Yesterday afternoon, after months of waiting and a few missteps, I assembled my new system - a Takahashi Mewlon 210 telescope, to which was attached a Baader Wide-field Mark V binoviewer. It being daylight, my goal was to see if I could bring distant terrestrial objects into sharp focus with my 2 sets of eyepieces - Docter 12.5mm EPs and APM 24mm UFF EPs.
I screwed a Baader 1.25x glass path corrector into the binoviewer, then attached a Baader T-2 Quick Adapter. All this was then attached to my Takahashi 1.25" prism diagonal, itself attached to the telescope.
I pointed this setup to a ridge that I estimated to be ~3/4 mile away, covered with pine trees. I assumed that if it could focus on objects that far away, it could likewise bring objects many light-years away into sharp focus. Turning the Mewlon 210 focusing knob as far as it would go in either direction, I still could not achieve sharp focus with either the 12.5mm EPs or the 24mm EPs.
However,when I removed the Tak diagonal and attached the binoviewer directly to the telescope, I did achieve sharp focus within the Mewlon 210's focusing range. When I re-attached the diagonal and pointed the telescope to objects closer to me, in this case pine trees about half the distance of the far ridge, I also achieved sharp focus.
[As an aside, several CN reviewers and commenters have experienced fits of ecstasy when observing celestial objects with the Mewlon 210, declaring the view only slightly inferior to what can be seen with the Hubble Space Telescope. That was not my experience. At best, my straight-on views of the distant ridge, using the 12.5mm Docters (193x), were soft, not as crisp as I expected.]
So what do I need to do differently to observe Jupiter, M51, etc. in all their glory? Was the binoviewer + accessories not assembled properly? Why could I not achieve focus with the diagonal attached, when I could get sharp images without it? Why could I see closer objects sharply with the setup, when it could not achieve sharp focus on more distant objects? I suspect I need a different glass path correcter, say the Baader 1.7x, or even the Baader 2.6x glass path corrector. Would that solve the problem?
Or should I go to Plan B, which is to chuck the whole setup and go back to using my APM binocular-telescope, the best instrument I ever ever owned for stargazing?