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I copy below an email that Dave Tholen sent to the
minor planet mailing list that highlights another
reason why one might want absolute timing of an
exposure to be pretty accurate. It is not just a
matter of locating a datapoint on a light curve!
EC16 is a fast moving asteroid; the timing requirement
is necessary to get good astrometry of such objects.
One other comment is worth making: the limiting factor for an object
like EC16 moving over an arcsecond per second is the accuracy of the
clock! In my images, for example, the RMS residual from the fit to the
15 or so reference stars was 0.2 arcsec, but the camera software writes
the time in the header to only integer seconds. I'm trying to find out
whether it rounds the time or truncates the time. The former case would
introduce an error of 0.5 arcsec for EC16 and the latter a full arcsec.
In other words, the clock is the dominant source of error!