ZWO CAA backfocus issue with Sky-Watcher Newtonian setup

Alex Nicholas · May 7, 2026, 12:26 AM

If you have the inward focus travel, then sure, there is no reason why you couldn’t, however, the coma corrector will not be able to protrude through the rotator, so you are going to need a LOT of inward focus travel… (Read: You’d need to have the width of the CAA + the total barrel length of your coma corrector worth of inward focus travel available in order to do this)

You could essentially ‘create’ this inward focus travel, by dissassembling the scope, and cutting the OTA length down by the required amount, however this will then require a larger secondary, and potentially still result in steep light fall-off/vignetting.

I find myself in the same situation as you in this… however, I am willing to sacrifice the OAG for the CAA…

What i meant is the coma corrector could go through the CAA somehow, not actually after it. Do you see what i mean?

Yes, and while you potentially COULD do this, attach the CAA directly to the focuser with a M54 thread, then make a custom adapter so the coma corrector threads to the male M54 adapter on the CAA, such that the coma corrector goes through the center of the rotator, and protrudes into the focuser barrel, and your camera attaches to the coma corrector as usual… While there is a possiblity that you could make this work, I think that the tolerances in the focuser and adapters would make this sub-optimal, as ANY decentering in any of the adapters would result in the coma corrector rotating slightly decentered in the focuser tube, which would no doubt cause friction and then damage.

You could replace your current focuser with a 2.5” focuser so that you had ample internal room for this, however, you’re going to get to a point where the cost of adapting the rotator to work the way you want it to work is far more prohibitive than resorting to an external guide scope…

I guess, it depends on how much you’re willing to spend to get electronic rotation to work, and what you’re willing to accept as the final outcome…

Obvious and easiest solution is to put the OAG aside, and replace it in the optical path with the rotator… Since that is not an option you’re considering… you will either need:

1 - An adapter to go from your focuser tube to the M54 female thread on the CAA.
2 - Either
(A) A sleeve adapter, that your coma corrector goes inside of, that threads onto the male M54 thread on the CAA - this adapter needs to hold the coma corrector in place, and will likely result in at least a 1mm change to your backfocus.
Or (B) An adapter that has an M54 female thread, and whatever thread your coma corrector has available on the inside edge (if it has one) such that the adapter threads into th underside of the coma correctors shoulder, and threads onto the CAA male M54.

These adapters are likely not going to exist off-the-shelf, so you’ll be looking at custom machining work, with tight tolerances, and the whole thing needs to ensure the resulting connection is flat, otherwise then you’re going to be in a world of tilt-induced pain…

I know you don’t want to lose your OAG, but honestly, for sanity, its going to be a case of drop the OAG or spend a lot of money to potentially get it working, providing you have 20mm+ of available inward focus travel.