Mirror Fabrication Requirements for the Canadian Large Optical Telescope Scott Roberts National Research Council, Herzberg Institute of Astrophysics

Transcription

1 Mirror Fabrication Requirements for the Canadian Large Optical Telescope Scott Roberts National Research Council, Herzberg Institute of Astrophysics Correspondence: Address: National Research Council Canada, 5071 West Saanich Road, Victoria, B.C., Canada, V9E 2E7 Telephone: ; Fax:

2 The Future New generation of large Telescopes Canadian Long Range Plan Coalition AMEC/NRC/CASCA

3 Canadian Large Optical Telescope Project Project Scientist Ray Carlberg, University of Toronto Project Manager Dennis Crabtree, Herzberg Institute (NRC) Science Steering Committee (Ray Carlberg, Chair) Technical Studies Research Collaboration between HIA and AMEC Dynamic Structures Scott Roberts, Technical Lead (HIA) David Halliday & Mike Gedig, Technical Leads (AMEC) Several University Groups (University of Montreal, McGill, ) Funded at ~$1M US/year as of April 2002

4 Technical Studies Optical Design System Error Budget Integrated Model Cost Estimate, Schedule, Plan Control Systems, Sensors and Actuators Adaptive Optics Telescope Structure Telescope Enclosure Instrument Concepts Silicon Carbide (SiC) Segment Design and Fabrication Operational Efficiency and Reliability

5 Optical Design Trade-Off s 20m RC design Trade off's - Primary focal ratio and secondary size Outside Naysmith design: For 20 meter aperture, Stop at primary, Tertiary 3m beyond primary vertex, quartiary 13m from optical axis, focus 2m from quartiary Unvignetted FOV 20 minutes diameter Telescope Parameters: Diff limited Image Quality (arcseconds) geometric-rms Primary F# Final F/# pri to sec Sec Dia Field Dia Field Curve field diam * 0' 1' dia 6' dia 14' dia 20' dia F/1 F/ m 4.1m 1m -4.4m 3.6' 0" " 0.033" 0.18" 0.37" F/ m 2.5m 1.73m -2.53m 3.6' 0" " 0.032" 0.18" 0.39" F/ m 2.0m 2.21m -2.0m 3.6' 0" " 0.032" 0.20" 0.63" (**) F/1.25 F/ m 4.5m 1m -6.0m 3.8' 0" 0.001" 0.030" 0.16" 0.33" F/ m 2.5m 1.97m -3.1m 3.8' 0" " 0.029" 0.16" 0.33" F/ m 2.0m 2.53m -2.4m 3.8' 0" " 0.028" 0.17" 0.46" (***) * Diffraction limited field diameter is definied as the diameter where the strehl ratio falls to 0.80 for a wavelength of 1 micron with the system focused for best focus at the center of the field ** 20' field spot size is larger due to the deviation of the focal surface from a sphere, if a conic section is used for the focal surface the spot size is 0.35", in line with the Y^2 scaling *** see ** Inside Naysmith design For 20 meter aperture, Stop at primary, Tertiary 3m beyond primary vertex, quartiary 2.5m from optical axis, focus 4m from quartiary Unvignetted FOV 20 minutes diameter Telescope Parameters: Diff limited Image Quality (arcseconds) geometric-rms Primary F# Final F/# pri to sec Sec Dia Field Dia Field Curve field diam * 0' 1' dia 6' dia 14' dia 20' dia F/1 F/ m 3.2m 1m -3.3m 3.1' 0" 0.001" 0.042" 0.23" 0.47" F/ m 2.5m 1.32m -2.5m 3.1' 0" 0.001" 0.042" 0.23" 0.47" F/ m 2.0m 1.7m -1.9m 3.1' 0" 0.001" 0.042" 0.23" 0.55"(**) F/1.25 F/ m 3.6m 1m -4.6m 3.3' 0" 0.001" 0.037" 0.20" 0.41" F/ m 2.5m 1.5m -3.1m 3.3' 0" 0.001" 0.037" 0.20" 0.41" F/ m 2.0m 2.0m -2.4m 3.3' 0" 0.001" 0.037" 0.20" 0.45" (**) * Diffraction limited field diameter is definied as the diameter where the strehl ratio falls to 0.80 for a wavelength of 1 micron with the system focused for best focus at the center of the field ** 20' field spot size is larger due to the deviation of the focal surface from a sphere as in outside naysmith design.

