Automatically pick gain/exposure settings for alignment

[bgottula]

Implement an algorithm to automatically choose the guidescope camera gain and exposure settings. This probably needs to be done on a per-location basis because assuming the sky is uniformly bright is probably a bad assumption especially in the presence of local obstructions.

Why: A single set of gain/exposure settings is not appropriate for all sky conditions. If settings that work well at night are used at twilight the image may be significantly overexposed causing plate solving to fail when it might have succeeded with other settings. If settings that work during twilight are used at night, plate solving may fail to detect some stars that are underexposed which may limit the number of positions on the sky where alignment can succeed.

[bgottula]

Notes from experiments performed on 27 November 2020:

• first successful plate solving near zenith: 5:17pm
• camera settings: gain 0, exposure time 0.5s
  • Default alignment gain setting is 400 with exposure time 0.5s. With these settings the images were very overexposed at the time I was first able to get good plate solves which is how I confirmed that some kind of gain control is required.

I was getting good plate solves at other locations on the sky somewhat lower from zenith with same settings over next few minutes after the first successful solution.

I also noticed that the same exposure time and gain settings in the ZWO capture program did not yield same brightness as in track software with camera in alignment mode. I think I have the same binning (4) and same gain (0). Perhaps the difference is due to using 8 bit in capture versus 16 bit mode (I think) in track when in single-shot mode?