Abstract
We discuss the use of the block adjustment method for determining astrometric positions from a mosaic of CCD frames taken with a long focus reflector telescope. The observations cover an area of 25 arcmin × 25 arcmin around the open cluster Rup21. The source of reference positions was the Tycho-2 Catalogue. The internal error in the measured positions is analyzed, and the external error is estimated from comparison with the Tycho-2 and USNO-A2.0 catalogs. In this comparison it is found that direct CCD images taken with a focal reducer could be distorted by severe field curvature. The effect of distortion presumably introduced by the focal reducer is eliminated with suitable corrections to stellar positions measured on every frame, but a new systematic effect on scales of the entire field is observed. This effect is modeled with polynomials. The final positions have an accuracy of around 0''.15.
