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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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