Date: Mon, 23 Nov 1998 16:07:36 -0800
From: Russell E Owen
Subject: Re: 3.5-meter night log 981122
> 	To make matters worse, we ran into problems with guiding and rotations
> early in the night. I am not sure why, but when the 'horizon' is rotated
> 90 degrees, the guider cannot deal with it. It continually makes huge
> offsets in Y. When the observer calculates the parallactic angle and
> rotates the 'object' the guider deals with it just fine. Perhaps I am
> just misunderstanding the 'Horizon' type rotations. For the rest of the
> night the observer just calculated the angle for each object.
I'm curious if this has been tried before, and if so, whether it worked.
Guiding should work fine using Horizon rotation. But if it's not been well
tested then perhaps there's some bug in the TCC.
One thing to keep in mind: horizon rotation is much more likely to have
problems than object rotation. Any error in pointing will result in the
object describing a circle of radius equal to the size of the pointing
error. This is a fundamental property of rotation about one point in the
focal plane (in other words, anything except object rotation), and is not
something an off-axis guider can correct.
This is discussed in detail in the TCC User's Guide under "Using a Rotating
Field". Please read that carefully before using Horizon rotation. Note that
this problem will also occur for the Echelle spectrograph, though guiding
will be able to correct it because guiding will be on the object itself.
-- Russell