THE ASTROMETRIC TELESCOPE FACILITY

David Black, John Dyer, Kenji Nishioka, Jeffrey Scargle, and Charlie Sobeck, NASA AMES Research Center, Moffette Field, CA Eugene Levy and Robert McMillan, Lunar and Planetary Laboratory, Univ. of AZ Michael Castelaz, George Gatewood, and John Stein, Allegheny Observatory, University of Pittsburgh Andrew Buffington, University of California at San Diego

SUMMARY

The Astrometric Telescope Facility (ATF) proposed for use on NASA's planned Space Station is similar in form and data output to ground-based long focus astrometric instruments. With a focal plane scale of 12.7 arc seconds/ram, the strawman design has a field size of 10 arc min square and a limiting visual magnitude fainter than 16. Output from an observation includes the X and Y coordinates of each star and its relative brightness. The targeted precision for the AFT is 0.00001 arc seconds.

Portions of the observing program will be made available to members of the astronomical community.

INTRODUCTION

The Astrometric Telescope Facility (ATF) will provide the astrometric community with an instrument with an internal relative positional precision of 0.00001 arc seconds. Designed and built primarily for the discovery and characterization of planetary systems, the instrument is similar, in general form and in the nature of its data output, (X and Y coordinates for the target and each of the 30 reference stars) to the long focus instruments now widely employed in ground-based astrometry.

Because the application of the ATF could obviously extend well beyond the task for which it was initially conceived, the design of the strawman instrument has been made sufficient to achieve both its original goals and to provide a significant fraction of its observing time for general astrometric studies.

BACKGROUND

The ATF evolved out of a long series of NASA sponsored workshops, systems studies, and reviews. The earliest workshops (six SETI meetings between 1974 and 1976) reviewed existing planet detection capability and examined likely near-term instrumental improvements. These were followed in 1976 by two additional workshops on ground-based techniques (Greenstein 1976) and the summer study "Project Orion"

Astrophysics and Space Science 177:307-313; 1991. �9 1991 Kluwer Academic Publishers. Printed in Belgium.

308 D. BLACK ET AL.

(Black 1980) that considered a ground-based astrometric interferometer. Various workshops, at which a wide variety of instrumental techniques were examined, continued through the next several years. A summary of the ground-based- instrumentation studies is given by Black and Brunk (1980). During this time the basic concepts of the strawman design took form and the characteristics of the focal plane detector it employs were laid out (Gatewood et al. 1980). In 1982, NASA sponsored a comparative study (by Lockheed) of a spaceborne astrometric telescope and a spaceborne astrometric interferometer (Banderman et al. 1983). The study concluded that the telescope was feasible while the interferometer was too complex.

In 1983 and in 1985 workshops were held to refine the basic science requirements for the astrometric telescope. These included a review of the instrument's astrophysical capability. Lockheed completed an advanced project study of the astrometric telescope in March 1984. In January of 1985 the University of Arizona and the NASA Ames Research Center signed a memorandum of understanding forming a joint team to advance the ATF project.

The ATF has been reviewed by a number of peer group panels, including: the Committee on Space Sciences and Astronomy; the Committee on Planetary and Lunar Exploration; and the Solar System Exploration Committee. In each case it has been agreed that the ATF is a scientifically feasible and desirabie instrument.

The most complete description of the system to date is the two volume, "Astrometric Telescope Facility: Preliminary Systems Definition Study", NASA Technical Memorandum 89429 (Sobeck 1987). A number of other descriptions of various aspects of the ATF project have also been published. These include descriptions of the science and instrumentation (Levy et al. 1986, Dyer et al. 1987, Levy et al. 1987, Scargle et al. 1987) and descriptions and testing of the focal plane detector (Gatewood 1987, Gatewood et al. 1987).

THE STRAWMAN DESIGN

Limiting the precision of the planned instrument are: the errors arising from random variations in the photon count rate; the subjugation of the optical components to environmental changes; unmodeled changes in the positions of the reference stars; and astrophysical phenomena which shift the photocenter of the target.

The latter, star spots for example, would cause small but perceptible shifts in the photocenter of a star. The effects of large spots on nearby stars would be observable. The photocentric motions that these would cause are distinct in that they have simitar, probably short, periods and show for approximately one-half of a rotation. Analysis of their motion will reveal; the stellar rotation rate; direction of rotation;direction of the axis or rotation; and the latitude of the surface feature. Large numbers of spots would cause a quasi-random variation in the apparent position of the star that might mask the perturbative effects of small planets in small orbits.

