I am definitely wrong in how I initially describe this and making it confusing as such. I probably started off with the wrong hypothesis of how the system was working and mucked up the description.
Here is the process I am suggesting;
1) Collimate the secondary so the laser is centred in the primary mirror marker (no variable polarizer).
2) Collimate the primary via a Cheshrie or Barlowed laser.
At this point the system is set up with as I would normally collimate.
Then I put in the laser and the single variable polarizer. When I put the laser with the single Variable polarizer in, I see three laser dots on the primary:
a) The incoming beam dot from the laser.
b) A second slightly less bright dot to the side of the primary beam dot (This is Reflection #1 off of the back of the variable polarizer)
c) A third dim dot diametrically opposite Reflection #1 (This is the second reflection, Reflection #2, back off of the variable polarizer)
The picture below shows an exaggeration of what I see (but you can't see the second reflection because the camera does not pick it up). The second dot to the right of the brighter dot is "reflection #1
3) Then, I very slightly change the tilt/rotation of the secondary and the dots converge into a single dot.
4) Repeat 2 and 3 until complete the dots converge and the primary is fully collimated.
This sounds an a lot like what is happening with an autocollimator.
To quote Catseye..."An AUTOCOLLIMATOR - This is typically a 1" long, 1.25" diameter tube open at one end with a peephole at the other end. There is a round, flat mirror on the backside of the peephole inside the tube with the mirrored surface facing toward the open end of the tool. A small amount of the mirrored surface has been removed at the center of the mirror to allow sight through the peephole. This tool operates on the principal that 2 mirrors (the primary and the autocollimator) faced opposite each other will reflect an image between one another, and if the angle of incidence of both mirrors is "exactly" 90 degrees (i.e. collimated), the reflected "ghost" images will merge together."
Unless I am missing something, I am pretty sure this is what I am doing with the laser and the Variable polarizer.
If I twist a spider vane slightly I see the dots separate and when I let go they come back together (the same thing would happen with an autocollimator). This really sounds like the laser and polarizer are working together as an Autocollimator.
I would bet an actual autocollimator is still more accurate because the laser is so brigth and spot so large that it is hard to get the dots perfectly on top of one another... but I am pretty confident this system is working as an autocollimator.
Edited by MellonLake, 29 November 2020 - 02:55 PM.