Uniformity of UKSTU photographic plates.

It is pointed out that most plates taken with the UK Schmidt Telescope (UKST) are hypersensitized before use. The technique for the hypersensitization of IIIa-type emulsions has been described by Sim et al. (1976), and for the IV-N emulsion by Hartley and Tritton (1979). During the calibration of a sensitometer, it became apparent that the sensitivity of the hypersensitized IIIa-emulsion in use was decreasing as a function of time in the plateholder. Malin (1979) had previously commented on the nonuniform background of UKST plates when subjected to his high-contrast enhancement technique. He suggested that such nonuniformities might arise from differential desensitization of the emulsion in the presence of air trapped between the curved photographic plate and the flat filter. Investigations regarding the observed phenomena have been conducted. The UKST Unit (UKSTU) is now engaged in refurbishing the telescope plateholders in order to permit the nitrogen-flushing of all hypersensitized IIIa-type emulsions during exposure.