8" mirror Polishing, part 4: Seeing by Standing on the Shoulders of Plastic
To test the shape of a mirror, amateur telescope makers have a few tests which involve bouncing light off the mirror to see its shape. One cheap one is the Ronchi test, which sends light through a grating of fine lines, bounces off the mirror, and then you place the grating at the radius of convergence of your mirror so it blocks part of the light and reveals the mirror shape. Nowadays you can make a grating by using a high DPI laser printer! I didn’t have one but someone in the Observational Astronomy discord named chantepierre did, so he mailed me a Ronchi grating in an envelope from France two months ago. Thank you, chantepierre!
I set up a Ronchi tester using a 3D printed part, LED, resistor, and battery - I just taped the Ronchi grating to the front. But I couldn’t see anything when I looked, because the mirror was tilted wrong. So I printed a mirror stand! The mirror stand sort of helped, but the print warped, so it wasn’t as helpful. Finally I realized I needed my tester to be higher, so once tester v2 stood proudly on tester v1, I was able to see the LED’s reflection through the grating and take a photo!
These stripes tell me… that a fridge nearby is vibrating. Sigh. But at least I’m seeing stripes!
Total polishing time so far: 70 minutes