How to calculate the focal length of a SCT or MAK

Dave,

It is indeed scarry how a little change after the rear cell of the SCT can dramatically change the focal ratio of the system. This is something I first realized when doing work with a variable projection adapter where my camera at prime focus would have a moon that just fits perfectly or is way too big by just moving the tube of the variable projection adapter ever so slightly.

Anthony.

Anthony,
Next time I'm out and if I have the time, I'll try and figure out using skymap and a 26mm eyepiece where the F10 position is.

You can superimpose the field of view of an eyepiece in skymap, telling you what you can and cant fit into the field of view, so I'll try and match up the scopes image to that.

After pouring the Mapug archives I found this.
www.mapug.com

"it was felt that the best primary to secondary spacing was achieved when the focal plane for an object at infinity was about 3 to 4 inches beyond the back of the telescope back plate.
"

"Even then there are compromises in the design since not all aberrations come to their lowest values for the same spacing. "
which is no surprise.