CCD astrometry of asteroids in the extragalactic reference frame

This paper describes observations of asteroids made with the Flagstaff Astrometric Scanning Transit Telescope (FASTT), which is a 20 cm CCD transit telescope that can observe objects brighter than V similar to 17.5 mag directly in the extragalactic reference frame. This is accomplished with differential reductions using ancillary FASTT observations of VI,BI radio reference objects [see Stone et al., AJ, 111, 1721 (1996) for more details]. Positions determined with the FASTT are accurate to +/-0.14 and +/-0.17 arcsec, respectively, in right ascension and declination for a single observation and are considerably more accurate than most positions determined for asteroids. Currently, the FASTT is observing asteroids for a number of projects that require very accurate positions. Namely, the FASTT is supporting the NEAR spacecraft mission by determining positions for the targeted asteroids 253 Mathilde and 433 Eros, and previously, FASTT observations of the asteroids 951 Gaspra and 243 Ida were made in support of the Galileo project. When compared with accurate JPL ephemerides computed for these asteroids, differences between FASTT observed and JPL calculated positions were generally small (<0.08 arcsec), and the rms scatter comparing these results was typically +/-0.18 arcsec in both coordinates. Moreover, the FASTT is observing a number of asteroids that are gravitationally interacting with other asteroids, and hopefully with enough observations, masses can be determined dynamically for some of them. There are only a few asteroids with accurate mass determinations. Approximately 1000 observations of these asteroids have been taken with the FASTT with an accuracy of similar to+/-0.16 arcsec in each coordinate. Finally, the FASTT is observing asteroids and stars that might be involved in occultation events. FASTT observations are being used to create accurate ephemerides for the asteroids involved as well as to determine very accurate positions for the background occultation stars. In particular, FASTT positional data were used in making the predictions for several recent successfully observed occultations (e.g., 654 Zelinda and 85 Io).