How do you focus out change in aberration magnitude? You don't. But nothing prevents you from trying and, when it accidentally coincides with the drop in aberration magnitude, to think that you did it.
Btw. the "excellent seeing" on the graph is at about 0.5 arc second. You are probably much closer to the average, 2 arcsec seeing.
Forgot to add, the expression is exact, not an approximation.
Vla, I understand and agree totally that we cannot re focus an increase in aberration magnitude. It would be very nice, though, if we could. The futility of trying is clear. This is exactly what I expect with seeing and why I thought it odd we actually could chase focus with very small focuser movement and refocus the image at a different point. It may have coincided with a drop in aberration magnitude, but refocus was successful every time, apparently, not just once or seemingly coincidental with a drop in aberration magnitude. Maybe, though.
However, each time the image blurred I could adjust focus again and achieve good focus. Every time. I would not expect refocus to be successful in seeing, but it was always successful. I could chase focus over small time intervals, but I found doing so tedious and not worth the effort to continually adjust the focuser. That is not the seeing behavior I am used to or understand observing during our dry season when seeing is often very good. But, successfully chasing focus is why I thought my focuser was slipping under the weight of the eyepiece and focal extender combination. The behavior is not normal seeing where the image is blurred and no amount of defocus would never clean it up. Something else is going on.
Attempts to chase focus may have been coincidental, I cannot rule that out now that you mention it. Next time it happens, I'll pay closer attention. But, I have never really observed during our monsoon season with weather patterns markedly different than our dry season. I have not experienced this weird effect nor remember ever having to chase focus before (except possibly the time I focused on Jupiter to sketch, then found I could actually focus on it again for some reason).
Our seeing on most nights if much better than 2" arc in the way I understand average seeing. More often than not, the Airy disc is easily visible most of the time. It's better than average. On the nights in question, seeing was better than Pickering 5/10 which is enough to degrade the image in itself and very much normal. Whatever is happening, and similar to Don's description and anecdotal input form others, it's probably seeing related except we can refocus the image. The behavior is as if the focus is shifting more so than the image is degrading. I do see some image degradation, but during longer periods of degradation is what prompts refocus and it's surprisingly successful. It's almost as if a giant smooth "air lens" was defocusing the image or, as mentioned, the focuser itself is slipping. The latter was ruled out.
I could be wrong. Whatever the case, I decided not to chase focus, just wait for it to return to normal. It behaves like a momentary blurring of the image due to seeing. If I had not attempted to refocus, I may never have noticed it.
Edited by Asbytec, 27 July 2019 - 06:47 PM.