Spots

[iss at pvtnetworks.net]

Posted to CCD-world:
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Re,

> > > Can anyone tell me if such spots are common on a CCD?
> >
> > Not at all. I think You should use better CCD grade if possible. We
> > normally use best grade CCD and dont have similar problems.
> >
>I agree; good (or even poor) CCDs don't normally have such features- 
>especially if they are round, and hundreds of pixels across.  The spots 
>sound very much like 'dust rings' caused by out-of-focus dust spots on cover 
>glass, or fiters etc.  Reasonably sharp-looking annular rings can be caused 
>by contamination in front of the focal plane.  It depends somewhat on the 
>nature of your illumination.

We had a Star-1A Photometrics CCD at the NASA JSC Star Tracker lab 
a few years ago that had what I believe is a genuine defect of this type,
it was parabolic shaped and covered about 1/4 of the field
of view. It looked like a negative image of a comet and the pixels
in it were about 3-4% less sensitive than the rest of the field.
It was not contamination, several cleanings verified this. After some
'negotiations' with the mfr, they sent us a test image taken with that
particular Phillips CCD chip which showed the identical defect. Their
claim was that if the response uniformity was <5% the chip was acceptable.
Since there was no spec on cluster defects we had to accept it as is; it
did flat field out ok. This was not an 'engineering' or low grade chip.
So, in our experience, defects of the sort noted by Tom are quite
possible and if significant to your application, should be eliminated
by specification and TESTED independently (ie not by the mfr).

Andy Saulietis
Andy Saulietis / ISS Alt-Az-Fp Drive Systems
HDPE Worm Gears, Custom designed mount/telescopes & Machine work
CCD filter wheels & other custom accessories
39 Silver Fox Trail
Cloud Country Estates
Mayhill NM 88339
505-687-3067 Voice
505-687-3021   Fax
e-mail: iss at pvtnetworks.net
32 54 13 N  105 31 44 W 7300' elev

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