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I was going to buy a Ronchi eyepiece until I read
Siuter's "Star Testing.." book.
Suiter says what Marks says in that the variation in pattern produced by the Ronchi grating
used near focus is too subtle to produce a result suitable to distinguish between a so-so mirror and an excellent one.
The test is ok for course optics like a camera lens but not
for diffraction limited optics like telescopes. Some mirror makers use it as a gross error check during manufacture.
Suiter says Ronchi himself abandoned the test in the 1920's because of it's limitations.
gb.
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