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Old 04-01-2018, 01:16 PM
Stefan Buda
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Stefan Buda is offline
 
Join Date: Jul 2013
Location: Melbourne, VIC
Posts: 823
Last night I captured some extra focal images of Canopus and sent them over to Bratislav for Roddier analysis.
This morning he sent me the results and I'm happy to report that the instrument is free from astigmatism. However the spherical aberration needs to be cured.
So, I pulled out the Mangin and measured R4 as I suspected that it was the culprit. Sure enough the spherometer showed a sagitta increase of 5 micron, corresponding to an increase of radius of 1.5mm. The damn polishing lap was working a lot harder on the inner edge than on the outer, despite of me using rather long strokes on the polishing machine - never trust a polishing lap even if you have performed exorcism on it. Optical polishing remains a black art, but I could have noticed the problem by periodically testing the surface with the spherometer.
Anyway, I decided to go back to fine grinding rather than try to polish it back to specs.
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