Common Features
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All are fully-steerable altazimuth telescopes capable of pointing and tracking
over zenith angles from 1 to 60 degrees.
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All have apertures of order 20m, and an overall size that meets the dimensional
constraints imposed by the Mauna Kea Master Plan (130 foot maximum height
and width).
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All have segmented primary mirrors. They differ only in the size, shape,
number and packing density (filling factor) of the segments.
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All employ coherent phasing of the mirror segments.
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All require active control of the primary mirror segments.
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All provide a wide FOV, of order one degree or more, over which the image
quality will be seeing-limited.
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All support multiple simultaneous observations within the wide FOV.
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All have a high-resolution diffraction-limited FOV suitable for adaptive
optics.
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Principal Differences
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The HDRT employs a partially-filled aperture, with a filling factor (the
fraction of the circular pupil that is responsive to light) of f=0.52.
The pupils of the other two concepts are essentially filled, except for
a roughly-circular central obscuration, f>0.9.
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The HDRT segments are circular, the segments of the other two concepts
have linear edges with hexagonal alignment.
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The HDRT provides a Gregorian high-resolution focus that has only two reflections.
Accessing this field requires mechanical retraction of the wide-field secondary
mirrors.
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The HDRT and VWAT employ segments that are as large as possible, with current
manufacturing capabilities, or nearly so. The XLT employs relatively small
segments, of size comparable to or smaller than those of the Keck telescopes.
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The locations of the focal surfaces differ; each has advantages and disadvantages.
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