The Flagstaff astrometric scanning transit telescope (FASTT) and star positions determined in the extragalactic reference frame

This paper describes the methods used for determining star positions in the extragalactic reference frame with the Flagstaff astrometric scanning transit telescope (FASTT) from drift scan observations. A laser metrology system is used to correct for instrumental motions, and detailed numerical computations are made for atmospheric and room refractions. These and other methods have improved the accuracy of the observations 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 magnitudes accurate to +/-0.04 mag can also be determined. As discussed in this paper, most of the positional error is caused by anomalous refraction in the atmosphere which places a lower limit on the accuracy of FASTT observations around +/-90 mas in both coordinates. After the Hipparcos/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 observation. The telescope has been automated and is operated remotely. Current observing projects include the densification of star positions in large regions of the sky, linking the optical and radio reference frames together, and determining the positions of faint solar system objects. A discussionis given of the methods used for reducing FASTT drift scan observations to equatorial positions and magnitudes. (C) 1996 American Astronomical Society.