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RE: Focus indication
Chris,
Thanks for pointing outside the box. I am ready to throw in the towel for a
'quick and dirty' FWHM calculation, as my results are not as consistent as I
believe they should be.
First pass data of a central box 900,900 to 1100,1100 using your method
(mean of abs differences between 4 adjacent pixels) on night 1815:
V Darks 16-20 and 101
V Objects 34-52
I Darks 24-29 and 130
I Objects 45-88
The dark outliers are the first picture taken (dark) during the night.
Would the difference between the V-I Darks be a temperature differential
between the CCD's? I believe that the CCD's are serially cooled, correct?
Anyone have some out of focus FITS images from a Mark IV? I don't have the
notes file from night 1815 handy, so I cannot tell if anything odd happened
during this run.
Attached is a png image of the mean vs. fractional day.
Later,
Rob
> -----Original Message-----
> From: Chris Albertson [mailto:chrisalbertson90278@yahoo.com]
> Sent: Monday, August 06, 2001 10:16 AM
> Cc: tass@listserv.wwa.com
> Subject: Re: Focus indication
>
>
>
> 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
>
focus.png