Adaptive secondary mirrors for the Large Binocular Telescope

The two 911mm-diameter adaptive secondary (AS) mirrors for the Large Binocular telescope (LBT) are currently under construction. The design of the units has been based on the extensive experience made on the MMT adaptive secondary mirror during laboratory tests and telescope runs. Mechanics, electronics and control logic have been revised to improve performances and reliability. Each unit has 672 electro-magnetic force actuators. They control the figure of the Gregorian secondary 1.6mm-thick mirrors with an internal loop using the signal of co-located capacitive sensors. The improvement in the computational power of the on-board control electronics allows to use it as real-time computer for wavefront reconstruction. We present the progress of the final unit construction and the preliminary laboratory results obtained with a 45-actuator sub-system used to test the new features introduced in the electronics and mechanics of LBT adaptive secondary mirrors