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That's how I see it too Mark.
If F ratio makes no difference, well why not image DSO at F40 then with 8 inches of aperture. The extreme example helps show the point.
But the troubles with some ASA F3.6/3.8 scopes shows how critical everything becomes in quality required to achieve a practical scope at the faster f ratio. But see Wolfgang Prompers Namibia images using one and a FLI 6303 camera and look at how deep the images are and then look at how short his exposure times were. But they also aren't 12 inch RCOS closeup galaxy shots either and that scope would be no good for that sort of work. So it is hard to have scope that is all things to all people. Often 2 or 3 scopes are required to fit the sweet spot for different types of images. The Cervalo Astrograph is an attempt to capture both grounds fast F ratio for the wide field and slow f ratio
for the galaxies or closeups. You can see a difference in resolution between the 2 f ratios and that is from the same scope and same camera. A good example of this theory that it is a workable guideline but not a truth.
F5 -7 is not a bad range for imaging. The Tak 180ED is F2.8 and they routinely showed lots of faint nebula in the usual imaging targets not normally seen.
Greg.
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