry to obtain better trackers, build more rigid structures and make tighterattitude control systems. This approach results in an exponential costgrowth as a function of performance. Finding methods of reducing therequirements is a more satisfactory approach. For example, the trend overthe years has been to go to longer integration times as we look at faintertargets. This has made the stability problem more severe. If new methodscan be used to obtain shorter integration times for data collection thenexisting pointing system technolgy will give improved accuracies. Manyother requirements can be reduced by adding an internal control loop to theinstrument.

For future systems, there are many new technologies available to be used increative ways.

o — Weight and power are no longer primary drivers. Thismeans larger optics, more rigid structures, and more complex electronicscan be used. The biggest improvement is in the dramatic increase intelemetry rate that the Tracking and Data Relay Satellite System (TDRSS)has made possible. Using KU band, the TDRSS data rate is 300 Mblsec.

o I~2kers — New solid—state trackers have demonstrated resolutions of0.01 pixels. See Figure 4. A sun sensor developed by ESA can track theradiometric centre of the sun with a long—term stability of 0.025 arcsec/lO minutes.

o D~.ti~.tg~.g — Steady progress has been made in 2—D imaging photon countingdetectors. The Intensified CCD with a format of 32Ox256 pixels canoperate at an integration time of 0.03 seconds. The flexible formatMulti Anode Microchannel Plate Array (MAMA) is available with 256x1024pixels and a 1024x1024 array is in development. As the MAMAhas a pulsepair resolution of about l~ss per event, very fast position correctionalgorithms are possible.

44 W. A. Delamare, C. A. Hendrjkson and D. E. Schneible

o £~x2ni~i — The rapid advance of microprocessor technology hasprovided opportunities for onboard computing that is only limited by ourimaginations. 10’ or 10’ bit random access memories can be consideredfor future missions. New memory management technology permits deadmemory cells to be circumvented which opens the way to 10’ or l0~° bitmemories. Larger memory capacity allows for rapid position correctionin close to real time with photon counting detectors.

o — On board 2—dimensional image processing can becontemplated. It is not clear what advantages can be gained for thepointing system. One idea is for a solar pointing system to image asmall area of the sun in H—Alpha and to transform to the fourier domainto obtain very accurate error signals. This type of approach would beapplicable to a slit jaw camera system.

o — The Tracking and Data Reliy Satellite System makesground computers more accessible to the spacecraft. It is possible toinclude a ground computer in the pointing loop to perform absolutepointing correction calculations and to improve long—term stability.

o I1LL1~Qp.ti~I — This technology is maturing well for a number offields. Can it be used to further reduce pointing system errors?

I I I I I I I I I I I

125.1

xx xxx x

128.1 x Xx x

X

124. X ICIC IC

IC IC124. x x

IC IC

IC IC CENTER S.D.—l.6l4%

122.1 sICxxx xx

121. I x ~ ~ x

I I I I I I I I I I I I

128.00 128.81127.00 127.81128.00128.58128.OS 123.50150.00130.81131.00131.00

Figure 4 Sub—pixel Performance of the Digistar TrackerAn image was moved in a two pixel diameter circle on the CID

LIMITATIONS TO HIGHER ACCURACY AND STABILITY

There is always a continuing pressure for higher spatial, spectral andtemporal resolutions in all parts of the spectrum. At one time, it wasthought that the physics of Solar Flares could be resolved if an instrumenthad a spatial resolution of a few arc seconds in the ultra—violet and atemporal resolution of tens of seconds. Now a few tenths of an arc secondand a few seconds time response appear necessary.

The real limitations that we have for future experiments are probably nottechnical in nature. The first constraint is funding. Space Telescopepromises magnificent performance, but its cost is in excess of 500M. Fourexciting new programs are Solar Optical Telescope (SOT), Advanced X—rayAstrophysical Facility (AXAF). Shuttle Infrared Telescope Facility (SIRTF),and Starlab, but all of them are seriously funding limited. Only SOT isclose to starting.

Another limitation to higher accuracy is the clustered facility concept.Pointing accuracy and stability are compromised by having to provideaccomodation for many different types of instruments. A dedicated systemwith a single purpose will produce better performance. Facilities arepolitically popular because many people can share in the program. The use

Advanced Pointing Systems 45

of shuttle as an instrument platform is creating a host of new problems forhigh resolution systems. The difficulty in developing special pointingsystems to remove shuttle motions, such as IPS and AGS, has beensignificant. The Space Platform will provide a good base for solar pointedinstruments as the solar panels of the power system are always sun looking.For other astronomy instruments, an intermediate pointing system isnecessary.

The last limitation is the difficulty of coordinating the design of amulti—discipline system. Pointing system engineers rarely design telescopesand instruments. It is important to study the error sources in relation tothe alternative approaches possible. Most trade—off studies reflect thebiases of the people doing the study.

