Thanks so much for taking the time to understand my setup and help with a solution!
The EAF focus is very much a physical limit as I've focused manually before attempting an AF routine and can visually see the focuser being right up to the tube. Images were evaluated in broadband with a simple UV/IR filter in place.
Comparing my own setup with yours, it looks like the one thing that's different is the focuser and the CAA. My hunch was, and someone on SGL reported the same, that it's the CAA that's actually the culprit. The CAA is introducing a fair bit of spacing which is eating some of that focuser travel.
Thus, it seems like possible avenues of resolving the issue are:
- Move the CAA after the reducer and find some way to still reach the same backfocus (narrower OAG? ditch the OAG altogether?)
- Ditch the original focuser and CAA and move to a FTF2025BCR (Starlight say it's Rotatable) though a bit on the expensive side
- Replace the CAA with a narrower rotator (my CAA is 20mm thick so doubtful I could even find one that's half that plus adapters)
I'm very open to any other ideas!
I can’t shake that your configuration should work. I had it working myself with the ZWO EAF, and was able to run its focus routine without running out of room for travel. But I am not coming up with a reason why you don’t have enough travel. And you’re not doing anything strange, here. You’re literally using the telescope and the optical train as it was designed when you use the CAA.
I guess another basic question. When you started searching for focus with the EAF, did you start with close focus? It can go into a wider focus-seeking routine if it starts on the wrong side of focus, and it can seek even wider if it doesn’t start the routine in the right place. I’d double-check this if it wasn’t so much trouble to change things over from my current setup.
On the problem-solving side of things, you could drop the CAA and replace it with part number 81 in the system chart (TKP20006B) which, I believe, comes with the telescope. That gives you the F56>M56 connection afforded by the CAA and consumes less mechanical depth, giving you more room to focus, but you lose the CAA in favor of a basic (but well-machined) mechanical connection. That means you have your M52 connection and 56.2mm of backfocus to sort out 1) a rotator, 2) an OAG, if you want one, and 3) anything else you want. Seems like a recipe for headaches if you have any priorities in 3, which you do with a filter drawer. So that solution probably means dropping the OAG in favor of a guide scope. You do get a sort of half-arsed rotator with the ZWO OAG, I suppose. It can re-slot in some limited range of rotational positions. But that’s also not a great solution to introduce a rotator.
In my case I use a Baader M48 quick changer to couple my optical trains with my telescopes, and wanted to adapt that solution to my FS-60CB. It gives me what is, effectively, a rotator, and in combination with a filter wheel left me with a very fussy optical train that was hard to get dialed in exactly.
You are correct that the 2025BCR affords a "rotator." This, in the sense that you buy an adapter which can thread into the female connection of the Takahashi flattener or reducer, which affords a 2-inch fitting on the other side that seats in the 2-inch compression fitting on the 2025BCR. You can loosen the compression fitting and rotate the optical train that way. Which, to be fair, actually works well, as the telescope-side face of the flattener or reducer can also serve as a leveling surface to eliminate the possibly of tilt from the compression fitting and make the operation repeatable. So at least it gives you an option to have the M52 male connection with 56.2mm of backfocus with some sort of reasonable rotation solution accounted for. And yep—definitely on the expensive side, adding expense to an expensive telescope.
I was curious about how you were evaluating your backfocus, as when the telescope is pushed beyond its optical limits you may end up with what looks to be out-of-backfocus if, say, evaluating Hα with pixels that are so small. But it was a bit of stretch given how small that sensor is. It may well be able to handle even something that demanding since you’re so far inside the corrected image circle. But it sounds like you’re doing all of that in a sensible way.
This is pretty mysterious to me, on the note of that first paragraph.
Edited by James Peirce, 20 March 2023 - 09:44 AM.