CCD Window

[Stupendous Man <richmond@stupendous.cis.rit.edu>]

>   So there is a good chance that some or all my chips have
>   plain glass windows.
>   
>   I am wondering if this could cause our noise floor?

  I suspect not.  The main effect of reflections will be to
diminish the light reaching the CCD.  A typical piece of
uncoated glass may transmit only 92% of the incident light,
while AR coatings may improve the transmission to 99% or
better at selected wavelengths.  That doesn't really increase 
our limiting magnitude by much, of course.

  I would _guess_ that few stars are bright enough to cause
significant ghosts: light going through the glass, bouncing
off the CCD, bouncing back off the glass, and striking
the CCD a second time in a different place.  If this were
a major source of error, then we would see stars with high
scatter clustered around bright stars in the sky; but instead,
we see stars with large scatter distributed around the 
edges of a frame.  

  I would rate large-scale "flatfield" errors and CCD gate structure
ahead of the uncoated glass as sources of photometric error.

> I could easily remove the cover glass.  This at least removes two surfaces.
> Now that the cameras are working pretty reliably, I would not mind removing
> the covers.  However, one trades dust on the window which is out of focus
> for dust on the surface of the CCD which covers specific pixels and
> probably moves around a little.  Anyone had experience with this that can
> make a recommendation as to the trade off?

  It ain't broke, don't fix it.  My $0.02.

                                      Michael