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that could mean mean that you are cutting off some light (vignetting) on one side of the incoming light column - probably it is running into the aperture on the way in. Back projection may show you what is going on. If the beam is truncated on one side, the apparent de-centering will switch sides as you cross focus. Try centering the Poisson spot so that it is the middle of the shadow region - if the outer bright zone then looks lopsided, you have uneven vignetting somewhere.
If that secondary is not offset (mine wasn't) I think that you will need to do that before you get anywhere.
Last edited by Shiraz; 02-05-2015 at 10:16 PM.
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