I have discussed this question with ZWO and I don't have a good answer from them that I can understand.
The performance graphs show the horizontal axis as Gain (0.1 db) and in one of the graphs there is a vertical axis also called Gain in units of electrons per ADU. I know the Gain shown on the horizontal axis is the value I set in the camera driver, and I know what the Gain e/ADU is.
Can someone help me understand the horizontal axis?
Thanks in advance.
Horizontal axis is gain in Decibels (dB). For ZWO cameras, this corresponds directly with their gain "setting". So, if you use a setting of 139, or 13.9 dB of actual analog gain, that is "unity gain", and therefor your actual conversion ratio is 1e-/ADU.
The vertical axis in the second chart is the conversion ratio of electrons (e-) to ADU (analog to digital units, the numeric value saved for the given pixel in an image as a result of converting charge/voltage to a digital value), at a given gain.
So, in the second chart, you are mapping GAIN SETTINGS/GAIN DB to the CONVERSION RATIO of electrons to digital units.
Both values are important. One tells you a useful value, e-/ADU, with which you can calculate other useful things (i.e. swamp factor, so you can understand how much signal you need to measure in a sub (ADU) to know you are swamping read noise (which is usually reported in e-). The other tells you what camera gain setting to use in order to achieve that conversion ratio (gain) in e-/ADU.
Edited by Jon Rista, 27 October 2020 - 10:35 PM.