I use SharpCap to capture image using ASI1600mm-PRO COOL. I have some questions in the gain/offset/gamma setting while taking bias frame.
1) If I connect the camera directly, no matter how much brightness I set, the min value of my bias frame is always 0. My understanding is the brightness is the 'offset', which will be added to the pixel value. However, I set the brightness to the highest value (which is 60 in sharpcap), then tried it with different gain setting (such as 0, 139, 300). I took 32us exposure bias frame, and look at the min value or the histogram, it is always 0. I found I have to change the gamma from 1 to 50, then the min value will be larger than 0, but I'm not sure if that is the right way to take bias by changing the gamma value? I use MONO16 and take fits file.
2) If I connect the camera via ASCOM driver, it has some predefined gain setting (but not offset). With those setting, the bias frame's min value is 16. However, then I added some custom defined gain/offset setting, such as gain=300,offset=10 and gain=300,offset=100. Both of them gives the same min value in the bias frame, which seems to me the offset didn't have any effect. Is this the right way to set offset and the right expectation about the offset setting?
3) One question on how the 12 bit is converted to MONO16. When I use 12bit, and saves it in MONO16, my understanding is the camera will convert the signal to a 12 bit number which is 0--4095, then convert it to MONO16, in that case, isn't any pixel value should be a multiple of 16 (65536/4096), e.g. it can be 0, 16, 32, 48, etc, and can't be 17? However, I do see value that is not a multiple of 16, how does that happen?
4) One question on gain. when I set gain to 600, I assume it will multiple the signal by 60db which is 1000. The full well of the camera is 20k, so with 20e, it will reach to the max capacity. Does that mean the result image will only have at most 21 value (0, 1000, 2000, ... 20k) converted to 12 bit? However, when I set the gain to 600, I do see a smooth histogram, and doesn't seems to be only 21 different value. How does that happen?
Thanks!
