Re: Focus indication

[Andrew Bennett <andrew.bennett@ns.sympatico.ca>]

On Thu, 2 Aug 2001 18:44:08 -0700, Arne wrote:

>Andrew,
>  Are you sure the ellipse rotates? 

Yes: the polar axis is not perfectly aligned so that
images are elongated NS by an amount proportional to
the polar axis EW error and to the exposure time.

Therefore, if the RA rate is right, the fitted ellipse
(for a circular image) will be aligned NS. If the RA
rate is wrong, the ellipse is rotated.

> I can see that the major axis
>of the ellipse changes size, but the orientation of the axes should
>remain fixed, and go from elliptical to circular to elliptical as
>you move through the proper tracking rate.

This is what happens if the polar axis is correctly
aligned EW.

Andrew Bennett, Avondale Vineyard