Conventional wisdom is that you can't really put image erecting prisms (a little roof prism like you have in binoculars and SLRs to flip the upside down image right side up) into Newtonian reflector telescopes because they can't focus deep into the tube enough. Roof prisms bounce the image around a lot and add to the focusing distance greatly, making them unsuitable for reflectors. That is unless you have a 3D printer and a little design vision. (Pun intended :-)
For a while now I've had a 200mm reflector telescope (effectively a 1000mm f4 lens in camera terms). I went out and bought a relatively inexpensive telescope, but bought a really awesome Orion Stratus 21mm wide field (68 deg, part A) eyepiece that cost as much as the telescope tube - a lovely seven segment piece of glass with very little chromatic aberration to match the clear tube optics. While it is awesome for sky watching it always bugged me that I was never really able to use that lovely lens on ground subjects like boats at sea.
So on Newtonian reflectors (an awesome simple, relatively aberration-free two mirrors and a tube) to look at terrestrial subjects and invert the very clear but upside down image you get, one usually has to use a 1.5x magnification image erecting tube. The only one I found available was a cheap tube with a crappy single lens in it (part G in the photo) that introduced all kinds of aberration(blurryness, spherical distortion, chromatic rainbows etc...) in the image through that lovely Orion eyepiece.
The right way to invert that image clearly would be to use a roof prism - very clear without as much aberration as a curved lens because each face of the prism acts as a flat mirror. But roof pentaprisms bounce the image around so much inside the prism itself and add so much length to the focusing that for a reflector telescope it means you would have to put the focusing mechanism in the tube itself and block part of the image, which is why the conventional wisdom is that they are unusable with reflectors. No go. Or so I thought - until I figured out a way to do it.
And along the way I get to fix something that has always bugged me about my "inexpensive" (a.k.a. Cheap) Sky-watcher telescope: the rickety, clumsy, and shaky, rack and pinion focusing mechanism (part D) strapped to that nice tube with two fine mirrors. My solution consists of removing it and replacing it with a beautifully machined Orion Low Profile Hybrid Dual Rate Crayford helical gear focuser with wonderful action that can steadily support much heavier lenses and takes up a much smaller portion of the focusing distance than the old crappy focuser (part C). I've removed the base and mounting plate from the Orion focuser (part E with quite a thick mount engineered for an Orion tube that didn't fit my bolt pattern, once I puzzled out how to detach it with the four little mostly hidden grub screws that attached it) and I'm going to 3D print a mounting plate that insets the focuser as close to the tube as possible and matches the existing bolt pattern. While this shortens the focusing distance somewhat it's still not in range of using that 21mm eyepiece with a Celestron 95112 Image Erecting Diagonal (part B) - but it's close.
While it's not possible to push the focusing mechanism further into the tube without impacting the image, the nice, crisp image Celestron prism needlessly wastes a bunch of focusing distance with the 1.25" eyepiece mounting tubes (now removed, above and below G) and its threaded case, so what I'm going to do instead is build the prism into the focusing mechanism to further shorten the focusing distance using the prism and bring it in to range of use with many eyepieces. I'm going to remove the prism from the trapezoidal plastic housing the tubes screw into and design and use my 3D printer to print a replacement 2" to 1.25" eyepiece adapter like the one that comes with the focusing mechanism (part F). The change is that the replacement adapter will also have a section with a plastic cut out to mount the roof prism Into. This effectively puts the prism into the focusing mechanism itself and bypasses the need to inset the focuser into the scope, overlapping the segments of the focusing distance they occupy and allow it to be used with most eyepieces, with the end result allowing my telescope to provide gorgeously clear, very high magnification images that are also right side up for observing terrestrial targets.