Concept studies partially supported by the CFHT have been undertaken by three groups:

Interim reports were received from the groups at the time of CFHT21 and final reports under the CFHT study contracts are due in mid-May.  The following summarizes key features of each concept. [I am indebted to Paul Hickson for the commentary below.]
The High Dynamic Range Telescope
  • A 2-3 mirror design with 6 off-axis 6.5m diameter primary mirrors taken from a 22m parent parabola.
  • Two distinct optical configurations:
    • Wide-field: 6 deployable 2.3m secondary mirrors direct light downward to a monolithic 7m tertiary, in a Paul-Baker configuration, that yields a 1°x1° fov
    • Narrow-field:  the wide-field secondaries are retracted and the light passes to a Gregorian focus employing 6 small secondary mirrors, yielding a 3'x3' fov.
  • This desiign was chosen to minimize the light diffracted from the core of the PSF.
 HDRT Interim Report [pdf]

 HDRT site at UH

A Wide-Field High-Angular Resolution Telescope (XLT)
  • A 4-mirror design with the primary filled by a large number of relatively small hexagonal shaped off-axis segments 
  • For  NG-CFHT, an aperture size of 20m is proposed but extension of this design to 30m appears feasible.
  • Near-diffraction limited images over a fov of diamter 0.7°. 
  • The central region of the wide-field is obscured but light from this region can be directed to instruments at a separate focal plane.
  • a novel, spherical enclosure is proposed to house this telescope.
 The WF-HART (XLT) Report [pdf]
 Science Case for the XLT [pdf]
 The HIA XLT site
The Large Petal Telescope (LPT)
  • The original concept (illustrated) is a filled-pupil 4-mirror telescope employing a segmented primary mirror composed of six large segments cut from 7.2m off-axis mirrors that have linear edges oriented with hexagonal symmetry. The aperture is approximately 16m. This design is discussed in the Interim Report [pdf].
  • In their final report, the LAM team has extended the design to a 20m-class aperture using 8 off-axis aspheric segments cut from 8m-class mirrors.
  • Good image quality achieved over a large focal plane located behind the primary. 
  • A unigue feature of the design is a rotating instrument platform, attached to the telescope structure, which provides field de-rotation for all instruments.
  • This concept was formerly called the Versatile Wide-Angle Telescope (VWAT).
 The LPT Final Report [pdf]

The LPT site at LAM

Common Features
  • All are fully-steerable altazimuth telescopes capable of pointing and tracking over zenith angles from 1 to 60 degrees.
  • 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).
  • All have segmented primary mirrors. They differ only in the size, shape, number and packing density (filling factor) of the segments.
  • All employ coherent phasing of the mirror segments.
  • All require active control of the primary mirror segments.
  • All provide a wide FOV, of order one degree or more, over which the image quality will be seeing-limited.
  • All support multiple simultaneous observations within the wide FOV.
  • All have a high-resolution diffraction-limited FOV suitable for adaptive optics.
Principal Differences
  • 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.
  • The HDRT segments are circular, the segments of the other two concepts have linear edges with hexagonal alignment. 
  • 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.
  • 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.
  • The locations of the focal surfaces differ; each has advantages and disadvantages.

This page was last modified on: Mon May 14 18:32:26 2001
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