I just started using APCC and a Mach2, and my usual imaging program is NINA with ASTAP doing platesolving.
My other mount is a Celestron CGEM and there is a pretty simple process to sync it to the sky when started up. You pick a star in the mount driver's pseudo-planetarium software (CWPI), go to, center it in eyepiece, then click sync. I generally did the centering with platesolving, and in NINA always had the "reslew to target" and "Sync" options turned on. I never knew exactly what was happening, but didn't really care. It just worked.
With the Mach2, there's both a "Sync" and a "ReCal" button in APCC, and the manual explains you only want to "Sync" once per session, and you want to be pretty careful about where you're pointing when you're doing it. I can't see my mount from where I control it, so that got me wondering whether the NINA "Sync" option is doing a "sync" or a "recal". And then I started wondering what exactly is happening.
So first question, is it useful and safe to have the "Sync" option turned on in NINA's platesolve section?
Second question, how am I supposed to actually use the ReCal thing in APCC? For example, last night I wanted to center a particular star as part of an alignment routine. I used Stellarium to command the mount to the star. It wasn't quite centered, so I platesolved with the "reslew" and "sync" options on. To my surprise, it wouldn't center. And to my even greater surprise, when I tried to figure out why I noticed that APCC, Stellarium, and the plate solver all showed different RA/Dec for where the center of the frame was. There was about an arcmin difference between them.
It was late and cold at that point, so I shut down, but now I'm perplexed about how APCC and Stellarium can show different values for the mount's RA/DEC (whether using JNOW or J2000). I'm not surprised the platesolve gives a different RA/DEC because the star in question isn't actually centered. But I'm not sure what the process is to rectify it.