The situation was that there were three telescopes. Each with the same 20″ cellular mirrors but one with a slightly different cell than the other two. Two were easy to collimate while the third one could never be fully collimated. It appeared that there was a bit of astigmatism that was preventing easy collimation. So, the problem was how to quickly test a mirror for astigmatism.
A Ronchi test was suggested but I didn’t have a grating or even a tester. What to do? Print one?
The first step was to find a grating. Since I couldn’t wait for a proper grating to arrive, I decided to try to print a 100 line/inch grating on transparency film with a laser printer.
The second step was to make something to hold the grating and a light source. First I cobbled together something out of scrap plywood and, although it worked, it looked like something that the dog dragged in. It was awful. That being said, it showed that a Ronchigram could be produced with a low-quality laser-printed grating. So I decided to 3D print a small tester.
What you need
- A 3D printer that can print support material (use PVA)
- 1/4-20 nut
- LED or flashlight
- laser-printed grating
Thingiverse link:
Putting it together
Once you’ve printed out the tester, it’s simply a matter of dissolving the PVA and trimming the grating so that it slides into the slot in the tester. Then lightly sand the front end of an LED and push it into the adapter and then push that into the big hole on the back of the tester. Glue the nut into the bottom of the tester and then mount it on a tripod.
Using the tester
Place the tripod at the radius-of-curvature of the mirror, turn on the light source, and look through the upper bit of grating. You’ll need to move the whole assembly around to find a sweet spot, but once there, you should see the characteristic Ronchi patterns that you’re after.