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I attached a Ross Null lens to my Ronchi tester and carried on the parabolization with the 8" lap until the outer 2 inches or so looked flat. After that I started using a 4" figuring lap so that I don't over deepen the 70% zone, which would be a pain to correct.
After a few hours the lines were straight enough to make test results difficult to interpret. See attached Ronchigram.
So it came time for setting up the Bath interferometer. The first noisy interferogram is attached and the processing shows that my conic constant at the moment is -0.74, and the figure exactly where I expected it to be.
After a few more iterations I should be approaching the diffraction limit.
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