Without proper modeling, the nonlinear motion of a reference star can cause an apparent motion in the target. At the precision of the ATF, each galactic reference star will exhibit parallactic and proper motions. Indeed, many will reveal the orbital effects of unseen companions. However, these motions can be easily detected by studying each reference star individually. Then, once a nonlinear behavior has been detected, that reference star can be removed from the frame until its motion is sufficiently well understood.

Characteristic

ASTROMETRIC TELESCOPE FACILITY

TABLE 1 THE ASTROMETRIC TELESCOPE FACILITY

Value

309

Aperture

Wave Length Range

Focal Length

F Ratio

1.25 meters

4000 to 9000 A

16.25 meters (prime focus)

13.0

Focal Plane Scale 12.7 arcsec/mm

Mirror: Design Substrate Surface quality

Field Size 10.0 X 10.0 arc minutes

Prime focus paraboloid Ultra low thermal expansion 1/20 wave at 4400 A

Mount: Type Motion:

Articulated, from down, to perpendicular to velocity stream

Setting accuracy: Absolute Relative

Guiding accuracy: Vibration Isolation

+ or - 1 arc minute + or - 3 arc seconds + or - 1 arc second 40 decibels

Tube: Type epoxy graphite Structure 1.89m by 21.49 m

Detector MAP Ruling:

Type transmissive, 50 lines/mm Active area 50 X 675 mm Motion: scan rate 10 line pairs/see rotation 270 degrees

Substrate fused silica Sensors CCD or diode arrays

Perhaps the best understood of the limitations to astrometric accuracy is photon statistics. Being purely random in their effect, one need only collect more photons to overcome this limitation (Gatewood et al. 1985).

Finally, there are the limitations dependent upon the design and engineering of the

310 D. BLACK ET AL.

instrument itself. At first glance these would seem to be the bane of the entire effort. However, if we return to the basic principles used in current long-focus astrometry, and if we are careful about our execution of these principles, there is a very real likelihood of success.

Not unlike existing long-focus astrometric systems, the strawman design owes its potential precision to several factors:

1. The use of a single stable optical system for the simultaneous formation of the images of the target object and a moderate number of reference stars;

2. The placement of all major optical components at the pupil of the system; 3. Simultaneous focal plane detection of the apparent relative positions and

brightnesses of the target and reference stars; 4. The reduction of the observations using conditional equations with sufficient

flexibility to model the significant effects of the conditions affecting the optical parameters and star positions.

Conditions 1 and 2 determine the character of the strawman's optical system, a single paraboloid with the mirror shaded so that each star fully illuminates every portion of the mirror. This single surface is the system's pupil, thus optical variations effecting the apparent position of the target object also affect the apparent positions of the reference stars. In this way, the effects of such variations may be modeled out using parameters as simple as a change in the zero point or field scale of the observation. Condition 3 applies to the nature of the detector, in this case a MAP (Gatewood 1987). Condition 4 is familiar to all astrometrists and is here, as always, applied at the full precision of the system.

I & ~ I MISSION AND SYSTEM DESCRIPTION AND REQUIREMENTS

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ASTROMETRIC TELESCOPE FACILITY 311

A schematic of the so-called Strawman design (the initial design to pass all of the conceptual and engineering criteria set for the ATF) (Table 1 ) is shown in Figure 1. The primary mirror is the only optical component between the Ronchi ruling and the reflected star field.

The Ronchi ruling is coincident with the focal plane and is driven slowly across it during an observation. The modulated light is then reimaged to the side of the tube, via relay optics. Fiber optic pickups then direct each cone of light onto a prism and then into a set of moderate width bandpass detectors.

Breaking the spectrum of each star into several bands lessens the instrument's susceptibility to color effects. These arise out of the variation in the diffraction spot size and the system's slight coma. In the strawman design, the effect is moderated considerably bythe use of 2 broad bands covering a range of 5000 Angstroms. Under consideration is a modification which employs 10 bands (in each of which every star has virtually the same effective wavelength) to essentially eliminated the effect of "color coma" (Kovalevsky 1984).

As currently visualized, the proposed instrument requires state-of-the-art electronics and advanced detector technology. But it does not depend upon the development of any new technologies. Indeed the proposed system utilizes detectors, electronics, and computer systems that are similar to those now in use on the Thaw Refractor at the Allegheny Observatory.

Mounting the ATF on NASA's planned Space Station has a number of advantages, the most obvious is the relaxed size requirements of the focal length as well as aperture. An on-station instrument is also more serviceable, both in case of failure and from the standpoint of refurbishment. This greatly reduces the design costs since backup systems may be implemented very simply, minimizing system complexity and cost. In addition, the instrument will not require its own telemetry. Power as well as coarse pointing is also provided.

