strange patterns on tek CCDs

[a.walker x305]

The CTIO Tek 2K #3 CCD mentioned by Mike Lesser does not
show the "crocodile pattern" when imaged using flat fields.
There has been some damage to the AR coat (presumably) though, 
about one percent of the pixels, distributed randonly over the 
CCD, have QE a few percent less than the surrounding pixels.
In most cases these are "single or few pixel events" rather than 
"blobs".  This CCD suffered some severe contamination early in 
its life and its present condition may be a consequence of this.
 
A more recent 2048 (#4) has surface in pristine condition, despite
having at times small amounts of visible surface contamination
after being cold for several weeks.  This has always been able to be
pumped off rapidly after the CCD has been warmed to room temperature.

Tek have stated that water can damage their AR coatings, which
is unfortunate since this is one of the more likely contaminants
found to condense on a CCD.

Alistair Walker