I replaced the 1960s antique Edmund Scientific spider (which had no collimation screws) a couple years ago with a very nice modern one that uses four 10-32 screws. I have never been very successful at collimation of the main mirror. When trying to diagnose the problem, i noticed that the round part through which the four 10-32 screws pass had a stripped thread.
The screws go through the round part to tip the mirror holder relative to the optical path. Obviously a stripped thread is not going to do that very well.
Fortunately, the round part with stripped screw hole was a pretty easier part to machine. The original was 1.53" diameter, .41" thick with five 10-32 tapped holes (four for adjustment; one to attach it to tge mirror holder). I had a piece of .5" thick Delrin which was easy to machine into a copy.
Once I put the new part in the spider, I discovered another nuisance: the torque to the adjustment screws was enough to repeatedly rotate the mirror holder so the laser beam would go everywhere it should except the center of the main mirror. The only solutions would be to add a lock nut to the mirror holder where it connects to the shaft or figure out how to make the adjustment screws zero friction through the center piece. I already had it in the tube, so i adjusted for center on the mirror,:rotating the mirror holder back each time to get back to mirror center. The final collimation step only requires turning wing nuts on the mirror cell. If I have occasion to remove this again, i may tackle one of these fixes. For now, the mirror is collimated.
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