Unlike the Hubble Space Telescope, pointing and guiding are not very critical, nor is the ATF very sensitive to the vibrations of a manned station. All of these points result in an instrument that is a predominantly scientific payload and is thus more adequate for the task for which it was designed.

CONCLUSION

The scientific potential of the ATF almost transcends intuition. Consider for example what tangential motions are greater than ten microarcseconds: a tangential displacement of a light year at the edge of the known Universe; a one astronomical unit perturbation as seen from the other side of the galaxy; the annual parallax of any star in the galaxy; or the signal imparted to the motion of virtually any star within 500 parsecs by a planet of the mass and period of Jupiter. obviously the ATF has wide potential applications in the various problems of distance and mass calibration, some examples are listed in Table 2, but its most profound applications may come in areas not yet obvious to current potential users.

312 D. BLACK ET AL.

TARGET PARAMETER

TABLE 2 SOME OBSERVABLE QUANTITIES

ASSUMED AFT VALUES dist/error

parsecs

stars parallax

IR dwarfs frequency

Jovian planet frequency

star clusters internal motion

galactic dynamics -

proper motion

star-Jupiter light bending

star spots rotation, etc.

x-ray stars, black holes,

quasar photocentric

shift

very distant reference points

primary mass = 1 dwarf mass = 0.02

orbit of 2 AU

mass = 0.001 suns orbit of 5 AU

10 year proper motion study

5000/5%

400/10%

125/25%

65000/10%

30 km/sec 200000/10% quasar references I

averages 1 in 4 fields

near occultation

0.0001 AU photocen, shift

0.1 AU photocentric shift

intensity related shift of 0.01 parsec

0.5%

5/50%

1000/10%

50 m pc/50%

The values assumed are given in column two. The values listed in the last three columns are only meant to suggest the potential of the instrument.

REFERENCES

Bandermann, L., Bareket, N., and Metheny (1983). "Comparative feasibility study of two concepts for a space based astrometric satellite", NASA Contractor Report 166403.

�9 Black, D. C. (1980). Project Orion, A Design Study of a System for Detecting Extrasolar Planets. NASA SP-436, 174.

ASTROMETRIC TELESCOPE FACILITY 313

Black, D. C., and Brunk, W. E. (1980). "An Assessment of Ground-Based Techniques for Detecting Other Planetary Systems", NASA Conference Publication 2124 (in two volumes).

Dyer, J. W., Nishioka, K., Sobeck, C. K., Gatewood, G., Levy, E. H. (1987). "Conceptual Design Considerations for the Astrometric Telescope Facility", IAU Colloquium 100, Fundamentals of Astrometry (edited by I Pakvor and H. K. Eichhorn).

Gatewood, G. (1987). "The Multichannel Astrometric Photometer and Atmospheric Limitations in the Measurement of Relative Positions", Astron. J., 94, 213.

Gatewood, G., Breakiron, L. A., Goebel, R., Kipp, S., Russell, J. L., and Stein, J. W. (1980). "On the astrometric detection of neighboring planetary systems I1", Icarus, 39, 205.

Gatewood, G. D., Castelaz, M. W., Stein, J. W., Levy, E. H., McMillan, R. S., Nishioka, K., Scargle, J. D. (1987). "A Prototype Detector for the Astrometric Telescope Facility", IAU Symposium 133.

Gatewood, G., Stein, J. W., DiFatta, C., Kiewiet de Jonge, K., Breakiron, L. (1985). "A Preliminary Look at Astrometric Accuracy as a Function of Photon Counts", Astron. J., 90, 2397.

Greenstein, J. (1976). "Minutes - Second Workshop on Extrasolar Planetary Detection", NASA-Ames Research Center.

Kovalevsky, J. (1984). "Prospects for Space Stellar Astrometry", Space Science Reviews, vol. 39, 29.

Levy, E. H., Gatewood, G., Stein, J. W. and McMillan, R. S. (1986). "Astrometric Telescope of ten microarcsecond accuracy on the Space Station", Proceed of SPIE 628, 181.

Levy, E. H., McMillan, R. S., Gatewood, G. D., Stein, J. W. Castelaz, M. W., Buffington, A., Nishioka, K., and Scargle, J. D. (1987). "Discovery and Study of Planetary Systems Using Astrometry from Space", I~,U Colloquium 99.

Scargle, J. D., Nishioka, K., and Gatewood, G. (1987). "NASA's Astrometric Telescope Facility", IAU Symposium 133.

Sobeck, C. (1987). "Astrometric Telescope Facility: Preliminary Systems Definition Study", Vol I and II, edited by. NASA Technical Memorandum 89429.