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Raymo (bless him) has it wrong here -- sub length for astrophotography has absolutely nothing to do with aperture (but it does [kind of], along with focal length, affect the image scale...). The only relevant term for exposure length is the f/ratio. The Esprit 100 at f/5.5 will put more photons on your sensor per unit time than the f/7 Esprit 120 (indeed, equivalent subexposures will require 62% less time on the f/5.5 optics). Dunk is correct that the larger aperture will have better resolving power, but this can be 'defeated' by careful use of dithering and drizzle stacking. If this is your first astrograph, I'd go with the 100 @ f/5.5
Last edited by Amaranthus; 14-09-2015 at 10:35 AM.
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