G'Day Frank, Great start to imaging with a QHY9. I have attached a section from the nebulosity user manual off the Stark Labs Nebulosity Manual. It explains the method of setting and forgetting your gain and offset figures. Well worth the read. For interest sake I run my QHY9 at Gain of 12 and Offset of 110. Somewher to start I guess.
Good luck with your new gear.
Cheers
Doug
5.4.4 How should I set my gain and offset to set it and forget it?
The best value for your camera may not be the best value for other cameras. In particular, different makers set things up differently. For example, on a Meade DSI III that I recently tested, running the gain full-out at 100% let it just hit full well at 65,535 ADU. Running below 100%
and it hit full-well at 40,000 or 30,000, or 10,000 ADU. There's no point in running this camera at anything less than 100% gain. On a CCD Labs Q8-HR I have, even at gains of 0 and 1 (on its 0-63 scale), the camera would hit 65535 on bright objects (like the ceiling above my desk).
There's no point in running this camera at gains higher than 0 or 1. Why is there no point? The camera only holds 25k e-. If a gain of 0 or 1 gets me to 0.38 e-/ADU (so that those 25k e- become 65535), running at 0.1 e-/ADU will only serve to limit my dynamic range. Each single electron already comes out to more than 2 ADU.
So, to determine the gain and offset to use:
1) Take a bias frame and look for the minimum value in it. Is it at least, say 100 and less than a thousand or a few thousand? If so, your offset is fine. If it's too low, boost the offset. If it's high,drop it. Repeat until you have a bias frame with an offset in, roughly 100 - 1000. Don't worry about precision here as it won't matter at all in the end. You now know your offset. Set it and forget it. Never change it.
2) Aim the camera at something bright or just put it on your desk with no lens or lenscap on and take a picture. Look at the max value in the image. Is it well below 65k? If so, boost the gain. Is it at 65k? If so drop the gain. Now, if you're on a real target (daylight ones are great for this) you can look at the histogram and see the bunching up at the top end as the camera is hitting fullwell.
Having that bunch-up roughly at 65,535 plus or minus a bit is where you want to be. If you pull up just shy, you'll get the "most out of your chip" but you'll also have non-linearity up there.
You've got more of a chance of having odd color casts on saturated areas, for example, as a result. If you let that just clip off, you've lost a touch but what you've lost is very non-linear data anyway (all this assumes, BTW, an ABG chip which all of these cams in question are). Record that gain and set it and forget it. Never change it.
By doing this simple, daytime, two-step process you've set things up perfectly. You'll be sure to never hit the evil of zero and you'll be making your chip's dynamic range fit best into the 16-bits of your ADC. Again, all the cameras in question have full-well capacities below 65,535 so you are sure to have enough ADUs to fit every electron you record into its own intensity value.