THE FLAGSTAFF ASTROMETRIC SCANNING TRANSIT TELESCOPE (FASTT) AND STARPOSITIONS DETERMINED IN THE EXTRAGALACTIC REFERENCE FRAME

This paper describes the methods used for determining star positions i n the extragalactic reference frame with the Flagstaff astrometric sca nning transit telescope (FASTT) from drift scan observations. A laser metrology system is used to correct for instrumental motions, and deta iled numerical computations are made for atmospheric and room refracti ons. These and other methods have improved the accuracy of the observa tions which are currently +/-140 and +/-170 mas, respectively in right ascension and declination for a single observation. Objects as faint as V similar to 17.5 mag are routinely observed with the FASTT, and ma gnitudes accurate to +/-0.04 mag can also be determined. As discussed in this paper, most of the positional error is caused by anomalous ref raction in the atmosphere which places a lower limit on the accuracy o f FASTT observations around +/-90 mas in both coordinates. After the H ipparcos/Tycho catalogs become available in a few years, all the FASTT reductions could be made differentially in small fields, wherein the expected accuracy is +/-40 mas in both coordinates for a single observ ation. The telescope has been automated and is operated remotely. Curr ent observing projects include the densification of star positions in large regions of the sky, linking the optical and radio reference fram es together, and determining the positions of faint solar system objec ts. A discussionis given of the methods used for reducing FASTT drift scan observations to equatorial positions and magnitudes. (C) 1996 Ame rican Astronomical Society.