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Refer to attachment.
Assuming we have an autocollimator where the mirror is not perfectly squared to the barrel. When we stack reflections P & 2, the autocollimator axis (same as focuser axis in this case) will point off-center. If we insert a quality laser collimator, the beam will strike the primary mirror off-center as shown.
Now let us rotate the autocollimator 180 degrees. Reflections P & 2 will unstack. The distance between them is 8X the axial error. We can estimate the distance between reflections P and 2 then divide the distance by 8.
In the link I provided, I estimated the distance between reflection P and 2 to be around 10mm after the 180 degree rotation. Dividing the distance by 8 will give me the true error. 10mm/8 = 1.25mm
Jason
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