I am working on processing a 3-hour stack of images (not my own) of M45 to California nebula that should be producing a very nice image showing a good level of dust. The sky was a Bortle 4 to 4.5, the guiding was well under 1 arcsecond for the entire time, and yet the final stacked image was incredible noisy and impossible to process into a good-quality image,
Could at least part of the reason be that the subs were 6 minutes in length with the resulting historgram was at the 50% mark producing very bright subs?
I have always under the impression that the optimal exposure for the subs has been reached when the histogram is at about 1/4 to 1/3 from the left edge. Is this not correct?
If a histogram reading at 50% is indeed too much light, what exactly happens to an image when it is overexposed? Does it degrade somehow to produce a noisy image?
A few things.
[if you know such, disregard, maybe some readers may find it helpful]:
#1 Samir is spot on to differentiate the difference between histogram from a consumer camera showing a histogram based on the (jpg-like) image that is displayed, v. the linear data of the image itself.
#2 keep in mind what a histogram shows:a distribution graph of the number of pixels(y-axis height) of a given luminance(x-axis left to right).
It will tell you the span and whether you are crushing the blacks or clipping the highlights, but it also very much depends on the subject matter. It is helpful you described the image (16 deg FOV?)-so lots of sky and not filled with bright nebula as if you were capturing CA nebula alone......IMO the histogram alone doesn't give you the full scope of the situation, though..
#3 visual noise is interesting, because sometimes it means you are seeing noise in the shadows due to the exposure/color correction,
which otherwise is still there, but would be masked/hidden with a lower exposure/color correction....
When an image is over-exposed (in areas of the image), you will lose details/color in the highlights/brightest parts of the capture.
In the shadows, if you haven't adjusted the image up, you would not have increased noise compared with a capture with all same except exposure time. [assuming no additional noise is added by the system]
^ of course depending on the types of noise affecting the system-say upper atmosphere moisture one wasn't aware of, thermal noise with the longer exposures, etc....
So what exactly does the noise look like in the stack? Is it in the highlights (nebulosity of pleiades and CA neb?) Is it in the sky and among the dust/dark nebula?
Here's the most difficult problem:
What might have happened with the individual subs that were exposed is that the darkest pixels might not be so dark and your lightest pixels are over-exposed, compared to the detail one was aiming for:
expose so dark space =pixels at low values, dust =mid, CA &M45 nebulosity bright.
From this point the capturing dynamic range of collection (was crushed) compare with the scene dynamic range (especially within the range one desired to capture)......
In so doing,
one will find the noise (grain) will have the same luminance values as the desired shadow detail (dust),
so one cannot just adjust the (offset) or other color correction tools to drop the noise down to the noise floor,
because in so doing,
you will be dropping the shadow detail desired (dust) down as well.....
your noise (grain) with the stack shares the same luminance values.....
Additionally, if you think of all the subs as if they were frames of a motion video,
[something that could be a thought-open about 30 subs in LR/Bridge, color correct them for decent subject visibility, and then open them in Photoshop (script>load files into stack) and make them into a short image sequence and look at the movie...Where is the dancing noise?]
consider the 'noise' is temporal and spatially dynamic,
while the shadow details of the target (like dust) should be static.........
This is another way in which one can tackle noise reduction.
But since you are looking at the stack,
what would happen if you went through and color corrected the individual subs so the darkest pixels are lowered?
Maybe you could find in the subs a differentiation between the shadow detail you want and the the noise you don't want in luminance values....
If so you can crush the noise out, keep the shadow detail,
and then the stack won't have the noise in the shadows you have now.......
But this is assuming the scenario where the noise is in the shadows, not the highlights,
and "bright" subs means you may have lost detail in the highlights=not recoverable.....
I intentionally refrained from using specific terms in the above,
as maybe some of the processing heavyweights will add in with troubleshoots they use with the particular AP software packages....