Canadian Very Large Optical Telescope Technical Studies Scott Roberts National Research Council Cana

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Canadian Very Large Optical Telescope Technical
StudiesScott RobertsNational Research Council
Canada, Herzberg Institute of Astrophysics(DAO)
http//www.hia.nrc.ca/pub/staff/cbt/XLT/Correspon
dence Email scott.roberts_at_nrc.ca Address
National Research Council Canada, 5071 West
Saanich Road, Victoria, B.C., Canada, V9E 2E7
Telephone [phone] Fax [phone]
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Author list
Scott Robertsa, Christopher Morbeya, Dennis
Crabtreea, Ray Carlbergb, David Cramptona,Tim
Davidgea, Joeleff Fitzsimmonsa, Mike Gedigc,
David Hallidayc, James E. Hessera, Glen
Herriota, J. B. Okea, John Pazdera, Kei Szetoa,
Jean-Pierre Verana aHerzberg Institute of
Astrophysics, National Research Council
Canada bUniversity of Toronto cAMEC Dynamic
Structures Ltd.
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Outline

• Project organization
• Context for a Canadian LOT
• Optical configuration
• Observational modes
• Telescope structure and dome design
• Pupil segmentation schemes
• Gap, segment size effects on EE, PSF
• Candidate mirror substrate materials, SiC study
• Integrated model of telescope

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The future generation of large telescopes
Canadian Long Range Plan Coalition AMEC/NRC/CASCA
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Canadian large optical telescope project

• Project Scientist Ray Carlberg, University of
  Toronto
• Project Manager Dennis Crabtree, (HIA/NRC)
• Science Steering Committee (Ray Carlberg, Chair)
• Technical Studies
• Research Collaboration between HIA/NRC and AMEC
  Dynamic Structures
• Scott Roberts, Technical Lead (HIA/NRC)
• David Halliday Mike Gedig, Technical Leads
  (AMEC)
• Several University Groups
• Funded at 1M US/year as of April 2002

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Context for a Canadian large telescope

• The Canadian Long Range Plan (LRP) for Astronomy
  calls for a 20 to 30 m telescope to be available
  to astronomers early in the ALMA/NGST era
  (2012). Faster deployment has a higher priority
  than the largest possible telescope size.
• Telescope should be superior to, or competitive
  with a 6 m NGST at wavelengths less that 2
  microns.
• Image quality is the highest, but not sole,
  priority. There is strong support for having the
  widest possible field of view for natural seeing
  observations.
• Possible location at CFHT site on the Mauna Kea
  summit ridge (ng-CFHT).
• Design fits within Mauna Kea Master Plan
  requirements.
• Designed for Mauna Kea environmental conditions.
• Canadian community expects to partner, but would
  prefer a second to none share of a large
  telescope.
• Baseline ? 0.36 to 2.3 microns, with extension
  to longer wavelengths

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Optical Configuration and Observational Modes
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LOT optical configuration baseline

• RC Design
• Segmented primary mirror, 20 m diameter, F/1
• Secondary mirror 2.5 m diameter
• 18 m back focal length (F/15)
• First fold beneath mirror support cell
• Instruments on 2 Nasmyth Platforms (vertical)
• Maximum 20 field of view (1.7 m dia, 2.5 m
  curv.).

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Observational modes

• Natural Seeing
• Maximum 20 field compatible with median MK
  seeing
• Degrades 50th ile MK seeing by no more than 15
• 10 Field with 1-metre refractive field
  corrector and ADC
• Degrades 25th ile MK seeing by no more than 10
• Low Order AO
• 6 field, low Strehl
• High Order AO
• 20 field, H Band Strehl 0.4
• Future upgrade path to MCAO

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Telescope Structure and Dome
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High performance, low profile structural design

• Large hydrostatic bearing wheels 12M diameter
• Monocoque primary support structure
• Short and direct load path for mass support
• Low profile azimuth platform
• Secondary support carried on main structure
• Elevation assy 880 tonnes, 1565 tonne moving mass
• Mirror cell 1st mode 13.4 Hz.
• Secondary support 1st 10.3 Hz.

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Structural design

• The load path from the telescope structure is
  passed almost directly into the azimuth support
  journal

• Main structure supports the mirror cell, bearing
  wheels and the secondary support structure

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Primary mirror cell (monocoque)
Sectioned Monocoque Mirror cell

• Modeled Performance
• Maximum deflections due to gravity lt2mm

Mirror segment access
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Telescope critical dimensions
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Primary F/ versus dome size
Diameter 104M Height 70M
Diameter 72M Height 48M
Diameter 75M Height 55M
Diameter 51M Height 38M
Elevation axis 18M above grade
APPROXIMATE ENCLOSURE SIZE REQUIRED FOR A 20M
MIRROR WITH FOCAL LENGTHS OF F1 AND F1.5
APPROXIMATE ENCLOSURE SIZE REQUIRED FOR A 30M
MIRROR WITH FOCAL LENGTHS OF F1 AND F1.5
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Calotte dome

• See 4840-73, Wed. _at_ 210
• 2 stages, base and inclined
• Circular aperture
• Wind attenuation
• Structurally superior
• Balanced stages
• Even power dissipation
• Safety
• Dome closed by 2nd internal stage

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Pupil Segmentation Schemes, EE and PSF
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Pupil segmentation schemes

• Hexagonal Segmentation
• Best system solution?
• Complex pupil boundary
• Families of 6
• Many spares
• 1.2 m, 348 seg., 58 spares

• Radial Segmentation
• Smooth pupil boundary
• 8 m central segment
• Phasing
• Commissioning
• Families of 20/40
• Economy in fabrication?
• Few spares (180 seg, 9 spares)

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Mirror fabrication study

• Canada, France and CFHT have jointly funded a
  study at Sagem to investigate technical, cost and
  schedule issues related to various segmentation
  schemes.
• Key results of study will be
• Fabrication risks, figuring errors, edge effects
  
• Optical test method(s) and requirements
• Budget and fabrication schedule

8 m segments
348x1.2?
150x1.8?
84x2.5?
Radial (1801x8m)
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EEs 2 m hex segment, various gaps, no spider
?1.2 ?m
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EEs for various hex segment sizes, 10 mm gap,
spider
444x1m 12x6m
114x2m 6x8m
30x4m 1x20m

• Hexagonal 20 cm wide support spider

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PSFs for segmentations with secondary obscuration
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EEs for various segment size and constant 10 mm
gap
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Mirror Materials
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Primary mirror candidate materials
 

• SiC offers lower areal density, simpler support
  systems, lower thermal mirror seeing effects

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Silicon carbide study

• Offers significant mechanical and thermal
  advantages over Zerodur, ULE substrates
• Isostatic Press, Machine, Light-weight, Sinter,
  CVD SiC front surface, grind, polish, ion figure.
  
• Trade-off stiff, 3 point support vs. low areal
  density whiffle tree support, 1 to 2 m
• Currently expensive to produce

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Integrated Modeling
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Integrated model

• Matlab Model
• End-to-End
• Optimize system
• Link to external optics engines and atmospheric
  models

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Summary

• Canadian designs for a Large Optical Telescope
  are progressing
• We would like to collaborate with other groups
  on technical studies
• See 4840-73, New approach to enclosure design
  for large telescopes, D. Halliday, M. Gedig, W.
  Brzezik, Y. Zhou, P. Evans, AMEC Dynamic
  Structures Ltd., Wednesday _at_ 210