camera calibration

[Gerard Luppino]

Hi Nick,

BY far the easiest and most error-free way to do this is
to use the Fe55 x-ray calibration. The drawback is you
need to buy or have access to a radioactive Fe55 source
(they are cheap and not dangerous, but it isn't the
easiest thing to find).

The technique works as follows: The K-kapture decay of
Fe55 produces a Manganese K-alpha x-ray of 5.89 keV.
This x-ray is absorbed in the silicon, producing a charge
cloud with size proportional to the energy of the
x-ray. The conversion factor is 3.65 eV/e-h pair.
Therefore the absorbed 5.9keV x-ray produces a cloud
of size 1616 electrons. The variance in the number of electrons
produced does not obey Poisson statistics, but rather is weighted
by bthe "Fano" factor, which is 0.1 for silicon. Therefore the
variance in the number of electrons is F*N = 160 and the rms
is the sqrt of 160 -- not a large number. To calibrate a CD
gain, one illuminates a CCD with x-rays (10 microcurie to 1 mcurie
source is plenty bright enough). You want to have a reasonable number of 
x-ray events but want to remain in the regime
where it is unlikely that many pixels have two events. One then reads out
the CCD and looks at the histogram of pixels. You should see a large
spike at the bias level value with a small peak at some ADU value
that corresponds to 1616e-. Reading off that number on the ADU axis
agives you the conversion of e-/ADU. There are nuances to this method, 
and it is EXTREMELY useful for diagnosing low level charge transfer
problems, but the above is the quick and dirty discription.

Aloha,

Gerry Luppino