Hello! When binning, how do different camera manufactuers handle full well capacity?
For example, when using 2 x 2 binning of 9 micron chips that typically have 100,000 e- full well capacity of individual pixels, is the binned full well capacity additive at 400,000 e-, or do different vendors handle this differently? Thank you and best regards.
It depends. With CCDs, binning is accomplished by first transferring the rows into the "serial register", which is like a CCD row except covered so light can't get to it. Each pixel in the serial register typically has 2x the well capacity of an imaging pixel. So binning 2x rows won't overflow the serial register. Binning 3x rows might overlow. Then, the serial register is moved, pixel by pixel, into the "summing well," which is what is sampled for the readout. Again, it looks like an imaging pixel, but with an opaque cover so that no incident light interferes.
For most scientific CCDs, like those from E2V, the summing well has 4x the well capacity of an imaging pixel. So binning 2x2 won't overflow the summing well. Binning 3x3 may overflow. For most commercial CCDs, the summing well only has 2x the well capacity of an imaging pixel. So even binning 2x2 may overflow that "pixel".
The best way to tell is to do a linearity curve and see where overflow begins. If you can also obtain the gain factor, then you know how many electrons are involved. If you look at Berry & Burnell's great book, you can get information on how to perform these tests.
Thanks! For some reason, I thought FLI and SBIG handled binning differently even though the same chip might have been used, but I could not remember. Best regards.
I believe that most SBIG camera do not change gain with binning, but the ST 402ME does. Below is a copy and paste from an ST-402ME spec sheet.
2.0e- binned 2x2, 3x3
(1.0e- and 1.4e- for ABG CCD)"
From what I understand about CMOS cameras, binning is done entirely in software. So if any of the pixels being binned is saturated, then the readings from that set of binned pixels are suspect.
"From what I understand about CMOS cameras, binning is done entirely in software. So if any of the pixels being binned is saturated, then the readings from that set of binned pixels are suspect."
That sounds logical, and may well be true. But I suspect that could be drawing a fine line (if you are suggesting that the hypothetical CCD image may be OK but the corresponding hypothetical CMOS image may not be), depending on what proportion of the pixels in a star image is saturated, and what the overall effect on the magnitude determination of the star is for that image.
For example, although the following relates to unbinned ZWO ASI1600MM images, it does illustrate that a small proportion of saturated pixels in a star image does not trash the result, and that using images from this camera right up to the top of the linear range of the sensor is fine. The photometry is of the delta Scuti star RS Gru, taken on the night of 23 August 2019, with the camera set on unity gain, exposures 150 seconds, through an 80mm f/7.5 refractor. The line profile is from the image representing the 1st peak in the light curve.
Hello! After reviewing some prior posts, it appears that SBIG does not change the camera gain with binning while FLI does.
Is this still the case?
If I interpret this correctly, if the gain does not change, the total number of electrons at 1x1 binning would be the same at 2x2 binning - just distributed among 4 pixels. However, if the gain changes (or goes down), then the four pixels may be able to store more electrons than at 1x1.
Is this a correct way of looking at gain and binning? Best regards.