[Aavso-photometry] CCD Linearity testing

[arne]

Gary Walker wrote:
>  Hello Tom
> 
> I assume that you are using a light box.? Are you using LED's or incandencent lights?? Arne has always cautioned me away from LED's, because they have a peak in the response curve.? If this aligns in frequency with a peak in the QE curve, we can have problems.? 
> 
Just to keep the cc list short, I'm replying to Gary and the maillist.

Tom is doing linearity tests, not flatfielding.  For linearity, you
don't worry so much about flatness or QE peaks - what you are looking
for are changes with respect to total exposure on each pixel.  LEDs
are fine for this, with the possible exception being Residual Bulk Image,
an effect that increases in strength with red light.  So you may get a
different result with, say, red LEDs, than with white LEDs.  Also, with
linearity tests, you generally don't need to have the telescope involved
at all - you can take the camera to the lab and work with it there.  This
keeps the temperature constant, and you work with completely defocussed
light, since there are no optics in the path.

One trick to guard against a systematic buildup of residual light is to
increase exposure until you get total saturation, and then ramp down
and see if there is any systematic difference between increasing exposures
and decreasing exposures.  I usually run several sets of "ramp-up"
and "ramp-down" cycles.
Arne