Spots

[Paul.Jorden at eev.com]

Posted to CCD-world:
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 > > Posted to CCD-world:
 > > -+-+-+-
 > > The Amateur Sky Survey now has several 2k x 2k CCD cameras running
 > > using the CCD442A.  We have bought the Grade 4 (lowest cost) devices.
 > >
 > > On one of the cameras, I see large round dark spots.  One is a few
 > > hundred pixels in diameter.  Others are smaller.  Most of these spots
 > > represent lower sensitivity of a few percent.  One has increased 
sensitivity.
 > > Being the tass list, these have sparked a debate as to their source. 
 We are
 > > running the present CCDs with covers.  (CCDs are now on order without
 > > covers.)  In my opinion, the edges of the spots are too sharp to be
 > > due to dirt on the cover or in the optics.  The spots are on the only 
CCD
 > > with a bad column, indicating that this is possibly a lower grade 
overall
 > > device.
 > >
 > > 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.

Regards,
Paul J
_______________________________________________________________
Dr Paul Jorden,  CCD Sensors, EEV
Waterhouse Lane, Chelmsford, Essex, CM1 2QU, UK
Tel:      44 (0) 1245 -453458 (direct), -493493 (switchboard)
Fax:      44 (0) 1245 453224
see also:-     http://www.ccd.eev.com/

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