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Hi Bert
Very near either side of focus star images are linear and the linearity shifts through 90deg inside and outside focus. Further from focus the donnuts are oval shapped with the major axis of the oval shifting through 90deg inside and outside focus. The out of focus donnut of a star in the centre of the field shows the secondary shadow off centre too with the off centreness shifting through 90 deg inside and outside focus.
At focus the stars appear like little triangles with what appears to be a hole in the centre..?
I'm not sure now but I think the last session I tested it the linearity is first parallel to the secondary then shifts to 90deg to this?
The astigmatism must be coming form the secondary but how?
Mike
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