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Thanks, Sean. Some great articles there.
So as near as I can tell, the factors involved in discerning an object are;
Surface Brightness - how bright it is across the surface area of the object
Size - Given the same SB, a larger object will be easier to detect than a smaller one
Contrast - How much contrast this level of SB has with the background skyglow
Exit Pupil - Obtaining a brighter image at the eyepiece gives your rods more photons to play with in making contrast assessments
Seemingly it's a balance between, on the one hand, using too little magnification where you get a bright field, but the object in question is too small and therefore doesn't stand out from the background sufficiently for detection, and using too much magnification where the entire image suffers from an exit pupil that doesn't deliver enough photons for your eye to work with.
Markus
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