Pivoting a beam about a fixed point with steering mirrors

This article presents a method of adjusting steering mirrors to achieve alignment in just two adjustments. The steering mirrors are adjusted in a way that creates a point that the beam always passes through. The beam pivots about this point as the pointing angle is changed. If the fixed point is located at the center of an aperture, the beam can then be pointed straight to a second aperture, thus completing the alignment. The pivot effect happens naturally when the second mirror is adjusted proportionally more than the first. If the extra amount was 10% and the adjustment to mirror 1 was 3 turns then mirror 2 gets 3.3 turns. The first three turns on both mirrors translates the beam sideways and parallel. The extra 0.3 turns the beam inward slightly in the direction opposite translation, so it crosses over the original beam line at a point that remains fixed. Further adjustments at this same ratio will change the angle through that point, and it appears like it pivots. The extra portion is the ratio A/B, which is the distance between steering mirrors divided by the distance from mirror 2 to the pivot. (C) 2018 Society of Photo-Optical Instrumentation Engineers (SPIE)