what would happen if 4 was aligned to the right of 1 instead of to the left? It seems to me the primary mirror axis would move to the left of the pupil?
Good point, Vic
I reran ray simulation and sure enough I can achieve two Stars of David when both focuser/primary axes are parallel using the off-axis-pupil-AC -- see attachment. The two stars of David can be achieved by either having the AC axis run through the primary center (desired) or by having the pupil axis run through the primary center (undesired). These are the only two cases I found.
I suppose you could verify the alignment by rotating the autocollimator. Properly aligned, the 2-4 hexagram should orbit the 1-3 hexagram--I think...
Another good point . Rotation will be the key to tell us if the AC axis (same as the focuser axis) coincides with the primary axis (perfect collimation). It will tell us if we have met the (desired) or (undesired) axial alignment as mentioned in the previous paragrpah.
Only and only when the AC axis coincides with the primary axis, 1/3 images and 2/4 images will rotate around their respective centers. In addition, 2/4 images center will rotate/orbit around 1/3 images center. In other words, the two Stars of David form will persist. If AC/primary axes do not coincide then the two Stars of David will disintegrate fast. This makes sense because image 1 is special in the sense it does not undergo any AC reflections. Therefore, unless the AC axis runs through the center of image 1, any AC rotation will give an orbital motion to image 1.
I still see advantages for the circular donuts. True that AC can be rotated to align the tips 1 and 4 triangles but that assumes focuser axial alignment â€“ it is a catch 22.
Vic, do you agree with the two fundamental additional benefits for above scheme (off-axis-pupil-AC)?
1- All images will persist upon perfect collimation â€“ no more disappearance acts
2- Image 3 alignment against image 1 for Primary axial adjustment has 4X sensitivity
Bottom line: 2X accuracy for focuser axial alignment (CDP) and 4X accuracy for Primary axial alignment and no more disappearance acts.
EDIT: Updated the illustration to show how to tell if the desired or undesird "8" shape was reached. The desired will maintain the "8" shape as the AC is rotated. The undesired 8 shape will maintain the two circles (or two stars of David) but the distance between them with vary with rotation.