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The focal ratio only comes into play with extended objects. For stars, which are point objects, aperture wins every time--you'll get fainter stars with the same exposure. Also the image size will be smaller in the 80ED (about 60% 0f the size in the 100ED), so yes you will need a longer exposure, but at the end of it you'll have a better image to work with. The other thing you can do is invest in a focal reducer to use with the 100ED, so you can get the benefit of both aperture and short focal ratio. IMO go for the 100ED.
Geoff
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