Active Thin-Mirror Telescopes

Abstract

Until to the late 1980s, large optical telescopes relied on sufficient stiffness oftheir structures and their mirrors as well as on mechanical design featuresenhancing their stability to achieve an image quality that was only limitedby the errors introduced by the free atmosphere. The only degrees offreedom that were controlled were the tracking and the focus. With thedevelopment of computers and electronics, it became feasible to correctthe optics of a telescope during the operation, a technique called activeoptics. Wavefront sensors equipped with CCD detectors could measure theoptical errors generated by the telescope with speed and precision, andthese errors could then be corrected during observations by realigning themirrors with respect to each other and by modifying their shapes. Inparticular, the large tolerances for the errors after polishing allowed forthe manufacture of large, thin mirrors with diameters of up to 8 m. Withactive optics, telescopes with mirror diameters of the order of 4 and even8 m could routinely achieve a seeing-limited performance. Active opticsis also an integral component of the next generation of extremely largetelescopes, which by necessity are systems with complex control of all opticalelements.