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In practice if you have your artificial star at least 20m away you won't rack your mirror forward off the baffle tube so you hit the retaining clip. Just use extension tubes at the back to reach focus. It's good enough for collimation. The optimum distance between the mirrors is there so you don't get spherical aberration which is important for viewing or imaging but doesn't matter for collimating. Visually you'll only see a difference in brightness in and out of focus but the rings will still be concentric.
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