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It all boils down to aperture. CCDs are linear in response meaning it will record at the same rate after 2 minutes as after 10 seconds until the wells are full. Your statement as you decrease F ratio you image with less time. No. You image with less time with larger aperture for the same focal length. Large aperture, short focal length images faster than same aperture and long focal length. Why? As I understand it is merely because when you narrow the field of view there is less light available to collect.
Faster F ratio means a wider field of view and thus more light to collect from the night sky.
I think exit pupil here in imaging should be changed to corrected field. That is the size of the circle of light that is corrected for coma and aberrations. That varies from design to design. That limits how wide a view that scope can image and what size sensor you can use. If you use a small sensor then you have the same situation as if you cropped a terrestial image and discarded the rest. A larger sensor simply gives a larger image with more light collected.
Hence in camera world, APSc sensors collect less light than full frame sensors. Same with telescopes.
So no you can't get say a 2 inch telescope at F1.8 and it will be way brighter than a 10 inch telescope at F8. It will simply be super wider field
but no brighter than the 10 inch. The 10 inch will be brighter because it collects more energy. Scopes merely collect energy not create it so larger aperture is always brighter than smaller.
Longer focal lengths are more susceptible to seeing than shorter focal lengths.
So if you live in an area where the seeing is poor then a longer focal length will not serve you well on most nights.
16 inch at F3.8 is 1546mm focal length and that is medium length focal length and I think in the sweet spot for most Australian imagers given seeing, local cloud, sensors available, difficulty of tracking.
16 inch F3.8 would be good. I use my CDK17 at F4.45 at tims (with its reducer) and that works very to focus more of the collected light onto the sensor giving a lesser imaging time. It works well and makes it less susceptible to seeing conditions.
Greg.
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