' ... what's the pain points between fixed and not fixed mirror ... '.
I spent time centering the object on the chip, you focus the scope and think great, I'm all set to get a series of exposures. Then over the next 30min-1hr, as the scope tracks in RA, the mirror moves! Your object may still be centered on the chip, thanks to guiding, but your focus is now out big time.
Also, I focus on something towards the East, then slew to the west and that object is out of focus and definitely not centered. Don't get me wrong, for visual uses the 210 is superb. But clearly there is a reason why the larger and more expensive Mewlons - 250, 300 etc all have fixed primaries and electric motors on the secondaries to reach focus.
In theory, a cheaper Newtonian would exhibit some of the same issues as it flops in the mirror cell, but look at what people can do with those! For me, also as a refractor guy, I just don't have the patience for it, and like to see things fixed and secure
