Hi Steve
To add to Dave’s comments, the Mersenne does not produce a focus on its own. It produces a parallel light beam. Clive used a very expensive apochromatic refractor to get focus. It’s visible in the pictures. Apparently when you view through a scope like this you see the mersenne field of view inside the refractor field. It would be interesting but unusual.
If you want a compact cassegrain design, I suggest you look at this one:
http://www.loptics.com/ATM/telescope...12in_cass.html
You can have the focus come out the centre of the altitude bearing if you want the eyepiece height to remain more constant.
There is also an old design called the ‘beaver tail’ cassegrain. It is essentially a Springfield mounted cassegrain. The eyepiece position never moves.
Cassegrain optics tend to be custom made and expensive. You need a bit of experience, preferably with faster mirrors, to make your own. You also need to read up on baffling which is essential in a cassegrain.
Rod.