Shortest focal length eyepiece with your Cat?

I'm researching information for some short papers/articles about choosing equipment. One of the puzzles I've come up against is that the focal length advertised by manufacturers of Maks and SCTs that focus by moving the objective appears to be the minimum possible, and mirror movement during focusing can increase the focal length by as much as 50% on average.

Among other things I want to recommend a good rule of thumb for calculating the longest and shortest useful eyepiece focal length for each category of telescope. It would be very helful if any of you SCT and Mak owners who have pushed this limit could reply to this thread giving a description of the telescope in question and the shortest focal focal length eyepieces you've been able to use consistently. A good sample will give me empirical data against which to check the validity of the rules of thumb I've looked at so far for use with SCTs and Maks that don't have an external focuser fitted.

Please state the type of scope, its objective diameter and advertised focal length, and the shortest focal length eyepiece you've been able to use consistently on DSOs, planets and the lunar surface.

John --

There's a great paper on the effect of a moving primary by Chris Lord. It can be found under "Calibrating The Effective Focal Length of Catadioptric Cassegrains With Moving Primary Focusing" here: www.brayebrookobservatory.org/BrayObsWeb...%20PUBLICATIONS.html (about 3/4 of the way down the page).

There's some ugly maths in it, but theres also a chart at the back which delivers what most folks need.

As for the data, I've used a 5mm effectively (once) with an 8", a 7mm several times with a 9.25", and an 8.8mm several times with a 16".

I've also used a 6mm on the 16" a couple of times, but only to separate a very tight double or confirm a very small galaxy or planetary as being non-stellar. The image at that magnification in our seeing certainly wasn't pleasant.

-- Jeff.