I am familiar, of course, with gain for DSLR cameras (where it is folded into the concept of ISO) and gain on scientific cameras, where it is a multiplier on the number of electrons in a pixel well.
I now have a QHY 128c camera which has "gain" numbers which run from 0 to 4000 (see https://www.qhyccd.c...=94&id=12&cut=1 .
It can't be a multiplier if the value 0 is a legal value.
There is a graph which relates the "gain" number to "system gain" which has units electrons/ADU. At gain 0, the system gain is 4.5 electrons per ADU, at 4000 it looks from the plot like it is about 0.2 . For what it's worth, the plot seems to be of the function
system gain = -0.001075* gain + 4.5
A typical use of ISO is to all the photographer to pick a different exposure. If you know the exposure at ISO 100, then you can easily calculate the equivalent exposure at ISO 50, ISO 400, or any other value. By equivalent exposure, I mean something that gives the same pixel values (ADU).
That does not seem to be the case here. if we have a scene in brightness B1 and gain G1, exposed for t1 seconds another at B2, exposed at t2 seconds with gain G2 the two shorts are equal if and only if exposure time t2 is
t2 = (B1*t1*(4.5 - 0.001075*G2))/(B2 (4.5 - 0.001075*G1))
That's not particularly intuitive or useful. Why not just make the gain look like ISO numbers? Since the same sensor is in Nikon DSLRs it is clearly possible to make that happen.
I am surprised that there isn't more about this in the QHY manual. Don't you need to know where to set the gain to take good pictures? Since many people have QHY cameras and I see some great pictures out there I have wondered if there is some secret of setting gain that I have just missed it.
Edited by nathanm, 16 September 2020 - 06:06 PM.