In your message dated: Sat, 28 Nov 1998 12:08:43 PST you write:
>"wonky"?? Is that a technical term <g>.
>My limited understanding of CCD devices tells me the primary difference
>between astronomical CCD cameras and "consumer" CCD's is the astro ones are
>electronically cooled to very low temperatures to increase sensitivity and
>reduce noise. While I'm positive Lee can explain more accurately, with an
>astro CCD one takes two exposures to get a picture - a "dark frame" and the
Actually, it's much more complicated than that. For best results you
take the following for a night's observing:
16 bias frames, shutter closed, 0 seconds. Average and filter
as above, and subtract from each image, including darks and
flats.
16 dark frames, at least as long as your longest exposure,
averaged and median filtered to get rid of cosmic rays,
and scaled to each image exposure time. Subtract from
each flat and image.
4-8 flats in each filter. Images of the twilight sky or
an evenly illuminated screen, about 5 seconds in duration,
averaged and divided into each image to take care of
vignetting and pixel-to-pixel variations in the CCD.
Once you do all this, you have an image that is photometrically
meaningful.
Cheers,
--Lee
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