Spots before our silicon eyes

[The Oregon Marsh's]

Posted to CCD-world:
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The CCD in the Star-1 series was a Thomson TH7883A.  Such "shading" defect
areas are 
relatively rare for front CCDs--on thinned devices, chemical spots due to
the thinning process 
and/or AR coating nonuniformities can sometimes produce spots with higher
or lower QE (depending 
on wavelength).  

It is difficult to spec these low-impact defects--if one wants perfect
imagers, they either must negotiate 
with the CCD vendor to assure they don't receive devices with defects they
are sensitive to or just get 
lucky.  With the real estate of CCDs being fairly large (and, the scrutiny
of scientific CCD users being 
so high), guaranteeing perfect CCDs will make them extremely expensive
(either for testing time or 
yield loss).  I've worked on both sides of the fence (CCD vendor and camera
producer), and open 
communication and specification agreement between the two can help make
both satisfied.  However,
it is difficult when sales volumes are low!

	Harry H. Marsh
	PixelVision of Oregon

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From: iss at pvtnetworks.net
To: CCD-world at cfht.hawaii.edu
Subject: Re: Spots
Date: Wednesday, April 21, 1999 9:57 AM

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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