Focus Assist?

I have a Pentax PF80ED 80mm ED spotting scope. I would like to be able to use a diagonal with it but when a diagonal is added, the focus travel is not long enough to get focus. Is there a device that will assist in the focus of such a setup? I think the additional length introduced by the diagonal makes it impossible to get the focus far enough "out" to match the focal plains...

The PF80ED uses standard 1.25 eyepieces with a collet type eyepiece holder.

Thanks

// Jerry

As a last ditch effort to fix the very same problem I did the following. Found an unused plastic barlow. Took out the glass. Used the leftover tube to lengthen my focuser. Its cheap and has been used 23 years on my 10 inch f/10 refractor. There of course are other remedies, but this one is quick and not asthetic.

Jerry,

You need to look for a diagonal which has a barlow built into it. I don't know that these are made any more, I got a used one with a short 80mm acromat I bought a few years back at a star party swap meet. This also came with a short style extension tube.