[Paul Haese]

This problem has got me stumped. Take a look at the two images. One is inside focus and one is outside focus. This is taken with the Wynne corrector in place on the AG12.

On first viewing it looks like collimation is shifting from one side of focus to the other. In particular the secondary shadow shifts. For reference the camera is orientated so that the top of the image is toward the opening of the scope and down is toward the primary.

I am using an Atlas focuser which not only measures correct with callipers under load from one side of focus to the other but also shows no collimation shift watching the cats eye collimation.

Is the shadow shifting indicative of the secondary not being centred correctly? ie up or down the tube.

Secondly, the out of focus shapes seem to indicate astigmatism. However star shapes at focus are round stars. These shapes could well be either rotation of the secondary (the sight tube seems to indicate otherwise), astigmatism introduced by the Wynne or position (rotating the Wynne indicates otherwise) of the secondary either up or down the tube. Is this shape again indicative of secondary position?

I have owned several Newtonians over the years, mainly slow ones and never seen this sort of problem. It might well be that the oversized secondary is making this harder to assess and collimate the scope and my lack of experience with a fast Newtonian is the main problem. I would certainly like sort the problem and am seeking sound advice.