Abstract
A 20 m space telescope is described with an unvignetted 1° field of view—a hundred times larger in area than fields of existing space telescopes. Its diffraction-limited images are a hundred times sharper than from wide-field ground-based telescopes and extend over much if not all the field, 40 arcmin diameter at 500 nm wavelength, for example. The optical system yielding a 1°, 1.36 m diameter image at f/3.9 has relatively small central obscuration, 9% by area on axis, and is fully baffled. Several carousel-mounted instruments can each access directly the full image. The initial instrument complement includes a 400 gigapixel silicon imager with 2 µm pixels (0.005 arcsec), and a 60 gigapixel HgCdTe imager with 5 µm pixels (0.012 arcsec). A multi-object spectrograph with 10 000 fibres will allow spectroscopy with 0.02 arcsec resolution. Direct imaging and spectroscopy of exoplanets can take advantage of the un-aberrated, on-axis image (5 nm RMS wavefront error). While this telescope could be built for operation in free space, a site accessible to a human outpost at the Moon's south pole would be advantageous, for assembly and repairs. The lunar site would allow also for the installation of new instruments to keep up with evolving scientific priorities and advancing technology. Cooling to less than 100E K would be achieved with a surrounding cylindrical thermal shield.This article is part of a discussion meeting issue ‘Astronomy from the Moon: the next decades’.
Highlights
Existing and currently planned large space telescopes have been designed to exploit the unique advantages of space—freedom from atmospheric blurring, absorption and thermal emission, but not over large fields of view
We outline the capabilities of these current and planned telescopes which form the context for a future large space telescope, which could be built on the Moon or orbited at L2
We explore the optical and technical constraints and show how such a 20 m telescope could be built for diffraction-limited imaging over a 1° field of view, matched to silicon and infrared arrays to cover wavelengths from less than 250 nm to 5 μm
Summary
Existing and currently planned large space telescopes have been designed to exploit the unique advantages of space—freedom from atmospheric blurring, absorption and thermal emission, but not over large fields of view. Its much larger 0.28 square degree field of view is to be imaged by HgCdTe arrays with 0.11 arcsec pixels, providing resolution and sensitivity in the near infrared like that of WFC3/IR. Wide-field imaging from space will be provided by Euclid [4], set for launch in 2022, with 1.2-m aperture, and a 0.5 square degree field of view imaged by large CCD and HgCdTe imaging array detectors with, respectively, 0.1 and 0.3 pixels. This telescope will operate at 130 K. Our hope is that showing such extraordinary technical potential will stimulate thinking by astronomers about the newly enabled science
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