Astrometric quality of the USNO CCD Astrograph (UCA)

The USNO 8-inch astrograph has been equipped with a Kodak 1536X1024 pixel CCD since 1995 June, operating in a 570-650 nm bandpass. With 3-minute exposures well exposed images are obtained in the magnitude range R approximate to 8.5-13.5(m). An astrometric precision of 10 to 15 mas for those stars is estimated from frame-to-frame comparisons. External comparisons reveal an accuracy of about 15 mas for those stars in a 20' field of view. For fainter stars, the error budget is dominated by the SM ratio, reaching approximate to 100 mas at R=16(m) under good observing conditions. (C) 1997 American Astronomical Society.