When "perfect" collimation is "close at hand", 4 center-spot reflections can be seen in the INFINITYTM autocollimator and are the result of multiple reflective interactions between the 3 mirror components of Primary, Secondary and Autocollimator.
In these simulations below, both Primary and Focuser optical axes alignment errors are present due to various Primary and Secondary errors resulting in the reflections being subsequently spread from their "perfect collimation" stacked position.
You might see something like this ....
or perhaps this ...
-
Reflection "P" is the "Primary" or "first" reflection and is the direct reflected image of the spot itself via the Diagonal mirror after 1 FL pass. Reflection "P" is what is visible "without" the autocollimator whereas images (#1, #2, & #3) are reflections of "P".
-
Reflection "1" is generated from reflection "P" after an additional reflection back from the A/C mirror, via the diagonal to be reflected off the Primary and back via the diagonal to the eye for a total of 3 FL passes from the original spot. It's sensitivity to Primary axis error is 4X and to Focuser axis error is 2X.
-
Reflection "2" (inverted) is generated from the "real" image response from the (parabolic) Primary mirror at its Center of Curvature (COC at 2 FL's away) from the spot reflection it sees in the Autocollimator mirror (which is 1 FL away). The trick here is that the autocollimator reflects (or "folds") the real image back onto the Primary surface where it is then seen via diagonal reflection by the eye (just like reflection "P") - it is a total of 5 FL passes from the origin.
The reflection "2" sensitivity to both Primary and Focuser axes errors is 4X each. Total distance between reflections "1" and "2" is the vector sum of the primary mirror and focuser axial errors. When the other axial error has been removed, this distance is either 8X the primary mirror axial error or 6X the focuser axial error.
-
Reflection "3" (inverted) is a reflection of reflection "#2" following a path like that of reflection "#1" (generated from reflection P) for a total of 7 FL passes to the eye from the original spot. When the faintest reflection (#3) is in view, the distance from "P" to "3" represents a sensitivity to focuser axis error of 2X regardless of the Primary axis error.
It is this unilateral sensitivity to the focuser axial error that allows us to use this reflection to "zero" the focuser axis with Vic Menard's "Carefully Decollimated Primary Mirror" protocol, leaving the primary mirror axial error, magnified 8X, visualized as the distance between reflections 1 and 2.
The distance and direction between reflections "#2" and "#3" is always equal to that seen between "P" and "1" and between the two A/C pupil reflections; thus, these two spot reflection "pairs" will always be arranged with their centers forming 2 separate parallel lines that are also parallel to the A/C pupil reflections.
*** Many thanks to Vic Menard and Nils Olof Carlin for their insightful contributions and editing assistance ***
|