A laser tracker active optics system for the Large Binocular Telescope

The Large Binocular Telescope consists of two 8.4m telescopes on a common mount. Its current active optics system uses measured mechanical deflections of its optics to compensate for misalignment due to changes in telescope elevation angle and analysis of stellar images to calculate compensating optic displacements. The process is iterated until the inferred wavefront is within tolerance. Due to the asymmetry of the distribution of thermal mass of the telescope structure, thermal gradients during the night cause the current active optics alignment system to require extra iterations to achieve alignment. A system is proposed which uses a laser tracker to measure optic misalignment resulting from the additional thermal influence. This provides a single measurement of relative optic misalignment and reduces the number of iterations required to achieve collimation. A single laser tracker location can be used to measure the rigid body locations of the primary mirror, secondary mirror, prime focus camera, and flat tertiary mirror for both telescopes. A set of reference points on the telescope structure provides a common coordinate system for measurements of optic locations on each side and to assist with binocular copointing. The laser tracker measures the displacement of the optics relative to their locations measured when the telescope is in collimation. Practical considerations integrating the system with the telescope will be discussed, as well as its expected performance.