Concepts for a large-aperture, high dynamic range telescope

This 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 a re computed and compared for four telescopes of the same light-gathering po wer 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 m unobstructed circular 6 x 6.5 mirrors that are identical off-axis sectio ns of a parent 20 m mirror, and 4. a square off-axis telescope (SOT) whose aperture is made of m off-axis m irrors. 4 x 8. The characteristics of the PSFs are examined in the diffraction- and seeing -limited regimes, assuming (1) perfect mirror figure and (2) realistic figu re errors (edge defects). The implications of field rotation with an altitu de-azimuth mounting are discussed in each case. The implementation of adapt ive optics (AO) and the properties of AO-compensated PSFs having a Strehl r atio 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 intui tively expected, the optical performance of all four telescopes is comparab le. With higher order adaptive optics and for coronagraphic observations, t he SOT and HOT are superior to the SMT. This distinction becomes larger wit h relaxed constraints on mirror edge-polishing requirements. A full optical design is presented for the novel HOT configuration, and optical fabricati on issues are briefly addressed. Finally, science programs and possible ins trumentation 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 i nstrument deployment.