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Hi, so far it looks like it's undercorrected to some degree - the question is how much. But no rings at all beyond focus and very strong outer ring before is indeed a bad sign. Here's some examples - look for Balanced spherical primary images in the unobstructed case. Shown are lambda/4 or...

Hi, well, I'm not a big fan of barrel focus drives, so take what I say with a pinch of salt. My old Kowa TSN-3 has a single focus knob and with that I can focus well at 96x - if the seeing allows it (this is another point, depending on the temperature you can get into a situation where heat...

Hi, both are actually a thing... as Henry stated, aberrations can make finding a clear point of best focus difficult at high magnifications - you have more of a wide range of least fuzziness, but no good focus. Star testing the scope can help to find out what aberrations are present. On the...