Figure 3. The secondary mirror sags with respect to the secondary cage by about 200 µm as the telescope moves from zenith (90 degrees) to the horizon (0 degrees). The circles show the measurements made while moving from the horizon to the zenith. The squares show the measurements made in the opposite direction. A hysterisis of about 15 µm is seen.

The functional behavior of the sag is somewhat slower than a cosine dependence on elevation angle which can be seen by the non-unitary values of m2 in the fits to the data shown above.

As will be explained in the main text, the transverse sag seen here is considerably larger than what one would expect from simple flexure of the central supporting post. These data were acquired at the secondary home position were the mirror is as close to the support cage as possible.