A SCHMIDT PLATE COMA-LIKE TERM

Astrometry from fast wide-field-of-view Schmidt telescopes is affected by complex structures that are not well represented by traditional pl ate modeling polynomials. For stars at the average magnitude of the re ference catalog, these structures may be handled with any one of a num ber of modern techniques, e.g., masks, subplates, or filters. Using da ta from the plate collection of the Guide Star Catalog (GSC) rereduced by such techniques, we show that there exist large amplitude systemat ic errors which are a function of magnitude and radial distance from t he plate center, i.e., computationally analogous to the coma term in a strographic work. The effect is small for radii under 2.degrees 5 from the plate center, then rapidly increases. The average offset of the f aint stars (15(m)) versus the reference stars (10(m)) is 0.'' 3 to 0.' ' 4 at a radius of 3.degrees 0 and increases to 0.'' 6 at 4.degrees 2. In the GSC plate material the effect occurs both on the Palomar and U K Schmidt plates. The effect for the GSC plate data is approximately i ndependent of telescope, emulsion, passband, and to a lesser degree, e xposure depth. (C) 1996 American Astronomical Society.