Hi All:
Thanks for the responses. Sorry to disappoint, but the issue shown has nothing to do with the gamma setting. The gamma was simply adjusted to give the best possible contrast of the data that was captured. Normally I leave the gamma slider to the right to see the brightest guide star(s) against a black background, but as I increased the exposure to show multiple stars I had to turn it down to have them show on the screen at similar magnitudes, but against a grey background. However, when reaching that last bad frame, I had to pull the gamma back to the right to see anything other than a solid white frame.
I won't have clear skies for a while yet, so I can't do a star test, but I did go back and run some tests with the ambient light in the observatory and learned a bit more about what is happening, but not why it's happening. Unfortunately the gamma slider DOES affect the saved image, so it's not possible to evaluate the camera by itself through PHD. The issue is a bit more insidious than that as well. I was able to take a few shots last night inside my observatory using the IR light from my monitoring cameras. That doesn't give me the ability to evaluate star vs. background, but did teach a little more about what PHD is doing.
So first off, extracting the raw dark from the dark library FITS file, the dark noise looks reasonable up to the 15 second exposure, so that shouldn't be an issue.
With the slider right, a two second exposure of the IR background gives this result.
Moving the slider left shifts the histogram to the right, indicating the slider acts like a typical midpoint adjustment.
Thus, as indicated, when the background is getting swamped as in the 5s exposure, the slider should be to the right to pull the background down as far as it will go.
With the slider to the right and no dark library applied, we get this noisier histogram that's even further to the left:
Now, here's where things get interesting. Turning off the IR cameras so that I just have whatever minor background is in the observatory due to a stray LED or two, I get the following with the dark library disabled. I think this was a 5 second exposure, but can't be positive as PHD2 doesn't appear to put that into the FITS header.
Applying the dark library, however, gives a really odd behavior.
The histogram narrows, which is to be expected when subtracting a dark from dark, but the screen function shifts the entire histogram to the right so it's full white instead of black. I don't believe the slider impacted this, but I'd have to repeat the test again tonight to be sure.
Unfortunately this still doesn't answer what's happening at the 5s exposure where I have multiple saturated stars (right of the histogram) and a white background as well. Is that real or some oddity of the screen stretch algorithm like the dark above? The only thing I can think of that might be a real effect is that since this started out as a quick test, I didn't bother turning off the observatory cameras. Normally that only introduces a small amount of "sky glow" when the scope is pointing up, but given how sensitive this camera is, and that it appears to be pretty sensitive to IR, that may have contributed to the 5s exposure problem. Unfortunately I won't be able to check that until we get clear skies.
Now, akulapanam, first off, I have to ask, where were you when I was asking about differences between ZWO and QHY!?!! 
I'm assuming you're referring to fixed pattern noise, in which case I'm happy if QHY is better, but I have to ask why two cameras using the same sensor would have different FPN. About the only impact I would think the camera hardware design would have would be thermal impact of the PCB and packaging. I wouldn't consider that FPN. This is of course assuming that they're buying the sensors from the same manufacturer, but given these are Sony IMX178s, I wouldn't think that anyone else has a clone (yet). Then of course there's driver functionality and any post processing they might apply, but I would be surprised if there's something there.
Thanks,
Beo
