>>>>>> My camera is a crop-sensor DSLR, so as far as I understand, I shall multiple the lens' FL by 1.6, which would give me 670 mm
Crop Factor strikes again......
Forget about the crop factor.
THink only of Field of View. The "crop factor" does not change the focal length of your image. It only crops out the outer edge of your image. Instead of looking out a picture window, you are looking out a porthole!!! (OK, that is extreme, but it gets the point across.) At any rate the tree in the distance, whether through the porthole or the picture window is precisely the same size. The only difference in the picture is just how much more of the landscape you see out there. The tree is in the center of both, and precisely the same size. With the porthole, you only see the tree. With the picture window, you also see the surrounding lawn.
An 0.8 focal reducer will make your porthole more of a picture window, by a factor of (the inverse of ) 0.8.
I suggest you go to Telescopius.com . Towards the bottom of the home page, you will see a "Telescope Simulator." Do what you need to do to designate your camera and scope combination under "Astrophotography" and ask the website to show you M31.
Take it from there.
Edited by Alex McConahay, 28 February 2020 - 06:08 PM.