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A note on accurate time and ISIS

A long-standing problem with ISIS has been the lack of accurate shutter timings. The current system uses clocks on the CCD controllers to write shutter opening and closing times to the file headers. The clocks are relatively inaccurate and can drift by a few seconds over the course of a night. The present solution is to synchronise the clocks with UTC at the start of each night, using the commands .DATE, .TIME, HH:MM:SS SET-TIME and DD:MM:YY SET-DATE on the engineering terminal. This system is clearly inadequate, especially when accurate times are required. On the JKT and INT, the shutter timings are obtained directly from the time service. Work is currently underway to create a similar system on the WHT and we expect it to be fully operational by the end of 1993. Until then, we advise all observers who require accurate timings to contact their support astronomer for further details.

Fri Jan 7 15:34:48 GMT 1994