[Merlin66 (Ken)]

You've seen the Ads on eBay and Orion etc. for these 60mm finders with helical focusers.....look like a great idea for finder/ guiders - right?

Well maybe - I bought one a couple of years ago to mount on my C11 as an electronic finder (replacing a 135mm telelens set-up). No major issues and it still works very well with my DMK51. (One issue - these finders definitely need a UV-IR filter fitted to the camera or you'll get bloated images. Not so easy to achieve with T thread adaptors).

I also picked up a "dud" 60mm finder last year - which appeared to give "carp images". Well, I've finally tracked down the problem. I star tested the finder and used the Ronchi grating on it - not perfect but I've seen a hellava lot worse.
I put the QHY5Lii on it and disaster - soft carp images even at (what I thought was the best focus). ??????
After many iterations, I finally traced the issue to the length of the camera nosepiece (gets worse when you fit a UV/IR filter) v's the available length of the 1.25" section of the helical focuser!!! You can check this easily by feeling inside with your finger - there's a "shoulder" about 30mm inside which blocks the nosepiece. What this means, is you think you're focusing but the camera is "fixed" by the shoulder and your adjustments are achieving nothing.

The end result: not capable of properly focusing cameras with long nosepieces but can be made to work with cameras using the T thread attachment.

Caution!!!