Effective F-ratio with star diagonal and focuser

I heard that the effective F-ratio of the C14 with a focuser and star diagonal is actually F/12 because the focus is moved back (because of the primary being put closer to the secondary).

Can anyone confirm?

Interesting name you have there, Dan,

As for your initial question, I have heard that the focal ratio shifts ever so slightly, but I do not believe it could change from F/10 to F/12. The only way it could do that would be if the focal length of the system changed, or if the aperture changed. And the change would have to be some 20%.

Maybe there is a way this could happen. But I could not see how.

Alex

The SCT secondary is a divergeing element, if moved forward in the optical path it becomes more effective.
Assumeing the SCT primary is some f/2.5 and the secondary acts like a 4x barlow and I'd guess the f.l. increases about 4% for every 1% increase in the distance of the secondary to the final focal plane.
Thus 10% or 20% more f.l. would not surprise me.

Michael