Abstract

For large optical systems, minimizing the weight is a very important design consideration. Adaptive control of the mirror surface figure permits the utilization of extremely lightweight structures. The success of such adaptive systems is critically dependent on the development of small, lightweight devices which can sample the surface figure of large, nearly flat mirrors without obscuring the mirror apertures and provide feedback to the adaptive control system. Our concept employs grazing incidence interferometers (GII) mounted along the perimeter of the mirror to sample the surface figure along narrow swaths. With enough swaths, the entire surface figure can then be reconstructed and the data used to control the mirror's surface figure. The GII is a Mach-Zehnder interferometer which illuminates the mirror at an angle of incidence of 89°. The reference beam is phase modulated at 20 kHz to permit noise suppression using narrow bandwidth digital filters. The GII performance is measured with a phase shifting interferometer in a standard Ritchey-Common test. Measurements were conducted in a large vacuum chamber to eliminate atmospheric turbulence effects. The phase resolution of the GII was determined to be λ/2200 rms (λ = 0.63 µm) corresponding to a surface displacement resolution of λ/76 rms.

© 1989 Optical Society of America