RE: Focus indication

["Creager, Robert S" <CreagRS@LOUISVILLE.STORTEK.COM>]

Hmmm, should of figured that IRAF would already of done this...

Tom has already indicated that I should take a sample of stars, so I'll
definitely do that (possibly from more than the central 200x200 region).

Thankfully, I don't like writing windows programs, so I'll develop this
thing on Linux, and then wrap the windows stuff around it.  I will take your
advise and make sure not to use CFITSIO directly, so some other poor sole
might be able to use it.  I think I see where this might lead to, so I'll
attempt to make it as generic as possible.  When I'm done, I'll at least
have a good understand of FWHM from one algorithmic perspective.

Thanks,
Rob

> -----Original Message-----
> From: Chris Albertson [mailto:chrisalbertson90278@yahoo.com]
> Sent: Wednesday, June 13, 2001 2:14 PM
> To: Creager, Robert S; tass@listserv.wwa.com
> Subject: Re: Focus indication
> 
> 
> 
> Focus is not something that you change in real time.  The typical
> way to focus is first to have a camera with repeatable focus
> settings.  Tom has this.  He can tell it to go 157 steps out and
> it does.  Next you make a series of exposures at various focus
> settings.  You give the set of frames along with the focus setting
> used for each to a program that computes a focus setting vs. sharpness
> function then finds the maxima of that function. 
> 
> "Sharpness" can't be determined by looking at just one star.  The
> CCD is likely not at 90 degrees to the optical axis and the lens is
> likely not 100% symmetric either.  The focal "plane" is actually a
> kind of sin wave and not a plane at all.  You have to look at 
> the whole
> frame or samples of it and do the best _avaerage_ focus.
> 
> It would be best if you did not look at cosmic rays, ice crystals,
> hot pixels or other defects.  Also "blended" double stars would
> not be good to use either.
> 
> This program outputs just one number.  You don't need 
> graphics for that.
> I'd suggest writing in pure ANSI C so the program will be usable by
> more people.
> 
> IMO focus belongs later in the pipeline. After you have detected
> stars and tossed out the clutter and noise.  It still could be
> done in real time, if you are runing a real time pipeline. But
> remember it takes 20 or 40 minutes to shoot the focus sequence.
> Don't make an interactive program.  Most TASS observers will
> run with the CRT switched off.
> 
> But wouldn't you know it, We are not the first group to need to 
> focus a camera.  THere is already plenty of software that does
> all of the above.
> 
> Here is a short note with links
>  http://iraf.noao.edu/iraf/web/irafnews/dec94/dec94-S-31.html
> 
> Here is what I would use:
> http://iraf.noao.edu/scripts/irafhelp?kpnofocus
>