Autofocus consistently off by one increment

I’ve noticed my AF is often off by one “increment,” which for my settings is 80 steps. I’m not sure how new this problem is, but now that I’ve solved other problems I ought to tackle it. I’d appreciate it if anyone could lay out a troubleshooting strategy.

This is a CPC1100 using a Rigel Systems focus motor on the stock focuser. So the mirror does shift, and there is backlash, so I use backlash comp in the AF routine, and let autoguiding settle before each exposure. I find the focus position selected by SGP is usually outside focus by one increment. I do an AF run after each slew, and each target seems to show this. I’m not sure it happens every single time, but I can say that I’ve never seen it wind up one increment inside focus, only outside.

Thanks for any advice.


Are you using a Bahtinov for establishing where actual focus is? Or some other method?


No, to set the initial focus I’m just eyeballing it, and then confirming with an AF run. But I think that part is working fine. Since I moved to AF, I never found much use for my Bahtinov mask.

But I think I may have called foul on this too soon. Looking more closely at my subs, at least some of them are elongated in RA, not just larger. So I suspect at least some of what I’m seeing is due to tracking issues, rather than focusing. And that’s not a big surprise, since seeing has been poor recently, with some wind. Let me dig a little deeper into this, and I’ll report back.


Can you post the focus files and your logs?