u/Accomplished-Slide52

Can someone give me the formula that give an acceptable time exposure for a S50 against the declination of an objet. I can't remember where I've seen a graphic but no very easy to read. TIA.

From a beginner. When you do multiple sessions of the same target how do you stack your images? Stack for each session and then stack the FITS files or merge all images and stack them in a single run. Why to choose a method rather than the other?