Field distortions in the CPC2 plate data

Residuals of a conventional plate adjustment of 5,687 plates of the Second Cape Photographic Catalogue (CPC2) using the Southern Reference Star (SRS) Catalog were analyzed to search for systematic errors depending on the location of the star images on the plate. Even after correcting for third order optical distortion, such coordinate depending systematic errors, hereafter called field distortion pattern (FDP), were detected which are nearly constant with respect to plate epoch, location in the sky and apparent brightness of the stars. The mean amplitude of these field distortions was found to be 57 milliarcseconds (mas), which is $0.57 mu m$ on a plate. Due to the favorable 4--fold overlap observing principle of the CPC2, the effect on the mean catalog position is only a fraction of that. But the FDP affects a block adjustment (BA) of the entire set of data on a 100 mas level at the border of the CPC2 sky coverage. A comparison with FDP's found in other projects was made and physical reasons for the FDP were investigated. Publication of a new release of the CPC2 is planned after the Hipparcos Output Catalog becomes available in 1996.Comment: paper for March AJ, LaTeX with 1 tabl