This was captured on July, 1 just after 12am, so definitely in time for this month's challenge! While one instance of Sharpcap was shooting darks and bias frames for the sake of science (see the flat research thread), another was pointed at M8.
Here it is after 10x60s as seen from my Bortle 8-9 balcony, L-Enhance filter, Altair 294c... and no flats as can be seen. It's still a beautiful nebula, even with no flats
Interesting coincidence -- I have been doing EAA with some dim nebulas with an L-eXtreme filter on my ASI 294MC Pro camera on an 8-inch SCT with 0.63x focal reducer and have experienced the same stacking histogram that you have pictured here. The histogram starts at the right side of the peak, so there is really no black level adjustment available. Yet the display histogram looks normal.
I have found that in most cases, if I move the color sliders (mainly red and blue) upwards, I can get to the left of the histogram peak and can adjust the black level. I also found that if darks are not applied, the entire histogram is shown in the stacking histogram. It is only when darks are applied that it looks like that.
Is there a reason why you did not apply flats? I found that applying flats only would over-compensate -- the image would have bad inverse vignetting. But if darks were also applied, the flats appear to work fine. Just the weird histogram issue. Another solution to the strange histogram that I tried was to increase the offset/brightness. That worked for the histogram shape, but then vignetting (or reverse vignetting, I can't remember which) would appear because the brightness level was then different from that used to create the dark frame.
It may be possible that longer exposures are necessary (I was using 20-30 seconds at gain 350 in my case), but I didn't have darks at longer exposures in order to try that.
Maybe you know what's going on with the stacking histogram here and can explain?
By the way, the image looks fine without further adjustment of the black level.
Edit: Never mind, I found the answer! (See https://forums.sharp...php?f=5&t=4226)
Edited by donstim, 08 July 2021 - 01:58 PM.