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 plate modeling polynomials. For stars at the average magnitude of the reference catalog, these structures may be handled with any one of a number of modem techniques, e.g., masks, subplates, or filters. Using data from the plate collection of the Guide Star Catalog (GSC) rereduced by such techniques, we show that there exist large amplitude systematic errors which are a function of magnitude and radial distance from the plate center, i.e., computationally analogous to the coma term in astrographic work. The effect is small for radii under 2°.5 from the plate center, then rapidly increases. The average offset of the faint stars (15m⁾ versus the reference stars (10m) is 0".3 to 0".4 at a radius of 3°.0 and increases to 0".6 at 4°.2. In the GSC plate material the effect occurs both on the Palomar and UK Schmidt plates. The effect for the GSC plate data is approximately independent of telescope, emulsion, passband, and to a lesser degree, exposure depth.