A box arrived today with a Blue Fireball M-03 adapter (M54x0.75 male to M48x0.75 male) among other adapter goodies.
The adapter threads cleanly into the M54x0.75 female thread at the camera end of the M63/M68/M54 rotator/adapter combo, ie into the second red ring. The M-09 adapter (M54x1) I ordered first has not arrived yet but when it does it is not going to work.
On the reducer/flattener side, it only screws in a couple of turns just like the M68 adapter I tested a few days ago. Looks like there might be a wonky thread in the front of the reducer/flattener nosepiece, although you would never notice it when only threading in filters. Anyways, a couple of turns seems to be enough for a solid connection.
Downsides of this approach are:
#1 - you lose the ability to thread a 2" filter into the front of the reducer/flattener, although since I am mostly using clip-in filters at the moment that is not a big issue and the M68 adapter tried previously did include filter threads
#2 - in-focus is *very* tight, seems like only a couple of millimeters when focused on "as close to infinity as I can get in a back yard with perpetually cloudy nights"... probably OK but the M68 option gave a lot more margin while still keeping the rotator in the path. The M63-M48 adapter from Stellarvue gives the most in-focus margin, but you lose the rotator
As long as the AT2FF nosepiece is still removable on future shipments then the in-focus issue is not a problem there. It's not clear if the nosepiece is removable on my Starfield reducer/flattener.
It's worth mentioning that this is a total non-issue for the new AstroTech reducer/flattener (ATRF72) from Astronomics since that is fully threaded from the start... only an issue for generic units like the AT2FF and ATR8, or my Starfield reducer/flattener. The ATRF72 also offers a 48mm T-thread on the back (like the Flat73a) rather than 42mm T which is an added bonus.
Anyways, in the same box was a WO 48mm T-ring for the Flat73a, so I set that up as well. The WO T-ring fits much more snugly into the T3i lens mount, with only a tiny bit of play and much more "friction" keeping movement from happening. With the inexpensive Celestron T2 ring the camera would almost rattle back and forth but you have to work hard to find any play with this T-ring.
The last thing in the box was a couple of T2 spacers - 3mm and 6mm. I suspect that I will get best results from the Starfield reducer/flattener plus AT72EDII with a bit of additional spacing between R/F and sensor but won't know without some testing.
First pic - Starfield reducer/flattener with Blue Fireball M-03 adapter threaded into M54x0.75 thread at back of rotator/adapter - very tight for focus. Compare with pic in post #13 using M68x1 adapter.
Second pic - Flat73a threaded into M63x1 thread at back of focuser drawtube - no nosepiece so focuser tube needs to be racked much further out
Edited by bridgman, 23 April 2020 - 09:38 PM.