[LEAPSECS] Telescope pointing

Gerard Ashton ashtongj at comcast.net
Fri Jun 8 13:47:29 EDT 2012

So summarizing what I find in Steve's paper, and only concerning the
pointing of the telescope and not the reporting results from the scope,
large telescopes with guidance systems from the 1970s and 1980s have a guide
camera field of view of 3 arcminutes, or a bit more. So if the telescope was
aimed to bring a certain object to the center of view, and the time were off
by the time equivalent of 1.5 arcminutes, the object would be out of view
unless other fortuitous errors brought it back into view. If the object is
on the celestial equator and the telescope has an equitorial mount, this
would be 6 seconds of time. In this event, the situation is salvageable but
it requires intervention by the on-site observing technicians.

In the case of a large fully-automated 21st century telescope, the APF, the
specified pointing accuracy is 10 arc seconds. If it were an equitorial
mount (which it isn't) this would correspond to a time error of up to 0.7 s.
If we consider that a measurement standard should have a tolerance 1/3 as
great as that which is being measured, and that in some directions the time
equivalent of 10 arc seconds, we might estimate that a time base tolerance
in the 10s of milliseconds would be negligible compared to other pointing
errors. The same telescope will develop serious problems within a year of
the discontinuance of leap seconds. I take this to mean that an error of 1
second in time is intolerable, which corresponds to an angle of, at most, 15
arc seconds.

This would suggest that when converting UTC to sidereal time, the older
telescopes could ignore the distinction between mean and apparent Greenwich
Sidereal Time.
Probably even for the APF the subtle differences in the definition of
Greenwich Sidereal Time that have been made in the last 30 years or so could
be neglected, right?
(That is, just for pointing, not for reporting results.)

Gerry Ashton

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