Techniques for Schmidt Plate Reductions with Application to GSC1.2

Abstract

Over the last few years there has been considerable progress in developing successful algorithms for obtaining astrometric quality positions from Schmidt plates which compensate for deficiencies of the polynomial approach; the sub-plate method (Taff 1989), the mask method (Taff et al. 1990), the collocation method (Bucciarelli et al. 1993) the filter method (Röser et al. 1995) and a filter weighting according to the method of infinitely overlapping circles (Morrison et al. 1996b.) However, none of these nor any other studies have investigated the magnitude dependence of the position estimates outside the magnitude range of standard reference catalogs. Often Schmidt plates cover the magnitude range from 6m to 19m (for the GSC it is 6m to 15m). Presently available reference catalogs, however, have a limiting magnitude of V ≈ 10m. Therefore, no magnitude dependent term for fainter stars can be reliably found by reducing the measurements based only on comparisons with reference stars.