I recently purchased a Glatter 1.25" laser collimator and TuBlug combination for my 8" solid tube dob. Using the collimator, I align the beam from the secondary in the centre spot of the primary, but then using the TuBlug to adjust the primary, when the shadow of the "donut" of the primary centre spot is correctly adjusted to fall in the centre of the TuBlug white screen, the red "target pattern" on the screen is way off.
I seem to be missing something crucial here, and it's probably quite simple, but can someone give me some guidance?
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.
Just bought one of your collimators, Howie, to go with my new Lightbridge 16. It certainly has made a huge difference to my collimation. I like the nice chunky machined aluminium build too!
