Linearity of CCD output amplifiers

[Michael C. B. Ashley]

Posted to CCD-world:
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Dear CCD gurus,

I am interested in knowing more about the non-linear behaviour of CCD
output amplifiers. I have made measurements of the performance of an EEV
CCD05-20 (770x1152 22.5 micron pixels), and have found that it is
non-linear at the 0.5% level below one-quarter of full-well capacity, but
deteriorates to 5% above this. Interestingly, the non-linearity can be
well corrected using a 2nd-order polynomial below one-quarter of
full-well, and a 3rd-order polynomial above this. There appears to be a
distinct "break" in the characteristics at around one-quarter full-well,
and I have been unable to find a single function which will represents the
entire range. The position of the break appears to vary slightly (with
temperature?). 

After correcting for the non-linearity, it remains stable to better than
0.1% from night-to-night, and about 0.2% over several years.  I measured
the linearity by using precision-timed exposures of a uniformly
illuminated screen, checking carefully for changes in illumination with
time. I guess there is the possibility of non-linearity in the remaining
analog signal chain, but I doubt it (this is with a Wright Instrument's
camera, which has given excellent performance).
 
Has anyone observed similar effects?  I am wondering if a knowledge of FET
characteristics would predict the behaviour I see, and perhaps suggest a
more suitable functional form for the correction, and reasons for the
variation in the break point. 

For those interested in the polynomials I found, here is some C code
to correct the raw ADU number "a". Full-well corresponds to 40000 counts
(we use a very low gain so that we can sample the full range of the CCD).

    if (a < 12000) {
      a =  a*(1-6.8e-7*a);
    } else {
      a = 806+a*(0.8586+a*(5.90e-6-a*3.12e-11));
    }

Regards,
Michael
--
Michael Ashley; Department of Astrophysics, University of NSW;
For further information:   "finger mcba at newt.phys.unsw.edu.au"

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