Optical Metrology and Wavefront Sensing and Control

11.1 Introduction Today, large-aperture modern telescope optical systems such as the ground-based Thirty Meter Telescope (TMT) and Giant Magellan Telescope (GMT) are divided into four parts: a large precision telescope radiation collector, a metrology and wavefront sensing and control (WFSC) system, the instrument that analyzes the complex wavefronts, and the detector that converts the complex wavefronts into intensity. Smaller-aperture space telescopes such as those used by the Wide-field Infrared Survey Explorer (WISE), Kepler, Stereo, and Ice, Cloud, and land Elevation Satellite (ICESat) do not require optical metrology and wavefront sensing and control because they are sufficiently small that their rigidity is obtained using mass, since the space system mass budget will accommodate it. In this chapter we expand the classical two-part astronomical optical system comprised of telescope and instrument into the much more capable (for large apertures) new three-part optical system of (1) the telescope, (2) the metrology and WFSC system, and (3) the instrument. At the heart of this new system is the two-stage optics system1 that collects, measures, and corrects the wavefront, and focuses it onto a field stop where the radiation passes into instruments that process the input complex wavefront for spectral, spatial (image), polarization, and radiometric information. In Chapters 8 and 9, we learned how scalar complex wavefronts propagate through an optical system to create an image, and we discussed the relationship between the wavefront and image quality. In Chapter 10, we learned that interferometry is a tool for analyzing the spectral (temporal) content and spatial structure of wavefronts. In this chapter, we show how wavefronts are manipulated to compensate for the following: • telescope fabrication errors, • the low mechanical stiffness characteristic of the new lightweight next-generation ground and space telescopes, and • atmospheric turbulence.