Concepts for a Large‐Aperture, High Dynamic Range Telescope

Abstract

ABSTRACTThis paper summarizes concept studies for a large telescope capable of wide‐field imaging and of the highest possible dynamic range for photometry and angular resolution.Point‐spread functions (PSFs) and scattered light levels at large offsets are computed and compared for four telescopes of the same light‐gathering power but with different pupil functions:1. a reference monolithic mirror telescope with a 17.4 m primary,2. a segmented mirror telescope (SMT) with a hexagonally segmented primary,3. a hexagonal off‐axis telescope (HOT) with a distributed aperture made of 6 × 6.5 m unobstructed circular mirrors that are identical off‐axis sections of a parent 20 m mirror, and4. a square off‐axis telescope (SOT) whose aperture is made of 4 × 8 m off‐axis mirrors.The characteristics of the PSFs are examined in the diffraction‐ and seeing‐limited regimes, assuming (1) perfect mirror figure and (2) realistic figure errors (edge defects). The implications of field rotation with an altitude‐azimuth mounting are discussed in each case. The implementation of adaptive optics (AO) and the properties of AO‐compensated PSFs having a Strehl ratio of 0.5, and of coronagraphic imaging, are also discussed for the four configurations. It is shown that, in the seeing‐limited regime and as intuitively expected, the optical performance of all four telescopes is comparable. With higher order adaptive optics and for coronagraphic observations, the SOT and HOT are superior to the SMT. This distinction becomes larger with relaxed constraints on mirror edge‐polishing requirements. A full optical design is presented for the novel HOT configuration, and optical fabrication issues are briefly addressed. Finally, science programs and possible instrumentation layouts with the HOT are briefly explored for different modes of operation. It appears that the natural “optical bench” configuration of the HOT can provide a remarkably versatile and convenient environment for instrument deployment.