söndag 15 oktober 2017

Optimising offset for the ASI 174MM-Cool

Used last night's cloud cover to do some testing with my cooled CMOS, the ZWO ASI 174MM-Cool.
The gain of CMOS cameras is variable, and to avoid clipping dark pixels, you have to add an offset or pedestal. Basically this means that each pixel value that is read is transformed by an amplifier and analog - digital converter (ADC) according to the formula

ADU = (pv + Offset)/Gain

The odd use of dividing by gain in stead of multiplying is because gain is defined as e/ADU. One would also expect the offset to be in ADU, giving the formula

ADU = pv/Gain + Offset

But I found that at high gain setting, any change in offset is much more critical than at low gain, so I think the proper formula is the first one.
First off, you have to distinguish between gain setting ( for the ASI 174, the gain setting can be varied from 0 to 400) and real gain (about 8 e/ADU at setting 0, 1 e/ADU at setting 189, and decreasing e/ADU the higher the setting goes).
I used this simple method to determine the best offset for various gain settings.
I took one bias frame at the shortest possible exposure time (32 microseconds) for each gain/offset combination. I varied the gain setting from 0 to 400 in 100 unit steps, plus unity gain (gain setting 189). I then varied the offset and used PixInsight image statistics to determine when the lowest pixel value (in ADU) started to increase. For some reason the lowest pixel value was at least 1, so I increased offset until I got >2 - 3 ADU as a minimum pixel value.
I did the tests at -15 degrees Centigrade, because that is the temperature I use for imaging at the moment. Offsets shouldn't be that temperature dependent any way.

The highest offset that my driver (INDI) allows is 240, which wasn't high enough for a gain setting of 400. The highest gain setting that can be used with the offset at 240, is about 365.
This exercise shows that in order to achieve the highest dynamic range in the ASI 174MM, you can optimise the offset for each gain setting to avoid pixel clipping. Using a higher offset than needed to avoid clipping will decrease the dynamic range of the camera.