| Popis: |
We describe the active support system for the 8.4 m LBT primary mirrors and the use of this system to optimize the mirror figure in the lab before installation in the telescope. We figured the mirror, mounted on passive supports, to an accuracy of 18 nm rms surface after subtraction of spherical aberration and several flexible bending modes that would be corrected with the active supports. After installing the mirror on its active supports, we optimized the 160 support forces based on interferometric wavefront measurements and a finite-element model of mirror bending. We verified the accuracy of the model and determined a scale factor-the model is stiffer than the real mirror-by measuring a number of bending modes using forces calculated from the model. We then optimized the forces to obtain an accuracy of 28 nm rms surface. The optimization included correcting 200 nm of spherical aberration (Zernike coefficient of surface error) with a maximum correction force of 26 N. |
