The Large High Dynamic Range Canada-France-Hawaii
Telescope
J. R. Kuhn,1
G. Moretto,1
R. Coulter,1
P. Baudoz,1
J. E. Graves,1
D. Jewitt,1 R. Joseph,1
R. Kudritzki,1
R. McLaren,1
M. Northcott,1 C. Roddier,1
F. Roddier,1 T. Owen,1
C. Shelton,1
A. Stockton,1
A. Tokunaga,1 D. Mickey,1 G. Luppino1
J. Tonry,1
B. Tully,1
R. Wainscoat,1
NGCFHT Studies
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Figure 1: The HDRT concept is shown
here in its wide-field (Paul-Baker) and narrow-field (Gregorian)
optical configuration. The HDRT design combines elements of previous
unobstructed and wide-field telescope designs in a scalable concept
that allows direct extrapolation of current telescope, mirror,
and adaptive optics technologies. (Click on figure for larger
version.) |
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Figure 2: This shows a
possible HDRT optical support structure design. (Click on figure for larger
version.) |
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Figure 3: These calculations show
the difference between a segmented (Keck-style) aperture (left) and the
22 m HDRT psf (right). The intensity scaling is linear. The angular field in each image is 1.1 arcsec and a faint stellar companion has been added
0.36 arcsec to the right of the
central star in each image. The hexagonal diffraction pattern on the left is
caused by the telescope segmented mirror. These panels show results for an
atmosphere with R_0=1m at 1 micron with a 400 degree-of-freedom AO system. (Click on figure for larger
version.) |
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1 HDRT Concept
The High Dynamic Range Telescope (HDRT) expands on the 6.5 m aperture
New Planetary Telescope which was devised as a replacement for the NASA
Infrared Telescope Facility. The concept described here benefits
from the ideas and from input from all three partner communities within the CFH
consortium. As conceived, the HDRT will provide unprecedented photometric and
angular resolution dynamic range. As the world's largest and highest
resolution optical telescope it will also provide wide-field
observations with an etendue significantly larger than even the
special-purpose survey telescopes now in their planning stages. Beyond
this, the HDRT allows unique opportunities for observing faint
astronomical objects in the near environment of bright sources.
The primary operational capabilities of the HDRT are
- Aperture: Effective resolution of 22 m (diffraction
limited over at least 1 x 1 arcmin), light collecting
area equivalent to a 15.9 m unobstructed aperture
- High photometric dynamic range: This unique capability will
be achieved through its unobstructed 6 x 6.5 m apertures. The
wide- and narrow-field low scattered light properties of the HDRT
will be unrivaled, both for direct imaging and coronagraphic applications.
- Versatility: Several operating modes involving wide (2o)
and narrow (1¢) fields-of-view (FOV). Its 6.5 m unobstructed subapertures may be coherently
combined or independently used with distinct detectors.
- High angular resolution: HDRT will achieve high spatial
resolution using adaptive optics while capitalizing on the unobstructed
pupil.
2 Science Drivers
- Extrasolar planets
- Star formation
- Kuiper Belt objects
- Weak lensing surveys
- Wide-field galaxy and redshift surveys
- "Origins" research themes
3 Key HDRT Technical Issues
- Off-axis optical design: Current technology allows 6.5 m
off-axis mirrors to be fabricated, and an unobstructed pupil offers
unique capabilities for imaging faint objects in the presence of bright
sources.
- Scalable
curvature AO system: This telescope design will use filled-subaperture
6.5 m curvature AO "units" that we know can be built and that
we understand very well. This is an effective route for achieving
an affordable, working, and maintainable AO system for a 22+ m telescope.
- Thin mirror technology: Trading steel for active optical
alignment mechanisms and software has proven to be effective in modern
telescopes. In addition to advantageous thermal and mass issues, a
thin mirror allows small conic changes that can yield very wide-field
performance from a parabolic primary. Building the telescope from
6.5 m unobscured off-axis mirror segments minimizes technical risks
and allows for a scalable design.
- Multimode optical configuration: The off-axis unit subaperture
design has fundamental advantages for flexible instrument implementation
schemes (since the optical path is accessible). Ultralow emissivity
IR performance, adaptive secondaries, and optimized instrument-secondary
mirror configurations are a natural advantage of this configuration.
The proposed off-axis Gregorian configuration has important advantages
for scattered light reduction and adaptive optics because of the accessible
pupil image.
- Compact beam alignment optics: Considerable work has been
done by the partners (following Roddier) in implementing membrane
mirrors and rotational shearing interferometers for achieving piston,
alignment, and focus adjustment of the subapertures.
- Telescope mount and enclosure: This will require imaginative
and cost effective approaches to meet the design goals of the facility
within the constraints of the CFHT envelope and Mauna Kea master plan.
1Institute
for Astronomy, UH
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