Design of a spaceborne astrometric survey instrument

We have investigated the design of a small astrometric and photometric survey instrument in the Hipparcos tradition. Such a mission will support a rich and diverse ensemble of scientific investigations. The design objectives, which have been met in this study, are to be able to measure 10(7) stars over the full sky, with an accuracy of 0.05 mas for mag < 9 and 20 mas for mag 15. A scanning survey instrument that uses CCD detectors is able to measure many stars simultaneously. As compared to a pointed astrometric instrument of comparable size, the survey instrument generally has much higher measurement throughput (sum over targets of the inverse variance), but on average, less scientific interest per target. An instrument for astrometry, unlike those for imaging, can be compact and yet scientifically productive. We report on a study of a Fast Astrometric Mapping Explorer (FAME-98) and an examination of standard approaches for scanning astrometric missions. We fmd some of these standard approaches wanting. We have concluded that a scanning astrometric instrument performs better if it spins faster, precesses faster, and has a shorter focal length than is conventionally expected. Further, we have shown that the use of solar radiation pressure instead of gas jets, as a means of precessing the spacecraft, yields a stronger astrometric solution, because the rotation is not broken into disjoint segments. Finally, we have shown that an elongated primary mirror is not advantageous, and that the central portion of the primary mirror contributes little to the astrometric accuracy of the instrument. A first look at the pivotal issue of systematic error has uncovered no serious problems, although careful engineering will be essential. Because the spacecraft is equipped with a solar shield, which keeps it in shadow, the temperatures of critical instrument components are stable at the mK level on the time scale of the spacecraft rotation period. The analysis has underscored three instrumental differences between a survey astrometric mission and a pointed astrometric mission. For a survey mission, the aperture shape does not matter, a large-area detector enhances the scientific throughput, and there is no advantage to an interferometer over a telescope.