I've not been able to image using my Dob for a while due to having an issue with a mirror (long story). Attempting to use all other methods (cap, cheshire, and laser) was not getting the job done, though it appeared that all was aligned. Found this tool and ordered immediately. As soon as I put the Concenter in my tube, I was able to see that the secondary was actually very much NOT centered under the focuser tube. This was so much easier to see due to the concentric circles in the eyepiece. I fixed this problem in less than 5 minutes. The true test will be tonight with a quick imaging session, but now that it appears that the secondary is correct, I should be good to go. Excellent tool.