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. Peter.