Ground and Space Borne Astrometric Searches and the Multichannel Astrometric Photometer and Spectrograph

George Gatewood
Allegheny Observatory, Univ. of Pittsburgh
Pittsburgh PA

R. McMillan
Lunar and Planetary Lab., Univ. of Arizona
Tucson AZ

A. Snyder Hale, D. Snyder Hale, T. Persinger, T. Reiland
Allegheny Observatory, Univ. of Pittsburgh
Pittsburgh PA

J. Montani, T. Moore, M. Perry, P. Smith
Lunar and Planetary Lab., Univ. of Arizona
Tucson AZ

The precision of both wide- and narrow-field astrometry has increased by more than an order of magnitude during the last decade and will repeat that feat in the coming decade. While much of this improvement has been achieved through the use of space borne instrumentation, a surprising amount of it has been achieved from the ground. With each advance, factors that could previously be ignored become significant sources of systematic error. Confining ourselves to very narrow field astrometry, we review a few of these areas of concern. We then present the details of an instrument designed to move astrometry to the highest ground based precision yet achieved in the characterization of neighboring planetary systems, the Multichannel Astrometric Photometer and Spectrograph (MAPS).

Designed to determine all 3 degrees of motion of the specific star systems known to have planets, the MAPS achieves an hourly astrometric precision of 0.1 mas and determines the radial velocity of the primary star with a precision of approximately 3 m s-1. The astrometric study utilizes stars found in the central 4.6 ¥ 4.6 arcminute field of the Keck telescope to establish a reference frame. The very high precision of the astrometric positions results from the near total correlation in the motion of the target and reference stars over this limited field. To achieve full correction for differential achromatic refraction, over a total band pass of 6,000 Å, each object is observed in two widely spaced wavelength bands.

Because the target star is many times brighter than the reference stars found in such a limited field, its excess photons may be channeled into the spectrographic part of the instrument. The long dwell times required to average down the errors in the astrometric observations make it possible to use a very high resolution, five times that of the high resolution spectrograph. This will allow the detection of astrophysical effects that could be misinterpreted as radial motion in the target star.