Telescope T-Thread is 42 x .75mm. I bought a T-ring for my camera but it left the innards exposed when not attached to the telescope. Had to print at .1mm resolution or the threads were no good.
Hmm. The 0.75mm pitch is touchy, but I regularly get usable threads on both my MakerGear M2s and my Wanhao I3 doing 0.25mm extrusion. The Wanhao definitely delivers better thread quality with thinner layers, because it's a lot less stiff than the MakerGear, but it really doesn't take the Wanhao much longer to do thin layers (volume through the extruder is the primary speed limit for it). Rough numbers, our I3 works best under 40mm/s, while the M2 produces slightly better quality at 120mm/s and still does fine between 150-200mm/s. How rigid is your Prusa i3 and how fast were you extruding?
I'm (very) new at this so I may have overcorrected. The first one I printed was the wrong diameter (I needed 42mm but configured at 43), and the threads didn't look great. So I reduced speed (from 50mm/s to 40mm/s) and reduced layer height (from .2 to .1mm). And outer perimeters (which would be the threads) are at 90% in Slic3r, so I actually went from 45mm/s to 36mm/s.
I'm still tweaking extrusion amount too. I ran these with a .98 modifier. And also, now that I've been using the cap I am noticing some wear in two opposite places, which indicates that my "round" cap may have been slightly oval.
I have no idea how to quantify or even judge the rigidity of my printer. It's a RepRapGuru kit, it's the only 3d printer I've used or even seen, and I built it. It's made of thin acrylic and it's sitting loose on my desk. So my guess is "not very rigid."
So do you run thinner layers faster on your i3?