I looked at this scope seriously last year but thought f/20 did not bring much benefit. f/15 would be more manageable for a modest increase in CO which would have no effect for imaging and be undetectable visually. I wonder why CFF doesn't offer an f/15 by default and build longer versions for who want them.
The point - but one could ask directly to them- could be that such an instrument wouldnt be anymore diffraction limited in the whole field. My ATMOS simulations show (assuming an f/3 primary as fixed) that the f/15 would be only few cm shorter, 5% more obstructed, and not diffraction limited on the edge of the field.
Edited by Vinny1980, 14 August 2019 - 03:18 PM.