window glow

[roger smith x294]

Alain,

Since you asked about windows I'll repeat something I have already reported 
to this group.  When buying fused silica windows be very sure to
specify the grade.  We have found it necessary to buy "UV Grade" which
is more expensive.  This has little to do with UV transmission:
eliminating the impurities that degrade the UV transmission seems to
deal with the emissions from the window too.

We had one fused silica window of unknown origin which we took out of
deep storage.   Radiation events were not excessive, but we found that
the "dark" current of our beautiful new CCD was 10-12 e-/pix/hour, or
about 3-4 times that expected.  After delivery to Tololo, reports came
back of 1200 e-/pix/hour, decaying to 30-40 over a day or two.  What we
(eventually) discovered was that the window would generate photons by
some mechanism after being exposed to bright lights.  Ironically, the
illumination was being provided by the instrument specialist poking
around the back of the echelle spectrograph with a flashlight to check
for light leaks!  The harder they tried, the worse the problem got so
I was considered the prime suspect.

To my knowledge, this is the only window we have seen do this.  I
was impressed with the long decaty timeconstant, and I think it would be
interesting to compare the dark current measured with a UV grade window
to an Aluminium dummy.  I don't know whether specifying UV grade is 
universally necessary or sufficient, but it has worked for us.

Roger Smith,   CTIO