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Old 05-01-2018, 08:14 AM
Stefan Buda
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Location: Melbourne, VIC
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Thanks Wavytone for the advice, but I did make my move yesterday on the Mangin. One single "wet" with 302 1/2 emery brought the RC back to what it should be and then I finished it off with 2 wets each of 1200 and 1600 emery.
I'm about 3 hours into polishing now and the RC is holding nicely.
One of the problems with the first polishing was that we had a heat wave and the ambient temperature was about 28 degrees - way too high for Acculap Standard hardness. Now I'm polishing at 22 degrees and another heatwave is upon us. Must finish it today or wait for the cool change.
I am not going to worry about 15nm of residual astigmatism - it is completely insignificant and quite small considering the complexity of the instrument and the lack of collimation adjustments.
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Old 05-01-2018, 09:17 AM
bratislav (Bratislav)
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Originally Posted by Wavytone View Post
Yup you have SA with a touch of astigmatism, the astigmatic axis runs from 10 o'clock to 4 o'clock. Could have told you that from the image, never mind the roddier analysis.


You know what you have to do... though to remove a high zone of 5 microns I'd be practicing the gentle art of hand figuring with pitch and rouge, pressing with petal-shaped card templates to control where the contact zones are... forget fine grinding - far too drastic, and even cerium oxide is too drastic. Rouge is far more gentle when it comes to figuring.
The residual spherical aberration can be turned off in software. ALL residuals (including astigmatism) then amount to about 22nm RMS (less than 1/25 wave) which is excellent even by visual standards. Astigmatism itself is about 15nm (1/36 wave RMS), just about undetectable.

And tackling 5 microns (10 waves!) with rouge would take better part of one's life. And most likely result in scratches (too many sessions and Mr Murphy doesn't sleep). Rouge is way to slow to do anything, and best left for massaging in those last tenths of a wave. Utterly out of place for an astrograph with more than 50% obstruction.

BTW this "high zone" isn't so much on glass - most of this residual third order SA comes from having a wrong radius at a wrong (actually WORST) place. Most of it could have been sorted out by respacing the components (at a cost of some softening of off axis images); but Stefan's design is already been cast in stone (and everything, including spacings) set to within a micron or so, so having R4 back to spec is the only solution.

Seeing that Stefan has already reground and mostly repolished the Mangin (I wanted to suggest using pads for raw polish as they won't flow at all, all but guaranteeing the steady curve on R4), it is all now academic ...
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Last edited by bratislav; 05-01-2018 at 09:39 AM.
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Old 06-01-2018, 11:53 AM
Stefan Buda
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Location: Melbourne, VIC
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Managed to finish the polishing of R4, yesterday, and the Ronchi figure looks much nicer than the first time round - most likely due to a more correct polishing temperature. The previous figure had slightly rolled down inner and outer edges.
Last night I put the OTA back together without aluminizing R4 to try recording some extra focal images. It was difficult to capture enough photons but during my tests I noticed that there was quite severe coma present, just like the previous time when I just ignored it. The SA looked definitely smaller but the coma should have been absent - clearly I had another problem to trace.
I got up at 6am this morning and frantically started measuring everything. It did not take long to discover that the small biconvex lens, which is almost symmetrical, was installed back to front. I took so much care to avoid this very problem and somehow still managed to screw up.
So by 8am the Mangin was in the vacuum chamber and by 9 it had a nice fresh aluminium coating. By 10am I had done the autocollimation test that shows nice, very gently curving Ronchi lines.
And now I ready to hit St Kilda beach with some good reading and try to survive the 42 degrees day.
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