Acra Astronautica Vol. 38. Nos 4-8, pp. 423425. 1996 Published by Else&r Science Ltd

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k, UR’P METAL MIRROR FOR OPTICAL MEASUREMENTS OF ORBITAL B

Paper No. IAA-95-IAA.6.3.05

Andrew Potter NASA Johnson Space Center, Houston, Texas and Mark Muirooney Lockheed Engineering and Science Company, Houston, Texas

Abstract

A telescope employing a liquid metal mirror that is three meters in diameter has been developed for the purpose of measuring the population of small orbital debris. The telescope has been installed at a location in the mountains of New Mexico, and is undergoing finat engineering tests. The limiting stellar magnitude of the telescope has been measured to be 21.5, with resolution of 1.8 arcsec. The highest sensitivtty for orbital debris detection is achieved by reading out the CCD detector at the same rate and in the same direction aa the debris object crosses the field of view of the telescope. In this mode, the telescope is canable of detectina debris as small as 2.5 centimeters at 900 & altitude, and less than 10 centimeters at geosynchronous altitudes. Improved performance for LEO altitudes might be achieved by the use of fast readout detectors.

‘Introduction

Radar measurements of the small orbital debris population using the Haystack radar have been under way for about four years. This radar can detect debris as small as 1 cm orbiting at 500 km. The measurements are statisticaf in nature, giving information about the debris flux at various altitudes? but without detailed orbit information for each ObJect that is detected. We have known for some time that radar and optical techniques do not see exactly the same debris population - some objects have hieh ontical reflectivities. but low radar refiectivities, Gd &e versa. Consequently, to get complete picture of the debris environment, we need to supplement the radar measurements wtth optical measurements. The best way to do that would be simultaneous radar and optical measurements.

Lacking that, we can measure the optical and radar populations separately, and compare their size and altitude distributions. Comparing those two sets of data will give a better understandiig of the orbital debris environment, knowledge that IS important for orbiting spacecraft, including International Space Station.

the planned

Aside from the obvious disadvantage of requiring dark, clear skies for operation, optical telescopes offer some advantages over radars for orbital debris

This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States * Chief. Space Science Branch

measurement. Telescopes can detect objects in high orbits, such as geosynchronous orbits, with smaller collecting apertures than radars. Objects in such high orbits can be observed most of the night. However, this is not true for objects in low earth orbit (LEO). For objects orbiting in LEO, observations are limited to a few hours after sundown and before. sunrise. In addition, the angular velocity of objects in LEO is high relative to those in geosynchronous orbits, which limits the integration time of the signal from the orbiting object.

Aomorch

A very large optical collecting aperture is needed for an optical telescope to approach the sensitivity of the Haystack radar. At least a three meter mirror is needed for this purpose. The cost of such a mirror fabricated from glass is conservatively estimated at five million dollars. Recent advances in mirror technology using a rotating pool of liquid mercury promised a low-cost alternative to a large glass mirror’. Liquid mercury mirrors cannot be steered or pointed.. However, this is not a problem for statistical measurements of the orbital debris population. function like

The liquid mercury telescope can

orbital debris the Haystack radar used to monitor in low Earth orbit, by ‘Staring” at a

particular slice of space. Operated in the ‘Staring” mode, the telescope observes any object that passes through its fiela of view.

Development of the liquid mirror telescope for orbital debris measurement was started in late 1992, and completed on the NASA Houston site in late 1994. The telescone mirror consists of a oarabolic dish with a diameter of three meters mounded on an air bearing to permit rotation about the central axis of the dish. ADDrOXkitelV twentv liters of liauid mercury are held in the dish.- To geierate the op&al surface, the dish is spun up to a rate of 10 revolutions per minute. Centrifbgal force then causes the liquid mercury to spread out in a thin layer (about _ 2 mm) over the dish, forming a reflectina naraboloidal surface. The rotation soeed is chose; io generate a surface with focal 1enGh of 4.5 meters. Rotation speed is controlled to better than 1 part in 100,000 by a crystal-controlled brushless DC motor. The real key to successful operation of the mirror is the air bearing that the dish rests upon. This bearing is extremely vibration- free. This is the enabling +echnology breakthrough, available only in the past few years. An important factor for use of mercury as the reflecting surface is

