Is it possible that focus is changing due to thermal contraction of the OTA?
I'm talking about the focus not holding steady immediately
after I set the focus. The focus slips
as I watch. That's exactly what it does. And when it starts slipping, it tends to repeat that performance whenever I refocus. Could this be due to thermal contraction of the OTA? It's not a gradual change in focus, but a sudden slip out of focus that is repeatable.
If your focus is changing though, and the image is not moving, perhaps you are having some other problem that has nothing to do with the mirror movement.
Why couldn't it be due to mirror movement? I do see a lateral shift in the image when I go from far-out-of-focus to a focused image. That's the nature of this little beast (in contrast to my 150mm Mak, which shows no lateral image shift at all when I turn the focus knob).
But that is not what I'm talking about. What I'm talking about is a spontaneous defocusing of the image. If I watch this long enough, there may be some lateral shifting of the image, also. But I don't let it progress that far before I try to refocus the image to sharpness again.
Try putting the scope out for a few hours before observing but if the temp is falling fast, it could be that if this is the problem (tube contracting) you may not resolve it.
I let the Mak acclimate on my porch for about an hour the first night I noticed this focus slip. That ought to be good enough for a 90mm Mak. This last Saturday night, I took the scope directly outside from my warm house. As I said, Saturday night I did not see any focus slip until I had been outside for about a half-hour. Again, that indicates this problem may be at least partly temperature related.
This was the reason Celestron shipped Carbon fiber tubes for a while. As I recall, they said that the Carbon Fiber would not expand or contract the way the metal OTAs did so that for long period images, the focus would not change.
Could be your problem??
You got me. I have no idea.
This is definitely a metal tube. No carbon fiber anything in my house. But to make myself clearly understood again, this does not involve "long period images." Here is what I see: I focus the image, and almost immediately the image slips out of focus (there is no obvious lateral shift of the image), and I need to turn the focus knob again to refocus the image. Sounds like the problem the OP had, in that the image in my scope slips out of focus, also.