Re: Focus indication

[Tom Droege <tdroege@veriomail.com>]

Jure is right about the tracking rate.  I used to do the focus run by 
taking a sequence of images.  This before Arne suggested the multi-image on 
one frame scheme.  This then allowed adjusting the RA rate by observing the 
motion of a star from one frame to the next.

Thanks also Jure for pointing out how to do the N alighment.

Tom Droege

At 01:02 PM 8/6/01 +0200, you wrote:
>Tom
>
> > 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.
> >
> > OK, someone of an analytical bent can analyze this problem.
> >
>
>I think there is a simpler and better method to determine tracking rate 
>than from
>a single exposure (which looks numerically sensitive to me).  Instead, you 
>make
>two exposures, 30 or 60 minutes apart.  So, take first exposure, keep tracking
>for some time, and take the second exposure.  On both images use a part of 
>your
>favorite processing pipeline which makes catalog matching.  Calculate sky
>coordinates of the central pixel.  If RA changes, you will know for how 
>much is
>the tracking wrong and there is no ambiguity about the direction.
>
>Also, if you do this test while pointing at the meridian, which is your usual
>mode of operation, you will be able to find out possible misalignment in the
>East-West direction of the polar axis.  If the second exposure has lower
>declination, move your mount to the East and vice-versa.  A similar trick 
>works
>for the altitude adjustment.  More on this at
>
>http://www.mindspring.com/~skyshooter/Star%25252520Drift.html
>
>Note that you don't need to pick up a particular star.  It is enough to 
>find the
>WCS coordinates.  You just need enough stars to make a match.  If you have 
>wrong
>both polar axis and tracking rate, you may need to iterate the whole process.
>
>                                 Jure