Re: Focus indication

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

On Wed, 01 Aug 2001 19:10:34 -0500, Tom Droege 
<tdroege@veriomail.com> wrote:

>Rob,
>
>Yep, the difference in x and y is a measure of the tracking.  One of the 
>problems for the Mark IV is adjusting the RA tracking rate.  There is a 
>built in DAC to adjust it.  One would like to be able to tweak the RA drive 
>rate.  The way the chip is mounted, the RA shows up in y.
>
>My present scheme takes an image with six different RA rates.  I then try 
>to look at the image and detect which rate is best.  Very hard to judge.  I 
>would like to see the y fwhm measured to .05 (a wild guess) pixel or 
>so.  This would allow fine tuning the RA drive.

If you fit a 2D elliptical gaussian, the axis rotates
as you vary the RA rate.

The NS component is fixed for a fixed misalignment and
exposure time.

The EW component varies with RA rate (and exposure time.)

So, ignoring the non-circularity of the basic image,
the wobble in the RA drive
and all the other little nastinesses other have pointed out,
the correct RA rate is the one that gives a NS orientation
for the fitted ellipse.

From the direction the ellipse rotates, somebody with
clearer visualisation skills than mine could tell
which way to rotate things to improve the alignment ...
and maybe even by how much. Left as an exercise for
the reader.

Andrew Bennett, Avondale Vineyard