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Old 10-05-2022, 08:57 AM
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Rod is offline
Join Date: Sep 2006
Location: Mount Martha
Posts: 359
Hi Steve

125 LPI should be fine for an F9 mirror. Make sure you understand how to do the test before wasting time on different line frequencies. 200 LPI is too much.

You check for astigmatism with the ronchi test and look for ‘clocking’ as you move the tester from inside to outside of focus. Clocking refers to the bands moving from one orientation to another. You can also sometimes see a S shape in the bands if the astigmatism is severe. There’s a photo and a description here:

Final testing on a slightly defocused star to ensure it remains circular inside and outside of focus is a good idea before coating the mirror. The definitive test though is likely a Bath Interferometer. I haven’t tried that yet.

Just watch for rust with the steel brush. You don’t want that falling into your lap.

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