Astrometric quality of the USNO CCD Astrograph (UCA)

The US Naval Observatory 8--inch astrograph has been equipped with a Kodak 1536x1024 pixel CCD since June 1995, operating in a 570--650 nm bandpass. With 3--minute exposures well exposed images are obtained in the magnitude range $R \approx 8.5 - 13.5^{m}$. 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 S/N ratio, reaching $\approx 100$ mas at $R=16^{m}$ under good observing conditions.