I have contacted Stellarvue and GSO again about the thread pitch - I didn't feel up to risking the $$ for a custom adapter without a bit more confirmation. In the meantime I took a hacky approach and ordered up a 2" nosetube with M56 thread on the end, which if I guessed right should thread into the M56 ClickLock I ordered previously:
These adapters seem to be popular but are normally used with a ClickLock "the other way round", ie M56 ClickLock threads onto the end of the focuser drawtube, adapter fits into the ClickLock, then M56 reducer threads onto the adapter allowing rotation and exposing M48 filter threads. I'm going to try putting the adapter into the AT1010 visual back then threading the M56 ClickLock onto the adapter.
I got curious about how much bearing surface there would be to keep the adapter aligned so took a look into the focuser drawtube to see if the inside of the tube happened to be 2" ID like the visual back (no, more like 2-1/8"). What I also saw was that a piece of plastic had come loose and was stuck at an angle in the drawtube. That probably explains the less-than-inspiring views last time I took the scope out. I was playing hide-and-seek with cloud cover but still couldn't understand why I was having so much trouble even getting a good view of the moon. Figured if the seeing was that bad it wasn't worth bringing out the AT115. D'oh !!
I pushed the plastic piece back to the end of the drawtube, guessing at orientation, then kicked myself for not measuring the inside diameter of the hole before doing so. I normally measure everything. Doing some very crude measurement using the picture I get somewhere around 34-35mm, which seems to make it more likely to be acting as a baffle rather than a mask.
Anyways, now that I have a fallback plan I figure it's more likely that someone at either Stellarvue or GSO will remember what the thread is. GSO appears to still be selling the focuser so there's hope.
EDIT - yes, the view through the scope is much better now that plastic piece is back where it belongs (thank you Captain Obvious). I guess that explains why I was having such a tough time getting my new laser aligned with the scope tube as well. Instead of a dot or a line (it was a bit foggy) all I was seeing was a series of rings. Now I see a nice tight dot.
Also just noticed that Agena has something similar to the adapter I ordered, although it's a two part solution while I believe the one from FLO is one piece:
BTW I realize that even with the ClickLock I will still be dependent on how well the original visual back holds onto the diagonal AND I have added another set of threads that could come loose, but the main thing that bugged me about the original visual back was that I really had to cinch down the screws in order to have any chance of it holding and didn't want to have to keep doing that every time I changed accessories. I can always drill a couple of partial holes in the adapter tube to keep everything in place.
Does anyone ever use blue Loctite on optical threads ?
Edited by bridgman, 11 April 2020 - 12:40 PM.