Quote:
Originally Posted by JohnH
Tj,
I am very glad you brought that subject up.
Is there a way to optimise the Gain/Offset settings for a particular CCD - the linked article addresses offset by taking bias shots and looking for the singnal "floor" if I am reading it correctly but the English is not crash hot. There is no mention of GAIN settings at all. In the case of the 145m I have set the gain to 400 (there are 1024 levels available). I did this based on the two attached graphs which show me S/N ratio increases with gain to at least that level.
I would love to confirm my interpretation, what is the best setting to use? I guess I will be trading some dynamic range for sensitivity if I go too far? What is a good rule of thumb 3/4 of max? Half?
Should I set it perhaps based on the sky glow level - eg set an exposure length I can track well to routinely like 240s and then adjust the gain to get the sky glow histogram showing nicely and then set the relevant offset?
Any advice welcome.  |
Hi John, for the QHY8 [but I think it can be generalised to any CCD cam] I use the minimum gain 0 or 1. I set the offset to have a sky ADU [pedestal] between 500 and 1000. The higher the offset the higher the pedestal. The higher the gain the quicker you'll white clip the right end of the histogram.
It will depend on CCD chips/Camera , sky conditions, temperature, DSO you're imaging, etc... The bottom line is to maximise the amount of data between the left and right of the histogram to get the maximum range. My QHY8 will saturate real quick on bright stars at gain 0/exp 15min and over so I never had to push the gain settings. Works for me so far.