Re: Running the Mark IV

[Chris Albertson <chrisja@jps.net>]

Tom Droege wrote:
> 
> 
> Another is to take a series of files that have been exposed at different
> focus positions and pick out the best focus.  I now have a program whih
> will start at a given focus position and step in arbitrary steps.  The
> program should take the starting position and the step size, examine the
> exposures and pick out the best position for the focus.

I have a program on my computer now that will do that.  It is called
"kpnofocus".  I suspect other people on this list are set up to run
it also.  Here is an excerpt from the documentation just to give the
flavor of it

   "A plot showing the variation of the PSF width and  ellipticity  with
    focus   is   shown   along  with  a  magnitude  weighted,  parabolic 
    interpolated estimate for the best focus.  One may delete bad points
    with  the  cursor  'd'  key.   To  exit  and record the results to a
    logfile use the 'q' key.  There are many graphical  display  options
    for  more  sopisticated  analysis  such as variations with position.
    The best thing to do is to ..."

It is an Interactive graphical program.  You have to click on the stars
to be used.  When you are done it displays a plot and the computed best
focus.

Tom,  Maybe it would be easiest to send a focus run exposure to someone who
is already setup to analyze it.  Make multiple exposures follows:

Repeat this about six times:
   1) Expose
   2) adjust focus
   3) move camera about 30 pixels

You can also use separate files where the focus is different in each file.
If I understand what you've written above this is what you've done.  The
multiple expose image is quicker to make as you only need one readout.

The program will attempt to fit a curve FWHM vs. Focus setting.  It will
them compute a minima for the curve.  So the computed best focus may not be
one used in one of the frames.  Documentation says the all your shots may
even be on one side of the best focus.  I would not trust this.  I'd bracket
it.



> I will shortly have a bunch of images.  I will be looking for someone to
> look at them and map what is wrong.
> 
> I know a few things already.  There is coma in the corners.  But no worse
> that the camera lens that I previously used.  The field is *much* flatter
> than the camera lens.  But someone should look at this and try to
> characterize it.

I think a "focus run" would help.  You can tell a lot by looking at the
image of a star at and on both sides of best focus


> Tom Droege

-- 
   --Chris Albertson             home: chrisja@jps.net        
     Redondo Beach, California   work: chris@topdog.logicon.com