[Rod]

Testing the mirror involved using a Foucault tester and an 8 zone mask. I had to observe pairs of zones darken at the same time and not their positions with the tester’s micrometer. This is easy at modest focal ratios but with a mirror this fast it’s difficult to see the zone pairs at the same time, particularly at the edge which is the most important part of the mirror. With practice I became better at testing this mirror. But I needed a way to verify the test results.

I had purchased a 12 inch optical flat from a friend a few years ago. It was uncoated and it was made by someone I trust. I had it aluminised and used it in the configuration shown below. Essentially it is a roughly made telescope set up in front of the flat mirror. A bright LED is mounted behind a ronchi screen in the eyepiece holder. As you can see the test showed straight lines, indicating a good parabola. Autocollimation is a double pass test so any errors shown are doubled in size, so it’s an excellent null test for a completed telescope.

The only error it does not fully test for is astigmatism. I hope to test for this with a star test once the tube is complete. If Melbourne’s restrictions ease up, a friend has offered to test the mirror with his newly completed Bath interferometer so I may also do that.