XLT

Canadian Large Optical Telescope Studies: 

Canadian Large Optical Telescope Studies Dennis Crabtree Herzberg Institute of Astrophysics

People involved: 

People involved Dennis Crabtreea, Scott Robertsa, Chris Morbeya, Ray Carlbergb, David Cramptona,Tim Davidgea, Joeleff Fitzsimmonsa, Mike Gedigc, David Hallidayc, 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.

Outline: 

Outline Context for a Canadian LOT Project organization 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

Context: 

Context Long Range Plan for Astronomy Finalized in early 2000 ALMA the highest priority Large Optical Telescope identified for construction after 2010 Timescale now thought to be sooner Efforts at both AMEC and NRC NRC now funded at ~$800K (US)/year NRC and AMEC now have joint research agreement

Project Organization: 

Project Organization Joint Research Agreement (NRC & AMEC) Science Steering Committee Project Scientist - Ray Carlberg (Toronto) HIA PM – Dennis Crabtree; PE – Scott Roberts AMEC PM – David Halliday; PE – Mike Gedig Involving Canadian University groups Collaborative NRC-CNRS AO study funded Coordinating technical studies with other groups

Requirements: 

Requirements SSC desire for a 20m telescope to be available to astronomers early in the ALMA/NGST era (~2012). Image quality is the highest, but not sole, priority. There is strong support for having a wide field of view for natural seeing observations. Baseline λ = 0.36 to 2.3 microns, with extension to longer wavelengths 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. Canada to be a “Second to none”, full and equal partner

Optical configuration baseline: 

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).

Pupil segmentation schemes: 

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)

Mirror Segment Asphericity: 

Mirror Segment Asphericity

Mirror fabrication study: 

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 348x1.2 150x1.8 84x2.5 Radial (180+1x8m) 8 m segments

Encircled Energies: 

Encircled Energies 2 m hex segments, various gaps, no spider =1.2 m

Encircled Energies: 

Encircled Energies Various hex segment sizes, 10 mm gap, spider Hexagonal 20 cm wide support spider

PSFs for different segmentations: 

PSFs for different segmentations

EE’s for various segment size and 10 mm gap: 

EE’s for various segment size and 10 mm gap

Slide15: 

  Primary mirror candidate materials SiC offers lower areal density, simpler support systems, lower thermal mirror seeing effects

Slide16: 

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 single mirrors are expensive to produce

Observational Modes: 

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 Multiconjugate AO with larger field

Telescope Design: 

Telescope Design Large hydrostatic bearing wheels 12M diameter Monocoque 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

Primary Mirror Cell: 

Primary Mirror Cell Modeled Performance Maximum deflections due to gravity <2mm Sectioned Monocoque Mirror cell Mirror segment access

Secondary support: 

Secondary support Quadrapod support In purely mechanical terms a Tripod or Quadrapod is a preferable form of secondary support when compared to a Triangular or Serrurier type truss. The structure has less inertia Less physical structure near the dome aperture to catch the wind More occlusion of the primary mirror. Tradeoff of stability against loss of collecting area Integrated modeling will be used to evaluate configurations Carbon Composite legs All steel peripheral structure Carbon Composite legs

Slide21: 

Main structure supports the mirror cell, bearing wheels and the secondary support structure The load path from the telescope structure is passes directly into the azimuth support journal

Enclosure size: 

Enclosure size Scale Perspective Diameter 104M Height 68M WEIGHT 3500-4000t Elevation axis 18M above grade APPROXIMATE ENCLOSURE WEIGHT FOR A 20M MIRROR WITH FOCAL LENGTHS OF F1 AND F1.5 APPROXIMATE ENCLOSURE WEIGHT FOR A 30M MIRROR WITH FOCAL LENGTHS OF F1 AND F1.5 Diameter 72M Height 48M Diameter 51M Height 38M SOME WEIGHT COMPARISONS CFHT 400t Gemini 500t Keck 700t Subaru 1500t Diameter 75M Height 55M

Enclosure types: 

Enclosure types Carousel (Subaru) Dome with slot & vertical shutters (Keck) “Calotte”

Callotte Design: 

Callotte Design Basic components Enclosure base with a supporting and driving mechanism Enclosure cap with a supporting and driving mechanism Aperture door with a closing mechanism

Cap and Aperture Cover Motion: 

Cap and Aperture Cover Motion apertures out of phase Inner cap Cover extended & locked Cap Inner cap Base apertures out of phase Enclosure closed for day-time Cap - base interface seals activated Cover - cap interface seals activated Cap Cap - base interface seals activated

Cap and Aperture Cover Motion: 

Cap and Aperture Cover Motion Apertures aligned Cover retraction followed by cap rotation Cover retracted & hidden Seals deactivated Enclosure opened for night-time Opening phase

Structural Configuration: 

Structural Configuration 72 ribs 615 nodes Horizontal beams Cap - base interface Aperture ring beam Base ring beam

Base and Cap Interface Sealing: 

Base and Cap Interface Sealing TOP - CAP SIDE Direction of wind and rain Dynamic seal - flap Static seal - inflatable Non-stick face Heating rod Heating rod Dynamic seal - labyrinth Drain trough

Cap Drive: 

Cap Drive Spring-loaded idlers Flanged wheels for up-lift capacity Standing by direct drive motors Rail Whiffle-tree arrangement of drive wheels Disc brakes Gravity-loaded drive units Equalized wheel forces

Power Requirements: 

Power Requirements Conventional design Calotte design

Wind effects are important: 

Wind effects are important

Wind Studies - Preliminary CFD Analysis: 

Wind Studies - Preliminary CFD Analysis Contours of turbulence intensity Cylindrical Wind Wall Flared Wind Wall

Simulated Air Movements - Central Vertical Plane: 

Simulated Air Movements - Central Vertical Plane VELOCITY VECTORS

Enclosure Facing 45 Degrees Off Wind: 

Enclosure Facing 45 Degrees Off Wind Velocity contours taken at the section shown above Contour plane Contour Section line Viewpoint 70-78mph 18-23mph 0-5mph

Particle Travel Traces: 

Particle Travel Traces In this snapshot, six particles are introduced on a plane cutting through the dome aperture. Note that those traces near the top are carried by air exiting the enclosure. The circulation around the inside skin of the dome is about 30% of the wind speed, but the air movement local to the primary mirror is in the region of 10% or less than the external wind speed.

Slide36: 

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

Areas of future work: 

Areas of future work Ground layer adaptive optics utilizing natural guide stars and/or Rayleigh beacons Instrument concepts Wide-field natural seeing optical MOS Deployable IFU cryogenic IR spectrograph M1 edge sensors, actuators, control system Wind studies -> optimized enclosure design SiC M1 segments Integrated modeling