6 Optical Design Trade-Off s Image Quality, in terms of diffraction limit and angular spot size is relatively independent of secondary size and final focal ratio, but is dependent on primary focal ratio and back focal distance. Slower primary F/# and longer b.f.d. = better image quality Comes at the expense of larger dome and larger secondary mirror, larger field diameter.

7 LOT Optical Configuration Baseline Primary mirror = 20 m diameter, ~2 m segments Maximum 20 field Primary F/1.25 Secondary mirror 2.5 m diameter First fold beneath mirror support cell 18 m back focal length (F/17) Instruments on Nasmyth Platforms (vertical) 1 m diameter field corrector and ADC

8 Observational Modes Natural Seeing Maximum 20 field, 1.97 metres diameter with a 3.1 metre field curvature Degrades 50 th %ile MK seeing by no more than 15% 10 Field with 1-metre refractive field corrector and ADC Degrades 25 th %ile MK seeing by no more than 10% 6 field, 0.1 to 0.2 images 20 field, H Band Strehl ~0.4 Low Order AO High Order AO

9 Structural 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

10 Elevation View

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

12 Integrated Model Star Field Atmosphere Disturbances Wind Gravity Thermal [F] Telescope Structural Dynamics [Q] Optics Model Wavefront Instrument + Detector Observation Actuator Error + Actuators Secondary Mirror Telescope Drive s Segments Control System Edge Sensors WFS Drive Encoders + Signal Processing Sensor Noise DM s AO Control

13 Primary Mirror Candidate Materials Standard Zerodur - glass ceramic manufactured by Shott Glass Technologies ULE - Titanium Silicate Glass by Corning (92.5% SiO 2 and 7.5% TiO 2 ) Borosilicate - Crown (Pyrex, E6, Shott Borofloat) Exotic Carbon Fibre - carbon fibres in an epoxy matrix (anisotropic, non-homogeneous) Beryllium - Light metallic element, Ni coated (space mirrors, light secondaries) SiC - Crystal Silicon Carbide - Similar application to Be (isotropic, homogeneous) Aluminum - Used extensively for cryogenic IR mirrors Fiducials Steel Copper - Exceptional Thermal Properties Invar - Low thermal expansion metal (36% Ni, <1% C,Mn,Si, Balance Fe)

14 Substrate Material Considerations SIC Beryllium (S-65H HIP) Material Property Units Density Kg/m^ Young's Modulus Gpa Poisson's Ratio Yield Strength Mpa Ultimate Tensile Strength Mpa CTE 10^-6/K Specific Heat Capacity Cp (J/Kg/K) Thermal Conductivity W/mK Carbon Fibre Steel (1015) Aluminum 6061-T6 Invar Copper Zerodur ULE Borosilicate (Pyrex)

15 Substrate Material Considerations SIC Beryllium (S-65H HIP) Material Property Units Specific Stiffness p/e Resonant Frequency (E/p)^ Thermal Diffusivity D (m^2/sx10^-6) Steady State Distortion alpha/k Transient Distortion alpha/d Polishable? Y Y* Y*? Y*?? Y Y Y Dimensionally Stable? Y Y?? Y Y?? Y Y Y Carbon Fibre Steel (1015) Aluminum 6061-T6 Invar Copper Zerodur ULE Borosilicate (Pyrex) Specific Stiffness = ρ/e Resonant Frequency = (E/ρ) Thermal Diffusivity (D) = k/(cp x ρ) Steady State Distortion = α / k Transient Distortion = α / D

16 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. Trade-off stiff, 3 point support vs. low areal density whiffle tree support, 1 to 2 m Currently expensive to produce

17 Segment Size and Gap 444x1m 12x6m 114x2m 6x8m 30x4m 1x20m Various segment sizes for constant gap (10 mm) Hexagonal 20 cm wide support spider

18 Segment Size and Gap

19 Primary F/Ratio 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

20 Optical Design & Fabrication Parameter Space RC, Gregorian, Korsch Primary F/# 1 to 1.5 Dome cost, sensitivities Hexagonal segments 1 to 2 m diameter Manufacturability, Support, Phasing, Segment Handling Zerodur / ULE / Silicon Carbide Stiffness/Deflection, Mass/Inertia, Thermal, Support, Cost # of support points per segment (3 to 27) Whiffle tree cost/complexity Secondary Mirror 2 to 3 m diameter B.F.L. 9 to 20 m