On Sat, 7 Jan 2006, Poul-Henning Kamp wrote:
> What Astronomers use UTC for, in your own many times repeated words,
> is "a convenient approximation of UT1", and consequently it follows
> that if instead of an approximation astronomers used the Real Thing,
> leap seconds could harmlessly be removed from UTC.
Too simple; many old telescopes, with equatorial mounts, such as the
historic telescopes at the Institute of Astronomy where I work, do indeed
use UTC as a UT1 approximation. The time error involved in this is a small
offset in one axis which you calibrate out on a "clock star".

Research-quality telescopes, in particular all the ones built in the last
few decades on alt-azimuth mounts, do of course use UT1; a 0.9s error
would be a complex ~10 arcsec error in both axes and give a quite useless
pointing performance.  However, UTC is often used as a UT1 delivery
system; because it's an international standard, and is widely available,
and DUT1 is guarenteed to be less than 0.9s, it's a natural choice for
supplier of time.   Interestingly, because control algorithms tend to be
rigorous, a large DUT1 probably would be ok in itself (there would be a
cost involved in checking that this would be so) but certainly in the case
of a couple of telescope control systems of which I have the required
knowledge, the DUT1 input method does a 0.9 second range check.


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