The instrument typically outputs two analog voltages that represent the tilt of the mirror in two orthogonal planes. A data sheet for a commercially-available autocollimator is shown in Figure 1. The fluctuations were measured in a bandwidth of several Hz and may be seen to be less than 0.5 arcseconds. A similar idea was worked on for the GBT and tests in Green Bank were made through a heated tube over a 60m distance. The results are shown in Figure 3 and are quite low.
Several companies make instruments for shorter ranges with sub-arcsecond accuracies.
A useful method of measuring displacement in two orthogonal directions perpendicular to the measurement beam is to use a quadrant detector. This device provides two electrical signals that are a measure of the position of the centroid of a light beam on the detector. Such a system is very useful for measuring X-Y shifts in the subreflector, for example. A quadrant detector is used in this way on the 12 Meter Telescope and over a 6m path turbulence results in a beam wonder of less than 10 microns with a bandwidth of around 1 Hz. It would be a simple matter to set up a test to more or less exactly simulate any conditions that may be defined for use on the MMA antennas.