This is an interesting topic, and there are a lot of different opinions on the subject. One of the more often expressed opinions is that aperture doesn't really matter for astrophotography, and that only f-ratio really matters. I am personally of the opinion that both does matter. I read quite a bit in different forums, and the conversation take a very intensive options that the element of finding a solution seems to dissipate alone the way.
This is an hypothetical example, but one that I which to find a reasonable solution.
System A : RASA F2.2, 11” (279.4mm), Focal Length 620mm, camera is a QHY (CMOS)268C with a 3.76um pixel size and 26megapixels
System B : Stellarvue 130mm, F7, Focal Length 920mm, same camera.
So, if we use the expression (F2.2/F7)^2 = nearly 10%
The f/2.2 scope will have the shorter exposure times. The required exposure time will be 10% of what the f/7 scope requires for the same image brightness.
So, if system A took an image of NGC7023 with a total integration time of 11.6hours, and if we apply the above expression using system B the total integration time will be 104 hours which is unrealistic.
The other factor is the angular magnification of both scopes, one is 620mm while the other is 920mm, (920/620 = 1.48) , there is a 1.5x differences and the area of sky covered of 2.25 ((920/620) ^2 ) times larger for B scope, also I believe that we need to take into consideration that there is a differences in F-Ratio.
I am just pointing out the different factors that I need to take into consideration to be able to come with the correct exposure time, this is my objective, finding a formula that I can rely on, I understand that the observation location with be different from one person to another and as we know the skyglow is also an important factor.
Taking into consideration that I have an excellent Polar Alignment and mount and without entering into an intensive discussion what would be a reasonable formula to calculated the exposure time based my points above, or if I wanted to experiment if I see an image from a fellow astrophotographer using the same camera??