Re: Focus indication

[Tom Droege <tdroege@veriomail.com>]

Yes, there is a program in the QBasic code that does just this.  (Adjust 
the RA) The problem is in picking the best point.  If you use long enough 
exposures so that you can see a one pixel difference, then you have to 
space them very far apart and they don't all  fit on one exposure.  There 
is also the problem that if the exposure is long enough to measure the 
error then it is so long that it saturates the CCD.  I would like to be 
able to use a short exposure and have some tool that will measure small 
differences in fwhm.

Then I would take separate exposures at different DAC settings and pick the 
one with the smallest EW fwhm.

I think it is hard for Arne to understand my problem since he has a full 
kit of tools installed on the computer he uses to examine images.  OK, I 
should have such tools, but I don't yet.  For a while at least, I will have 
to live with what is available in Windows.  I am not able to put a cursor 
on the ADJ RA image and measure the fwhm of the various points to see which 
is best.  That is what I need.

Tom Droege



At 06:44 PM 8/2/01 -0700, you wrote:
>Andrew,
>   Are you sure the ellipse rotates?  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.
>   The problem Tom has is determining on which side
>of 'correct' his current exposure resides.  Just as for focus
>exposures, where you offset in Dec, change focus, offset in Dec, etc.
>on the same frame, you can do something similar in RA.  Expose, go fast
>forward, set new VCO voltage, expose, etc.  Then just look for the
>image that has the smallest y (RA) fwhm.
>Arne