Affiliation
American Association of Variable Star Observers (AAVSO)
Thu, 08/06/2020 - 13:02
Someone was posting a question about what camera to buy. At some point in the thread, someone mention issues of linearity with CMOS and that they should try to buy a CCD camera because CMOS cameras have issues with linearity.
So, I did a linearity test on my ASI183MM-Pro using my homemade light box for flat frames. I took 13 sets of .FIT images, ten in each set, at an exposure interval betweeen 1 to 13 milliseconds in increments of 1 millisecond each. I recorded the average ADU for each .FIT file and then calculated the average ADU of the average ADU for each set of ten.
Nice. But one weakness of some CMOS cameras in photometry is that they are linear only with certain values of gain and other settings in effect.
Could you then post the gain and other settings you used for this test? Thanks.
I set my gain to 0. The way gain works in ZWO cameras is
(Full well capacity)÷(2A/D conversion-1)]÷(10Gain setting÷200)
For the ASI183MM, that works out to be
Gain=(15000÷4095)÷(100÷200)
Gain=3.663÷1
Gain=3.663 Electrons per ADU
This is divided by 16 to 0.2289 Electrons per ADU when converting from 12-bit to the 16-bit storage of .FIT.
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