I thought I would post some details on the 3D-printed 1.25" coma corrector I just made with some stock lenses for my little fast Newtonian.
I fully understand why there is no real market for a 1.25" coma corrector. There are very few fast telescopes that only have 1.25" focusers, and very few owners of such small telescopes are likely to put a several-hundred-dollar wide-field eyepiece on such a scope. But I had an old 4.5" f/8 mirror I acquired and turned into an f/3.4 mirror then mounted it in the OTA for an Orion Starblast (with the metal tube cut off to the correct length).
I primarily made the scope to fit around my 7mm Pentax XW eyepiece, but I also have a 15mm Celestron Luminos 82-degree eyepiece. On the 4.5" f/3.4 it would give a 3.3-degree FOV at 25x. Such a view would be great except that, as is, all you see is the most textbook example of coma I've ever beheld at the eyepiece! Yikes!
So I poked at some design ideas in Zemax using stock lenses and came up with something quite decent using just 3 elements: a plano-convex (PCX) and plano-concave (PCV) together acting as a meniscus lens, and a double-convex (DCX) lens. Ideally all three lenses would be BK7 glass, but I found the PCX and DCX in BK7 and the PCV in SK18 at Anchor Optics (part of Edmund Optics), all for about $50. I tweaked the design and it works fine with only a slight enlargement of the on-axis spot, but it isn't noticeable at only 25x.
The PCV is uncoated as no coated version was available that would work. My only concern with that is the two plano surfaces of the PCX and PCV lenses mate up. The best solution would be to cement the elements together. The lazy solution that I went with is to space the plano surfaces with shims by 0.25mm = 0.010". Tests on bright targets show no noticeable internal reflections.
The coma corrector reduces the focal length by about 2%, in this case from 377mm to 369mm. The corrector actually pushes the focus back about 9mm, so no extra in-travel on the focuser is required, which is good, because I didn't have a lot left. However, this short outward travel precludes mounting the coma corrector in a barrel with a 1.25" female fitting and set screw, like a Barlow lens; there isn't enough room. Instead, the lens needs to screw into the filter threads on the bottom of the eyepiece.
I originally planned to get the coma corrector's three parts (body, lens spacer, and retaining ring) machined, but then we got a 3D printer at work and I've done nothing but play for 2 weeks... The only thing holding me back from printing the coma corrector parts was how to attach it to the eyepiece since you can't print or otherwise easily cut such fine threads. Then I thought to make slightly flexible tabs that would push out against the female threads of the eyepiece barrel, which worked great. The coma corrector presses into the eyepiece and is held securely, but it is pretty easy to wiggle it back out. It doesn't seem to wear on the PLA type plastic, and it hasn't loosened up with repeated use.
The spots are significantly better off-axis, with no visible degredation on-axis. The edge stars aren't perfect, but are rounder and easily 5x smaller! It makes the 15mm EP completely useable, which was certainly not the case before.
The attached diagram shows the internal layout of the coma corrector and the placement of the lenses. The photos show the corrector by itself (you can see the tabs that hold it into the eyepiece barrel) and the corrector installed on the 15mm eyepiece. All in all a fun little project!