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 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 visiting observers).