Mirrors for optical telescopes

Abstract

There are two principal approaches to handle specific difficulties arising in design of mirrors tor space telescopes. The first, most radical one, involves the use of mirrors with active surface shape control (adaptive mirrors), whereas the second one realizes "rigid" mirror maintaining the optical surface shape within the specified tolerances for any conditions or manufacture and operation. The difficulties inherent in design of a segmented adaptive base mirror of a telescope for visible spectral region are due to very high control accuracy required in a wide dynamic range. Therefore, the problems of correction are usually tackled by means of a small auxiliary mirron conjugated to the base one. Owing to certain shortcomings of such approach, a design concept is discussed for a telescope with the segmented base mirror. Basic consideration is given to computer simulation of an adaptive optical system. The results obtained in this way make it possible to determine the requirements on accuracy of manufacture and alignment of mirror segments depending on criteria of image quality. Essential results were obtained with simulation of a system with a wavefront sensor and aposteriori image correction. In design of "rigid" mirrors, principal attention was drawn to the unique characteristics of beryllium, which ensures the required mirror weight-to-hardness ratio. In order to eliminate optical-quality processing in the technological procedures of beryllium mirror manufacturing, which evolves hardly controlled small-size dispersed particles, a technique has been developed for deposition of a glass coating layer on the beryllium. The mirrors have been built, featuring diameter of 1, with rms deviation of the optical surface from the designed shape within 0.02–0.06 mcm.