Secondary Mirror

The secondary mirror is a Ø800 mm concave ellipsoid which defines the telescope pupil. It includes slow alignment drives to keep the optical alignment and fast tip-tilt and focus drives to correct dynamic perturbations.

Assuming the open air baseline configuration for the telescope, the wind load on the telescope structure and primary mirror will be a dominant error source in the image motion and image quality budgets. Fast drives are needed to correct dynamic effects at the Nasmyth focal station, which cannot take advantage of the AO system correction. For the Coudé focal station, providing fast tip-tilt and focus capabilities to M2 will allow compensation of part of the wind effects, reducing the load on the AO system.

In the design study the feasibility has been evaluated to achieve a bandwidth of 150 Hz (-3 dB) for the M2 tip-tilt drives to use it as the main tip-tilt mirror of the AO system. In case a higher bandwidth is needed, the possibility exists to implement an additional faster tip-tilt mirror already foreseen in the optical train (M6).

The implementation of fast drives at the M2 with the required bandwidth requires an extremely light and stiff mirror. In case the open air configuration is not finally implemented in favour of a conventional configuration with dome, reducing drastically the wind effects, the necessity to include fast drives at the M2 will be reconsidered. In this case a more conventional mirror with lower dynamic requirements can be implemented.

In the design study the performance has been evaluated of a high stiffness light-weight silicon carbide mirror for the case of providing fast drives and a more conventional Zerodur mirror for the case that fast drives will not be provided.