Quote:
Originally Posted by Wavytone
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.
[snip]
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.
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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 ...