SGP Auto Focus for Newtonians

I have a general question about SGP and it’s ability to successfully perform autofocus with a Newtonian.

For reference, I have a AT65EDQ refractor that I use SGP’s autofocus feature on and it performs flawlessly.

However, using SGP’s autofocus on my Newtonian always gives flawed results. Using a Bahtinov Mask, I get a reference focus point (using a stock focuser and an nSTEP controller unit). Then, when I run SGP’s autofocus routine, I will typically get a focus point about 50 steps off the Bahtinov mask result. In fact, when I put the Bahtinov mask in place after SGP’s autofocus run, it clearly shows an asymmetric pattern. The other thing I note, is that I can’t get a symmetric U-curve when using autofocus. I feel like the HFD algorithm does not seem to perform well with Newtonian stars. I do have a Televue Paracorr Type II corrector and when the scope is in true focus, the stars as nice circular stars across the field of view.

Bottom line, my question is, can SGP autofocus work reliably with Newtonian’s using a coma corrector?

Certainly. Based on what you’re stating I would look for backlash in the focuser or enable backlash compensation and see if that solves the problem. It could also be slippage as well. Essentially for SGP (or really any auto focus to work) the focuser needs to be able to get back to where it was, and it sounds like this might not be happening.

Thanks,
Jared

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Jared,

Do you have any recommendations on initial backlash compensation settings. I’d like to try this and see if it helps but not entirely sure if there is a recommended method for measuring this?

Thanks,
Kevin