I began looking for a new focuser because I was running into the limits of the stock focuser with my imaging train setup: a QHY294M-Pro, CFW3M-SR, OAG-M with ZWO ASI120MM, and the Vixen Coma Corrector PH weighing in at nearly 3.2kg hanging off the focuser. I had issues with collimation shifting as the stock focuser had some slop in it and could visibily flex when loaded sideways, even with the focuser pointing downward. Adjusting the PTFE strips in the focuser did little to help this as it just pushed the collimation off or made the focuser so hard to move that my ZWO EAF or QHY Q-Focuser couldn't handle the additional load reliably. There is also some backlash but that calibrated out using the "overshoot" method in NINA well enough.
The stock Vixen focuser drawtube features an M60x0.75 thread and is supplied with a T-ring with a female M56x0.75 thread on the inside, into which the coma corrector is installed. The t-ring then adapts to M48 for the rest of the imaging train. Frustratingly, the Vixen coma correctors (CC PH and Extender PH) are slightly larger in diameter than a standard 2" accessory (being 56mm in OD) which means I would not be able to use the existing Vixen coma corrector or Extender PH if I went with an aftermarket 2" focuser. Judging from several forum posts regarding coma corrector options, the Vixen coma corrector PH and the Extender PH seem to perform very well, so I was loathed to have to look for another 2" coma corrector when I already had a perfectly good one. I also wanted to be able to use the Extender PH which adds a 1.4x barlow characteristic.
The Vixen focuser is very low profile, measuring only 56mm from the top of the focuser body to the scope tube, and my drawtube protrudes approximately another 10mm when achieving focus with a 55mm backspacing setup for my QHY294M-Pro. The stock focuser has about 37mm of travel. The vast majority of aftermarket focuser options that were short enough (e.g. TS optics had a few, Baader Diamond steeltrack, Feathertouch, etc) were only 2" which would necessitate purchasing a new coma corrector, and furthermore many did not have the lifting capacity I was after. There are limited 2.5" focuser options, and the Moonlite 2.5" CRL was the only one I could find that was both short enough and had an acceptable weight capacity. A 3" focuser is frankly overkill, many were too large anyway or too tall to achieve focus.
My scope in this writeup is the newer variant of the R200SS ("white tube version") with a 232mm OD tube.
So I ended up purchasing a Moonlite 2.5" CRL with the 1" stroke and the V3 mini integrated motor. I also purchased the M60 Vixen ring adapter from Moonlite. The parts arrived to me in Australia rather quickly, even with having to pay the customs import fees. The focuser came with an 8" and 10" flange adapter, USB cable, temperature probe with a 3.5mm 3-pole plug, instructions, 2" compression ring, and an M68 spacer ring.
I had a custom M68 T-ring machined by PCBWay, which incorporates the M56x0.75 thread for the coma corrector and adapts to M54. I had hoped to fit a Gerd Neumann CTU-XT tilt corrector into the mix, however I just cannot get a stackup that works - the Gerd CTU is 99mm OD and the largest object that fits under the QHY OAG-M is about 94mm OD. I might possibly be able to get it to work if I replace the filter wheel with the CFW3M-US, however I am considering just selling the whole QHY setup and replacing it with the PlayerOne equivalent which has a rear-adjustable tilt plate built in.
Prior to disassembly, I had collimated the scope one final time with the Catseye autocollimator system. Removal of the Vixen focuser was trivial, it is just 4x screws with nuts on the inside of the scope tube. The Vixen focuser hole spacing is four 4.8mm holes in a 72m x 72mm square, and a 80mm hole to clear the focuser.
The stock focuser looks tiny when compared next to the Moonlite. This thing is a beast and is a beauty to behold. It feels incredibly smooth to use by hand.
Mounting the Moonlite was rather straightforward, I just had to match drill the 4x 5mm holes into the scope tube. The Moonlite drawtube is 70mm OD so I did not need to cut a larger hole into the scope tube. I lined up the focuser using the Catseye collimator fitted into the focuser, and just nudged it around until the collimation was "close enough" by hand. I then marked the four screw hole locations with the focuser in place.
With the focuser installed, it was a simple case of collimating again and reassembling the imaging train. I also painted the fasteners with black paint inside the scope tube.
Of course it is going to be cloudy and rainy for the next week so I will report back once I have first light and can confirm everything works as intended. Driver installation was simple - I just downloaded the VCP driver, ASCOM driver and single-focuser program from the Moonlite website, and everything connected and worked just fine with NINA.
I will note, however, that the Moonlite 2.5" CRL has some substantial light leaks between the drawtube and the red machined body. I will need to solve this somehow, probably with a fabric sock.
I will update this post when I get some clear weather and can test the scope.
