Wavefront sensing and guiding units for the Large Binocular Telescope

The Large Binocular Telescope (LBT) will see first light with a single primary mirror in January 2003. It will be equipped with fully adaptive secondary mirrors from the beginning as well as a complete on-axis wavefront sensing and tip-tilt guiding system. Here we present the preliminary design of the Acquisition, Guiding, and Wavefront sensing system for the LBT. The system is divided in an off-axis system for target acquisition, guiding, and slow wavefront sensing, and an on- axis system for rapid wavefront sensing and tip-tilt guiding. The on-axis system operates on the optical light reflected off a tilted entrance window for the instrument science camera. In this way both the correction to the wavefront (done with the secondary mirrors) and the wavefront sensing (done on the light reflected off the dewar entrance windows) is performed without introducing a single additional optical surface in the science beam. In this way the design follows the lead of the upgraded MMT system. However, the present design differs from the MMT design, in that it does tip-tilt sensing in addition to the rapid wavefront sensing. To enable the system to use a tip-tilt guiding star up to 1 arcmin away from the science target, a significantly larger field of view is required for the on-axis system. The off-axis part of the system can do classical guiding and slow wavefront sensing in parallel which will enable the control system to maintain the optimum setting of the optical system during observations. It will also include a high resolution wavefront sensing mode which will allow quick and detailed checks of the secondary mirrors.