A CCD system for reading circles in wide-angle astrometry

This paper describes a CCD circle reading system developed for the USNO (Flagstaff) transit telescope. Circle readings are needed for the determination of declinations in wide-angle astrometry. The detectors are commercially available Philips CCD-TV imaging cameras, which can produce digital images every 1/15 of a second. The positions of division marks on the circle are determined by fitting a function to their profiles. This reading system is very accurate (+/- 0.007 arcsec), fast, and extremely reliable. Also, since the digital output is two-dimensional, the radial positions of division marks on a circle can also be determined, but with lower accuracy (+/- 1.2-mu-m).