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

[Arne Henden]

Pedro Pastor wrote:
> Hello,
> 
> I find this topic very interesting. I have always wondered how binning works
> with respect to the main CCD sensor parameters (linearity, gain, SNR, etc).
> And I end up thinking I have no idea how binning works.
> 
> Some time ago I was told a binned CCD camera is always linear. After Gary
> comment it seems to be not. How does it work? If I use 2x2 binning, what do
> I get: a) The sum of the electrons on 4 different wells; b) Only 1 well
> where all 4 photosites pour electrons together (up to ADC range)??
> 
The exposure is always made with the entire array in unbinned mode.
It is only during the readout process that binning takes place.
   Charge is first shifted from row to row, with the first row
being shifted into the serial register.  Vertical binning takes
place in this step, as you can shift more than one row into the
serial register at one time, effectively summing the charge.
   Charge is then shifted along the serial register, each serial
pixel ending up in the output summing well and read by the electronics.
Horizontal binning occurs here, as you can shift more than
one serial register into the summing well, effectively summing
the charge.
   For this binning process to work properly, the serial register
"pixels" usually have twice the well capacity of a typical array pixel.
Therefore, two nearly saturated pixels can be summed together in a
serial pixel without any loss.  Then the output summing well has to
have 4x the capacity of an array pixel in order for a 2x2 binning
operation to not have any loss.  These increasing well capacities are
common in science-grade detectors.  No detector I know of has 3x/9x
capacities in the serial and summing well pixels, so anytime you
do 3x3 binning, you always risk saturation of the summing process.
   Now, what happens if an array pixel gets saturated, but its neighbors
do not (for example, if you are undersampled so most of a star's light
falls on a single pixel)?  The 4 array pixels comprising a 2x2 binned
pixel get summed together; the resultant signal is less than 4x the
capacity of a single array pixel, so you assume the binned pixel is
unsaturated.  This can be a false assumption, which is what Gary
found out.
   Usually commercial camera vendors keep the same signal-chain gain (electrons
per ADU), no matter what binning takes place.  This means that summing
4 pixels, each containing 1/4 of their full well, can result in a
signal that pegs the ADC.  Many professional cameras have adjustable
signal-chain gain so that you can keep full-well performance if you wish.
   You can always do binning in software after the readout, of course,
and handle any kind of binning factor without saturation.  The
disadvantage is readout time, software time, and increased readnoise
since each array pixel is then read in unbinned fashion.
Arne