On Wednesday, April 01, 1998 6:38 PM, John Hoot

<<snip>>

> >
> >2. Any idea how many "hot pixels" are normal for a 216XT? Is 100 to 200
> >too many?
>
> Depends on what you mean by hot. There is always some difference. You
> should be
> able to expose 5 .. 10 minutes at -12C without having more than a handful of
> pixels
> reach saturation from thermal effects.


John Hoot makes an important point when evaluating a CCD chip.

Often, software will stretch an image so that many pixels look white,
though they may not be fully saturated. Much of this has to do with a
particular video sub-system's ability to display 256 true shades of gray
as well as the compression of the dynamic range to 8 bits. Most
RAMDAC's in the computer display subsystem must be set to a 24 bit
color depth to achieve 256 (8 bit) true gray shades.

I consider a pixel a "hot" pixel when that pixel produces considerable
more noise than adjacent pixels above factory tolerances for that chip
at a specific temperature. Specifications differ among chip makers,
but the Kodak 0400 chip allows a 6% difference among the surrounding
100 pixels at 70% saturation at 25 degrees C.


>
> John Hoot
> SSC
> Author of DOSPVIEW
>
>


--
Michael Hart
Husen Observatory

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