RELATIVE ASTROMETRY WITH THE STEWARD-OBSERVATORY CCD/TRANSIT INSTRUMENT

The CCD/Transit Instrument (CTI) can produce relative astrometry with 1-sigma errors of less than 40 mas for stars brighter than V = 17. This figure of merit is obtained from ten night's data. We obtain this precision, equivalent to 2.6% of the 1.55 arcsec CTI pixel, over a test region 8 X 12 arcmin using simple first-moment centering algorithms. Errors for stars with 17.0 < V < 18.8 range from 40 to 100 mas. We detect 3-sigma proper motions at the 15 mas per year level, using CTI positions in combination with positions obtained with the University of Texas PDS astrometry system from the Palomar Observatory-National Geographic Sky Survey. The test field sample contains three stars with proper motions larger than 20 mas per year. Four of the five stars in our test region with colors indicating spectral type M2 or later have statistically significant proper motions.