phew .. my head is spinning from some of the intellectual replies to this
.. must be the end of the day.
my understanding is pretty simplistic but works for me ...
Same F-stop = same exposure length, regardless of aperture, focal length or colour of the OTA
So if the 50" you're about to buy is F/6 and your 80mm is F/6, then exposure times would be the same to achieve the same image brightness.
If the larger scope you're looking at has a slower F-stop (F/10, F/12, rather than F/4, F/6 etc) then that larger scope will require longer exposures to get the same brightness of detail.
eg. If you go from an F/6 scope to an F/8 scope you'll go from 60 second to a longer exposure time (80 secs??) to achieve the same brightness of image.
Roger.