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the fact that mercury vapor is a hazardous substance. When mercury vapor is inhaled, it moves directly into the bloodstream, and can affect the central nervous system. As a result, we observe a number of safety precautions when working with liquid mercury. Procedures are in place for dealing with spills. The telescope mkor is covered when not in use, and mercury vapor monitors constantly measure the level of mercury vapor in the air near the telescope. When researchers prepare the mirror for use, they wear gas masks to protect them from the vapors. They remain near the mirror only long enough to prepare the dish and begin it spinning so that the mercury wiil fan out to cr&te the reflective surface. Once the surface is established. the researchers leave the area, and move to another room where the telescope -operation is controlled. Several hours after the surface is established. two things happen. First, the mercury appears to attach Itself to the surface below it, and begms to rotate at exactly the speed of the dish. Before that, the mercury does not rotate at exactly the same speed as the dish. Also, the concentration of mercury vapor drops very substantially, an order of magnitude or more. The latter effect is thought to be due to the formation of a transparent layer of mercury oxide.

The paraboloidal mirror focuses starlight perfectly only to a point. In order to get a large flat focal plane, it was necessary to use a field corrector lens located just below the nominal focal plane of the mirror. Our corrector iens provides a flat focal plane approximately 0.5 degrees in diameter. Specifications of the optics, detector, and data acquisition system are given in Table 1.

For data acquisition, the CCD is rotated to an angle matching the direction of motion of the objects of interest, and is read out at a rate matching their angular motion. For the case that stars are observed, the CCD is oriented east-west, and is read

Table 1. Specifications of NASA 3.0-meter Liquid Metal Mirror Telescope (LMT)

Optics: Mirror Aperture = 3.0 meters Focal Ratio = f/l .49 (primary mirror) Effective Focal Ratio = 81.68 (tier corrective optics) Flat Focal Plane = 0.5 degrees

Detector: CCD Detector, Ford LSP 2048x2048 15 micron pixels Readout rate: 5OOkHz 8.8 seconds to read full frame, no binning 0.6 seconds to read fbll frame 16x16 binning FOV of detector = 0.35 degrees Plate Scale= 0.6 arc set per pixel

Data Acquisitions & Analysis System: Controller: Photometrics VME 220A A/D: Photometrics CE 200A Head: Photometrics CH 220 (Peltier and Liquid Cooled) Computer: Spare IPC Operating System: Solaris Analysis and Acquisition software: IRAF Bus Conversion: BIT3 VME -5 Spare, 1 Mbyte/set Disks: 4.0 GB

out at the sidereal rate. Each star is exposed for 97 seconds, the time required to cross the-O.35 degree field of view of the CCD at the 33 deuree latitude of the Cloudcroft site. A similar procedure is used for measurement of debris objects. The CCD is rotated to the expected direction of motion of the debris, and read out at the angular rate of the debris object. A mechanical chopper is installed in the light path to provide timing signals. Theoretical calculations of the minimum size of debris detectable in this mode yielded values in the 1-2 cm range for altitudes up to 1000 km, even for mismatched angles up to 20 degrees. (Albedos are assumed to be 0.1 for ail objects).

Performance Tests

During 1995, the telescope was moved to a good optical site at a height of 9,000 feet in the mountains near Cloudcroft, New Mexico. Engineering tests and software development are nearly complete. The limiting visual magnitude was measured to be about 21.5. Image sizes for stars were measured to be 1.8 arcsec, fir11 width, half maximum. Measurement of cataloged satellites in low earth orbit have shown that the theoretical predictions for orbital debris detection in LEO were somewhat optimistic. The problem is apparently related to liitations on the readout rate of the CCD chip. In order to read out the chip at rates equivalent’to the angular rates of debris in low earth orbit. it was necessarv to ‘bin” up to 16 pixels together. The result-was that starlight added significantly to the effective noise level in the image, consequently lowering the sensitivity. This effect was particularly sign&ant during the period of these tests (July-August) since the galactic plane was in the field of view for most of the measurements. The result was a limiting size of about 2.5 cm in low earth orbit (900 km) for the existing detector system, as compared for the predicted value of about 1.6 cm at 900 km. The measured limiting size was determined by extrapolation of the signal from cataloged objects in low earth orbit. The limiting size for objects at geosynchronous altitudes remains at values of 10 cm or less, since the slow-moving objects at these altitudes are star-like in their motion. These objects can be measured at the full spatial resolution of the chip (no ‘binning” is necessary), and the performance of the system for star detection is a good measure of the performance expected for satellites at extreme altitudes.