A LOW COST EXAMPLEOP ACHIEVING HIGHER ACCURACY

The Interstellar Medium Absorption Profile Spectrograph (IMAPS) is a highresolution ultra—violet echelle spectrograph that will be flown on a rocketat the end of this year. See Table 3. This spectrograph has a spectralresolution of 2.4x10’ that could have required extremely stringent pointingrequirements. The instrument consists of a mechanical collimator, anechelle grating, a cross disperser grating, and an intensified CCD detector.See Figure 5.

PERFORMANCEREQUIREMENTS SPECTRAL COVERAGE 90-120 NM

FORMAT ECHELLOGRAM

SPECTRAL RESOLUTION 2.4 x 1O~NM

SPECTRAL SEPARATION 0.1 NM

SPATIAL RESOLUTION 3.3 ARC. SEC.

CONSTRAINTS - ROCKET FLIGHT

- LOW COST

POINTING REQUIREMENTS ABSOLUTE ACCURACY N/A

RELATIVE ACCURACY + 3 ARC. MIN.

LONG TERM STABILITY 10 ARC SEC/SECOND

SHORT TERM STABILITY

Table 3 Interstellar Medium Absorption Profile Spectrograph

IMAPS could use the obvious approach of integrating the signal in thedetector and reading it out every few seconds. We found that it would benecessary to add an internal image motion compensation system to overcomethe limited performance of the rocket pointing system. This approach wasbeyond the funding limitations of the program. The alternative approachthat is being implemented uses the detector in a fast readout mode and theS—band transmission link to transfer the raw data to the ground. A smallmirror on the echelle reflects light from the target star to a non—ruledarea on the focusing cross disperser and forms a star image on the detector.The detector is read out with a standard television format and is recordedon the ground. The rocket pointing system is being modified to open up thelimit cycle and to reduce the reaction jet thrust. The angular velocity ofthe payload will be below 10 arc sec/second, so in 0.03 seconds (theintegration time) only 0.1 of a pixel motion will occur. The short—termstability will be negligible as the integration time is so short and thepayload inertia is so high. Because the pointing signal is an integral partof the scientific data, the image motion will be removed in a computer onthe ground. Another major advantage of this approach is that the detectoroperates at ambient temperatures instead of being cooled to —90C.

This solution has permitted the rocket program to take place. Without itonly a much lower resolution instrument could have been flown within theCost constraints.

46 W. A. Delamare, C. A. Hendrikson and D. E. Schneible

CROSS PHOTON COUNTINGSTAR DISPERSER DETECTOR

TRACKER

MECIIANICALCOLLIMATOR TV FRAME RATE

TRANSMISSION

GROUND IMAGEMOTION CORRECTION

POST FACTOFigure 5 Ground Processing Simplifies Pointing Requirements

for a High Resolution Spectrograph

CONCLUSION

Higher resolutions can be achieved in space as there are no fundamentaltechnical limitations. The scientist desiring the higher performance shouldactively participate in the design of the experiment. In particlar heshould specify carefully ~he basic parameters of accuracy (absolute andrelative), stability, and integration time, and he must communicate theseclearly to the system designers early in the program.

Higher temporal resolution may be achieved using high sensitivity photoncounting detectors and larger collecting apertures.

Higher spectral resolution for stars is easier to obtain than for extendedtargets. These normally require improved spatial resolution as well.

The spatial resolution improvement requires a benign environment.Decoupling the disturbances of a multi—use spacecraft can be exceedinglydifficult and expensive. The use of a free—flying instrument avoids thehigh frequency components that degrade the instrument resolution. The fineguidance sensor must share the focal plane with the instrument detector.

ACKNOWLEDGEMENTS

Ed Jenkins of Princeton University is the Principal Investigator for theIMAPS rocket instrument and it has been under his inspiring leadership thatthe creative ideas for pointing have been generated. John Lowrance and PaulZucchino, also at Princeton, and Murk Bottema, at Ball Aerospace, madesignificant contributions to the pointing system design.

The IMAPS program is funded by the NASA.

REFERENCES

1. Shuttle Pointing of Electro Optical Experiments. Proceedings

of the S.P.I.E. Volume 265 February 1981.2. The Advanced X—Ray Astrophysics Facility. •M.V. Zombeck, SmithsonianAstrophysical Laboratory. Report SAO—AXAF-~8OOO7.

3. SIRTF Fine Guidance Sensor Imsger Study. NASA/AMES Report 901—18January 1982.

4. Apollo Telescope Mount of Skylab: an overview. R. Tousey Applied

Advanced Pointing Systems 47

Optics, Vol. 16, no 4, April 1977.

5. Solar Optical Telescope Observatory. NASA Request for Proposal5—63469/279. February 1981.

6. HEAO—3Post Launch Evaluation Report. TRW report no 26000—630—031October 1979.