SCT + focal reducer, working out the f ratio, lost link

[Tim]

With the sky full of viewable planets at the moment I have mounted my C9.25 in the obsy for a bit.

I have fitted the f6.3 reducer/flattener. I used to have a link to a formula or table that showed the f-ratio achieved with various spacings, other than just the standard 105mm for f6.3. (With 225mm for instance you can achieve f5 with an F10 SCT, and shortening the assembly to 50mm gives a 0.7 reduction to f7.)

Does anybody happen to have or know of a link to such a table?

Thanks

Tim

[Tim]

Found it:

http://media.skyandtelescope.com/images/Linked-1.gif

[barkis]

Why would you reduce the focal length to image a planet Tim,

Unless there is a very,very, close conjunction coming up?

Ron.