The Holy Grail of collimation for me is a simple and cheap procedure which can be done during the day, inside, and produces optimum results without a star test. See how close you think I've gotten here.
My previous effort gave me good results, but I tried it again recently with a different laser and it didn't work as well. It didn't converge on a final result well and it resulted in a mediocre collimation, I think because it was hard to interpret the laser position.
So I went back to the internet and found the "hall of mirrors" test here, attributed to Jared Wilson. The test itself is not a full collimation, but it is a very sensitive and precise way to verify that your two mirrors are coaxial. I put it together with a Cheshire and tilting focuser to produce a procedure which is quicker, easier, cheaper, more precise, and didn't require a final star test. I'll describe it below, but first some philosophy.
Leery of lasers I have become leery of lasers in collimation, either the spot or the hologram varieties. "What's not to like about a laser?", I hear you ask. "They're so straight!" Very true. But that does you no good unless they are exactly centered and aligned when installed in your scope. The best ones are good at this, but there are still problems. A semiconductor laser is a bar, not a point. In order to approximate a point light source you need to stop it down with a circular aperture, but that's not perfect. Especially with hologram patterns, any residual non-point shape is magnified and results in a pattern which is not rotationally symmetrical. The lines or circles it projects vary in thickness, which makes it impossible to judge position accurately. The hologram itself also needs to be exactly collimated with the laser. If it isn't, it's as bad as if your laser were off center. And both holograms and points suffer from shimmer, dazzle, and flare, especially when reflected off a convex secondary. It's difficult to decide where the exact center is, especially when the flare is asymmetrical. You can sometimes reduce your brightness to tame this, but then the image becomes faint and difficult to judge. A plain old Cheshire is much easier and cheaper to machine to at least as good tolerance as a well-adjusted laser, and the visual feedback is clearer.
Star tweak? A final defocused star test is usually the gold standard for collimation. It's sensitive and tests the system with point light sources. I've always found it to be nearly impossible though. Finding and keeping a rich enough star field near the zenith not long after sunset, waiting 30 s or so for each defocused image, and trying to interpret the fuzzy results and what to tweak is difficult, time-consuming, and frustrating. If you had a daytime, indoor procedure where the smallest adjustments have a visible effect and the metrics were optically correct and easy to discern, how could a star test improve on that?
(next slide please)
Edited by TinySpeck, 30 July 2021 - 11:56 AM.