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P R Jorden has supplied the following notes about the operation of the Compur focal plane shutter which is used in both the INT and JKT CCD cameras. The sequence of events for an exposure (of N secs) is shown below;
Thus a 1.00 second exposure could give a 0.4 % change in exposure from edge to centre; the status should indicate the average time. Exposures of less than 1 second are not recommended.
The shutter timing has been checked and appears accurate and repeatable; in particular the shutter status accurately reflects the actual duration of shutter timing. The true exposure time for a CCD exposure appears in the packet headers on each CCD frame (except in multiple exposure frames). The value given does not exactly correspond to that requested (it is usually several milliseconds more) but it does reflect accurately the true exposure time.
C D Pike has made an observational check of the linearity of the JKT CCD shutter. The results are shown in Figure 7.