Jon Brewster wrote:

> > having problems with hot pixels. If I cool the camera down to -20c by
> > placing it in the refrigerator, the dark frames show about half as many
> > hot pixels as when the camera is only -10c. If this is normal, I was
>
> oops sorry everything below is for my 416, but I still think
> the point(s) hold.
>
> My average dark pixel is around 3800 at -10C. A typical
> PictorView contrast stretch shows "hot pixels" that are anywhere
> from 4000 to 4500. I even found a couple at 6500. However,
> I don't consider these to be "hot pixels" even though they show
> as bright white spots on the dark. I don't have a complete
> controlled experiment but maybe this data is useful:
>
> A dark at -10C
> average pixel 3800
> possible hot pixel 386, 247 = 6320
> possible hot pixel 121, 53 = 4365
>
> A flat at 0C (and 9 months earlier, sorry)
> average pixel 41000
> possible hot pixel 386, 247 = 41658
> possible hot pixel 121, 53 = 41133
>
> As you can see the "hot pixels" in the dark frame are nicely
> sensitive to light and have a good dynamic range. Since they
> are consistent, they can be processed into good images.
>
> In short Robert, you may not have a problem. I wouldn't attempt
> tweaks to improve darks. Darks are done to improve the end product.
>
> I would be interested in yours and other folks numbers, that way
> I could discover just how wrong I am.
>
> Jon B

This is excellent data I believe. With the 216 cooled to -5 C. I usually
get bright pixels of 2500 to 3500 counts. Remember that bright stars on this
same scale are 10,000 to 65,000.
When looking at dark frames always check the absolute values. It would be
nice if all pixels not exposed to light were a few hundred counts but that is
not to be.

Doc G <http://www.mailbag.com/users/ragreiner>

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