I just treated myself to a laser collimator for my Newtonian telescope. I’ve never had one before. It’s basically a small laser pointer, with an inch wide oval plate mounted in front of it, set at a 45 degree angle to the laser axis. There’s a small hole in the center of the plate for the beam to pass through. The whole thing is set in a tube that fits a telescope focuser, where the eyepiece would go. You turn it on, and the laser beam goes through the plate, down the focuser axis, hits the secondary mirror, is reflected 45 degrees down to the primary mirror, hits it in the exact center, is reflected back up to the secondary, is reflected 45 degrees back into the eyepiece and passes through the hole in the plate to hit the laser itself.
That’s what happes if all the optics are aligned perfectly. If not, you may see that the beam is not hitting the primary in the exact center (most primary mirrors have their center marked). So you adjust your secondary mirror until the spot is at the center of the primary. Then, if the primary is misaligned, you see that the spot isn’t returning to hit the hole in the oval plate perfectly, but is off to one side and is visible on the plate itself, which is marked with a helpful reticle. (There is a window in the side of the collimator tube so you can see the plate.) So you then adjust the primary until the spot moves into the plate hole and quite noticably disappears.
The really awesome part is you can see the plate when you’re crouched down at the bottom of the scope adjusting the primary. Every alignment method I’ve used before involved looking through the eyepiece, trying to memorize what you’re seeing, move to the bottom of the scope and make an adjustment, run back to the eyepiece and decide if it got better or worse, repeat… Ugh. It was a tedious 10 minute process. With this thing I can see the result in real time while I’m turning the adjusting screw. It’s less than a minute and doesn’t involve any guesswork or judgement. OMG this is awesome.