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Hi Neil,
Thanks for your purchase of my products, and also thanks for posting your question, because your post was the final inducement for my overdue registration on iceinspace, so that I could reply.
When Barlow collimating Newtonian primary mirrors, you should only pay attention to the shadow position, and not at all to the characteristics of the red backround illumination on the screen. Diffraction effects caused by dust or dirt on the collimator or barlow lenses can produce Newton's rings-like patterns on the screen surrounding the shadow. They have no effect on the accuracy of the Barlowed shadow adjustment.
Also, when Barlow collimating, make sure to remove the 1mm aperture stop from the collimator, as it will also produce diffraction effects, and lower the shadow contrast.
Clear skies, Howie
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