There are several possible approaches to improving the performance of the existing detector system for LEO debris. One approach might be the use of the Hough transform to analyze images collected at the sidereal rate. The Hough transform operates to transform a line in Cartesian coordinate space to a noint in noiar coordinate space. In an image collected at the sidereal rate,- passage of a small satellite across the field of view will leave a streak, but perhaps only at a signal-to-noise ratio of 0.1, comoletelv invisible in the imacte. After a Houah t&foe this line should appear as a detectable point, since ail the points on the line will have been summed*.

Performance of a CCD detector for detection of fast-moving objects can be optimized using the approach defined by Zook et al. The exposure ttme of the CCD to the s is limited to the ‘tiwell time” - the time required k? or the image of the satellite to

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cross one pixel. In this way, the signal-to-noise of the object 1s maximized, while the background signal from stars and the sky is minimized. A series of images are obtained as the object moves across the field of view. These can be viewed as a video sequence, or analyzed by a the three-dimensional equivalent of the Hough transform. Application of this techniaue to the existina CCD will require binning of ‘the pixels to achi<ve sufficiently high readout rates of the detector.

Improvement in performance could also be achieved by use of a CCD detector capable of much higher readout rates, such that ‘binning” of pixels would not be necessary, and it would be possible to operate the chip in a video mode, whereby several short exposures per second could be taken. A fast readout CCD suitable for this purpose has been developed by Williams and Redford specifically for satellite observations.

A low-light-level video detector is also capable of detecting fast-moving objects. Recent advances in microchannel plate image intensifier technology have led to levels of sensitivity and resolution previously unattainableJ. However, the quantum yield of these devices is significantly less than CCDs, which makes them less attractive.

Ho et ~1.~ have been developing a detector system that is optimized for the problem of detectinn fast- moving debris objects. -Their instrument &es a microchannel plate detector as a photon counter. Data from the detector can be used to generate images at any desired time resolution. The image sequences can be stacked into an image cube, in which the satellite signals will fall on lines in three- dimensional space.. The positions and lengths of the lines will depend on the position angles and angular velocities of the satellite signals. The objective of data analysis is to locate the lines formed by satellite passage. This is a formidable computational problem. In principle, this technique could be very effective. Ho et al.’ report some limited success in initial trials of this technique.

Conclusions

It has proved possible to construct a large (3-meter diameter) optical mirror of excellent quality using liquid metal mirror technology. Cost savings were substantial. Combining this mirror with corrective

optics and a large CCD detector, we have develo ed a system is capable of detecting objects as sm 9, 2.5 cm in low earth orbit (900 km), and less than 10 cm at geosynchronous altitudes. Performance improvements for low earth orbit operations are possible with improved detectors.

Acknowledements

Bertrand Freeman of LESCO,. as well as Paul Hickson of the University of Bnttsh Columbia made it all work.

References

I. Hickson, P., B. K. Gibson, and D.W. Hogg, “Large Astronomical Liquid Mirrors” Publications of the Astronomical Society of the Pacific, Volume 105, pages 501-508, 1993.

2. Duda, R 0. and P.E. Hart, ‘Use of the Hough Transformation to Detect Lines and Curves in.Pictures” Commun. ACM, 15, 1, January 1972,

3. Zook, HA. and A.E. Potter, ‘Dptical Detection of Large Meteoroids in Space” In: Prooert es Interactions of Interolanetatv- (R.H. Giiese ‘it P. Lamy, Eds., pages 293-298 (1985). See alsc H.A. Zook, ‘Dn the Optical Detection of Meteoroids, Small Near-Earth Asteroids ant Comets, and Space Debris” In Lunar and Planetary Sci. XIX (1988), The Lunar and Planetary Institute Houston, pages 1329-1330pages 1 l-15 4. Williams, C.. and S.D. Redford. “GEODSS 5. Upgrade Prototype System (GIJPS)“, Proceedings of the 1994 Soace Surveillance Workshoo. S-7 April 1594, MIT Lincoln Laboratory, Projkct Report STK-221, VolumeI, pages 181-189

5. Sturz, R.A., “Advances in Low Light Level Video Imaging” International Symposium on Optical Science, Engineering and Instrumentation, San Diego, CA July 9-14, 1995, Airborne Reconnaissance XIX, ‘95 Conference 2555.

6. Ho; C., B. Priedhorsky, P. Higbie, S. Kleban, K. Tran, ‘Detecting Small Debris Using Ground-Based Photon Counting Detector’: Proceedings of the 1992 Space Surveillance Workshop, 7-9 April 1992, MIT Lincoln Laboratory, Project Repon STK-193, Volume I, pages 161-167