In post 133 Ronchigrams were taken of the wavefront reflected off the off centre half of the C8 secondary to see if they could pick up the astigmatism expected from an aspheric surface. There was only a tiny amount clocking between the inside and outside of focus images.
To check whether this could be due to insensitivity I repeated the experiment using small parts of the C8 corrector plate but this time using transmission rather than reflection.
As I maintain that a secondary figured to correct for coma should be under-corrected by approx 2.75 waves wf I tried to find a part of the plate that would be equivalent.
As the original test was carried out at f34 I chose my 75mm f15 achromat stopped down to 33mm (so also at f34) to deliver the focused rays to the grating and camera.
The precision pinhole was situated at the focus of a Meade 8'' sct the collimated rays from which were received by the achromat with its mask.
Between the Meade and the achromat the rays passed through a section of the C8 corrector which could be moved sideways to present a section either near the edge or near the centre.
As spherical correction is proportional to the 4th power of the aperture radius I imagined that that each annulus of corrector could deliver correction equal to that of its OD minus that of its ID. And that a 33mm circular aperture would be a small part of that annulus with the same asphericity.
It happens that the innermost annulus just outside the baffle with an ID (that should be IR) of 33 and an OR of 66mm has an ashericity of 2.8 waves.
I take the full sized corrector (with r = 1) to correct for 16 waves, assuming a spherical 2ndry:
(0.66)4-(0.33)4x16 = 2.84 waves
whilst the outermost annulus has around 13 waves.
The first two images were taken near the baffle with the other pair closer to the edge.
Inside of focus is cw.
Whilst the optics here are more complicated and I could well have misunderstood it, this test seems equivalent to the one using reflections off the C8 secondary and which showed so little.
Here the clocking between inside and outside is obvious and lends yet further support to the 2ndry being a touched up sphere.
If these Ronchigrams clearly show astigmatism due to off centre aspheric surfaces, then the C8 secondary, whose off centre surface showed virtually no astigmatism, must surely be essentially spherical.
That's me about done with tests. All of them have firmly said sphere to me but if anyone can take on the test plate job that certainly would be the clincher.
Edited by davidc135, 23 September 2023 - 03:07 PM.