[CCD-astrometry-photometry] [Aavso-photometry] SXV-H9 linearity

[Arne Henden]

Pedro Pastor wrote:
> On the other hand, your explanation raises a new question (to me): Where the
> increase of sensitivity comes from in a binned CCD??
> 
> (You even said -from the accumulated signal point of view- binning could be
> equivalent to software pixel post-summing).
> 
> I would said the increase of sensitivity came (in a 2x2 binned CCD) from
> having an equivalent photosite of 4 times the surface BUT with 4 times well
> depth, that is to say, as if the chip "created" a new big well from the
> union of the 4 individual ones. (In that particular case no individual pixel
> saturation effect would appear).
> 
Yup, but that is not what is done.  As I said, the pixel array itself
cannot be binned (the electronic gate structure prevents it), so all
binning has to be in the readout process.

> And finally, that reasoning leads me to the point of asking myself what's
> the definition of "Gain" for CCD's. From manufactures' specifications, it
> seems like it is the electrons to ADU's ratio (a mere conversion factor).
> How can I work out the gain of my CCD on different binning configurations??
> 
I don't know quite what you mean by increased sensitivity.  A binned
CCD will _appear_ to be more sensitive since you are seeing 4x the signal
in each 2x2 binned pixel, but the QE certainly cannot increase.  If you
are properly sampled (say, the star fwhm is spread across 2-3 pixels),
then you don't need to bin (in fact, it will in general hurt your
photometry).  I only bin under very specific conditions.
   You will have to ask your camera vendor how they treat binning,
but I would be surprised if they changed the camera gain.  Yes, gain
is defined as the number of electrons per ADU, a conversion factor.
You can work out the gain by using the two flat/two bias technique for
determining readnoise and gain (I think it is described in Berry/Burnell).
Likewise, a simple check to see if the gain factor changes with binning
for your particular camera is to use a constant light source, set the exposure
time in unbinned mode so that you get a few thousand ADU per pixel, and
then keep the light source and exposure time fixed while varying the
binning.
Arne