Date: Tue, 19 Jan 1999 09:24:05 -0800
From: Russell E Owen
Subject: Re: 3.5-m obslog 990116
> JH02 (Zheng): "It has been a smooth observation run, with seeing
> improving with time until early morning. The position offset appears
> to be not exact, with different results if moving from up or down
> direction. Does it suggest a mechanical gap?"
Any idea what's going on here? There should be no noticeable
hysteresis in the telescope, and we know the math doesn't care.
One guess on my part: they're moving sideways as well as up and down
and using Object offsets and using the crude math that the sideways
amount is scaled by one over the cosine of the altitude (something
that Remark may be doing). In which case for a large enough offset
there will be a noticeable difference between up-and-sideways and
down-and-sideways.
If that is indeed the problem, possible solutions include:
- use arc offsets, to some extent that's what they're there for
- pay attention to the math (something a user should not have to do)
- use boresight offsets (if using object rotation this is safe)
-- Russell