A few notes:
The Ronchi testing procedure performed by Celestron almost certainly was restricted to the axial condition only. Repositioning the various components of the test setup to examine the correction at some off-axis angle would have consumed time, and would do nothing to augment assessment of the all-important axial wavefront.
The speed of application of 'aspherizing' touch-ups, this being measured in seconds, is strongly supportive of localized application of correction. Not to say that a more global correction on a hyperboloidal (or some such) figure couldn't be performed quickly as well.
By strict definition, any departure from spherical, even of the most localized and narrow zonal correction, qualifies as aspheric. 'Marketing' speak can easily take advantage of this, vagueness of description being used to imply more than is warranted.
Daviddc135's Ronchi tests documented a page or so back lends credence to a basically spherical figure. The difference in the patterns is certainly smaller than that expected for a coma correcting secondary.
The recent Celestron literature describing the differences between the classic SCT and the Edge system, in which the spherical secondary is admitted, is potent confirmation. I would accord it greater weight than old literature from the 60s-70s in which the strong hint of the implying-more-than-warranted marketing speak leaps from the page.
Today we are conditioned to take mention of astrographic performance as implying good outer field performance. In those olden days of decades past, the same level of demand and expectation from a commercial instrument was not necessarily the case. I would be loathe to read into any mention of astrophotographic performance as extending beyond the field center.
The field curvature of the SCT is very considerable. Correcting for coma in the presence of the notable de-focus provides a gain that is arguably not commensurate to the effort.
When figuring the 300mm aperture f/2 elliptical Dall-Kirkham primaries for the Ceravolo astrographs, I spent about 5 full days on each primary applying the local figuring. The test was done via Ronchi on a system mock-up using the optional f/9 corrector in place, in double pass autocollimation. Figure, test, figure, test, for a total such cycles that would number perhaps about 50. (The fused silica substrate facilitated a quick turnaround due to the low c.t.e. obviating any real waiting to equibriate.) Testing was done ONLY on axis; the adherence to design tolerances guaranteed to result also in good off-axis correction.
When getting close to finished, and some particular narrow zone was under treatment, just a single revolution on the spindle was all that was required, with a tiny 1" diameter pitch tool swirled by hand with gentle pressure on that zone as the mirror slowly turned, for a dwell time on any one spot of about 2 seconds. And that's with the harder fused silica, which works more slowly that does, e.g., Pyrex.
A spherical primary, with corrective action applied to a tinier secondary, would have been almost languid hedonism by comparison!
Back to daviddc135's earlier Ronchigrams of his secondary mirror tests. Those 'grams remind me somewhat of the starting condition of a pair of 50mm f/3.3 bino objectives I aspherized for one of my projects. They were probably about 2 waves undercorrected, which I attacked with a soft lap of the classic 'petal' form so as to remove the most glass around the 70% zone. The before and after Ronchi patterns were VASTLY more different for those ~2 waves of correction than the very minor differences in David's tests. This is very strongly supportive of those SCT secondaries being fundamentally spherical of figure. I can't accept them as being aspherized to anything near to the degree required for correction of coma.
Edited by GlennLeDrew, 20 September 2023 - 11:02 PM.
