Figure 4. This shows the axial sag of encoder A as a function of telescope elevation angle. The circles show data collected while the elevation angle was increasing from the horizon (0 degrees) towards the zenith (90 degrees) and the squares show data collected in the opposite direction. The hysterisis of actuators A and B are similar with a magnitude of approximately 7 µm. The maximum sags of actuators A and B are quite similar. As can be seen from this figure, A shows a sag of at about 70 µm. The sag of B is slightly larger at about 80 µm.

Similar to the functional behavior of the transverse mirror motions, the axial sag of the secondary is not modeled well by a simple cosine dependence on elevation angle. In the case of the axial A and B actuators, the variation is a little faster than a simple cosine dependence. This can be seen in the m2 values for the fits to the data shown in the graph. The differences between the values of m2 measured when the elevation angle is increasing and that measured when the elevation angles are decreasing is real. On average, m2 measured with increasing angles is 1.5 times that measured with decreasing elevation angles. This statement is equivalent to saying that the hysterisis seen above is real and reproducible.