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.