Figure 2. The mirror actuators were pistoned a known amount while reading both the Heidenhain encoders and the Mu Checker linear gauges. Note that only two Mu Checkers were available and these were positioned to within 0.5" of the B and C Heidenhain encoders.

The crosses and the triangles in the figure above show that the agreement between the Mu Checker measurements and those that we infer from the C Heidenhain encoder are in good agreement. There are no measurable scale problems with the Heidenhain encoders. Note that in this series of measurements it appears that the C actuator slipped between the initial position where the encoder readings were zeroed and the very next reading. The amount of the slip is given by the difference between the symbols and the solid line which shows a perfect mirror response. The slip above amounts to about 3 µm.

We tested the sensitivity of the readings to vibrations of the secondary vanes. When a small impulse of a few pounds is given to one of the secondary vanes, vibrations in actuator C were seen with an amplitude of approximatley 2 µm. These motions are quite consistent with that required to explain the image motions jumps which we reported on earlier this year.

Note also that the agreement between the x's and the squares again illustrates good agreement between the Mu Checker and Heidenhain measurements.