[Aavso-photometry] Linarity of SBIG ST-8 camera

[Lionel Catalan]

I've been doing tests to determine the linear response range of my SBIG ST-8
camera. The tests involved exposing the camera (without telescope) to a
relatively uniform light source for varying times. I used a device that I
had previously built to take flat frames. I found that the deviation from
linearity is less than 1% if the ADU values are kept below 50,000. If the
ADU exceeds 50,000, the deviation from linearity increases very quicky and
reaches 12% at 60,000 ADU. Since this is a NABG amera, I had naively
expected that the linear range would extend up to 65,535 ADU (the full ADU
range of the camera), but this is not the case. Then, after reading the CCD
detector specifications, I found that the full well capacity is about
100,000 electrons. Therefore, with a measured gain of 2.17 e/ADU, the full
well capacity should be reached at 100,000/2.17 = 46,000 ADU. So this seems
to be consistent with my measured linearity range (I hope this makes sense).

My problem is that I've been using this cameras for several months now,
collecting thousands of images, and in some pictures, the maximum ADU values
of my target or comparison stars sometimes exceed 50,000 (although I always
kept it below 60,000). It would be a big pain now to find these images and
to eliminate them from my photometric analyses. Has anyone any idea on the
possible error of having the maximum ADU in a star profile slightly above
the camera linearity range? My intuition is that the error may not be very
large because the large majority of the pixels in the star profiles are well
within the linear range. Any comments/ideas would be welcome. 
 
Lionel Catalan, CTE