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I didn't mean to pour cold water on your idea Rumples - I just thought it might be simpler to go straight for the defocused star. One potential problem you can run into with looking for concentric circles down the front of the tube is if the optical elements are not all centered along the same axis. In this case when collimation is good on a star you will not see concentric circles down the front of the tube. I don't know how common this is but I have seen one sct in this condition - it still performed quite well.
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