Each new batch of plates is subjected to an extensive series of tests to determine the optimum hypersensitising procedure, and the progress of hypersensitising is monitored by the routine exposure of test samples in a laboratory spot sensitometer. Hypersensitised IIIa-type emulsions lose sensitivity when exposed to air of high humidity. Furthermore, since that date, it has been the practice to store exposed plates in an atmosphere of dry nitrogen until they are processed. Some of the common filter and emulsion combinations and their limiting magnitudes are give below. I 90 19.5 Users should note that exposure times of greater than about 120 mins can be significantly affected by trail.
Plates can be photographically or digitally stacked to achieve greater depth.
The exposure time given is only a nominal value. The actual exposure time depends on the speed of plates which varies from batch to batch. Reducing the exposure times to about a third of the quoted times results in a plate with limiting magnitude about 1 magnitude brighter than those quoted. In this case some moon can be tolerated with no further loss of information. This filter combined with the very fine grain 4415 emulsion is being used to carry out a survey of the Milky Way and Magellanic Clouds in H-alpha.