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Good question. I think it improves all areas of the image particulary the coloured subs which tend to be noisier.
Even high signal areas will have some noise in them. Short exposures will have shot noise in them which is the randomness of light causing unevenness in an exposure.
But the background and dim areas are usually the weak areas of an image for sure and improve the most from extra exposure. Bight areas will respond better to sharpening if its longer exposures. Even bright areas often have dark sections in them of dust areas etc.
The total exposure needed for an excellent image of our average astrophotography targets does vary depending on aperture, f ratio of the scope and QE of the camera but even with the fastest of both you are still looking at really 8 hours minimum, more like 12 hours for a fast system and upwards of 20+ for a slower system.
Bright targets you can get away with less but if you are going for the best possible quality then the above minimums come into play.
Greg.
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