UT in the FITS Headers
The UT start time of an exposure made at the WHT or INT
is recorded in the UTSTART
FITS header, and has accuracy ~ 0.1 sec rms.
This UT value comes from a computer which has Network Time Protocol
(accuracy probably a few msec) but communication delays,
and the finite response speed of CCD
shutters, conspire to limit the absolute timing accuracy to ~ 0.1 sec.
The time recorded by the UTOBS FITS header comes from the
telescope control system (TCS) and is less accurate, rms ~ 1 sec,
hence the label 'approx' in the description of this entry.
This time is also reported
on the main TCS information display, but the
update frequency is only 1 Hz, and there may be significant time lags in the
UT and ST clocks are provided for convenience on the shelf above the
instrument-control screens in the WHT.
They are not synchronised to any master clock, and may drift
significantly. ING staff occasionally reset them (and will be happy
to do so whenever this is requested by visiting observers).
14 August 2012
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