I don't have the number right now but I remember over 100 to 110mm actually.
I do have a big wynne corrector that should be about to handle that, but all that weight up front will probably require some major mods.
I know Val uses the AP CCD67T reducer. I wonder if the Optec 0.5 or 0.3x would be possible with a small sized chip (doubt it though).
I have the optic reducer. Would be interesting if it would work with the F20? I also have the AP reducer. Will be playing this weekend should the weather cooperate. You go to love scopes like this. They are what dreams are made of just when you look at them