Re: Focus indication

[Chris Albertson <chrisalbertson90278@yahoo.com>]

If you want a focus indication why not choose a method that is
applicable to all images, not just images of the night sky?
Auto focus SLR cameras don't detect stars or compute the FWHM
of anything.

An in-focus image has higher spatial frequency content then an
out of focus image.  Or equivalently, an out of focus image is
like a focused image which has had a low pass filter applied to
it.

One thing to do is to compute the mean delta between adjacent
pixels.  Every non-edge pixel (All four million of them) has
four adjacent pixels.  Take an average of all 16 million
differences or maybe just use the central 500 x 500 pixels.
This should allow you to focus on anything you point the camera
at, the sky or even your dog.


--
   Chris Albertson             
   chrisalbertson90278@yahoo.com
   Redondo Beach, California
   home: 310-376-1029
   cell: 310-990-7550