Timing in PF-QHY Headers
DATE-OBS represents the UTC of the start of the exposure, and is
accurate to ~0.1sec (rms). Together with EXPTIME/2 it should be used
for accurately timing the mid-point of exposures.
UTOBS, JD, MJD-OBS and STSTART are approximate, and they should not be used
for accurate timing.
The following information comes from the old 2012
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 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 system.
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