Mirror Fabrication Requirements for the Canadian Large Optical Telescope

Scott RobertsNational Research Council,

Herzberg Institute of Astrophysics

http://www.hia.nrc.ca/pub/staff/cbt/XLT/Correspondence: Email: scott.roberts@nrc.ca; Address: National Research Council Canada, 5071 West Saanich Road, Victoria, B.C., Canada, V9E 2E7 Telephone: 250-363-8314; Fax:

250-363-0045

The Future

New generation of large Telescopes

Canadian Long Range Plan

Coalition AMEC/NRC/CASCA

Canadian Large Optical Telescope ProjectCanadian 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

Technical StudiesTechnical 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

Optical Design Trade-Off’sOptical 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 quartiaryUnvignetted FOV 20 minutes diameter

Telescope Parameters: Diff limited Image Quality (arcseconds) geometric-RMSPrimary F# Final F/# pri to sec Sec Dia Field Dia Field Curve field diam * 0' 1' dia 6' dia 14' dia 20' diaF/1 F/8.6 16.0m 4.1m 1m -4.4m 3.6' 0" 0.0011" 0.033" 0.18" 0.37" F/15.0 17.6m 2.5m 1.73m -2.53m 3.6' 0" 0.0009" 0.032" 0.18" 0.39"

F/19.2 18.1m 2.0m 2.21m -2.0m 3.6' 0" 0.0009" 0.032" 0.20" 0.63" (**)F/1.25 F/8.6 19.5m 4.5m 1m -6.0m 3.8' 0" 0.001" 0.030" 0.16" 0.33"

F/17 22.1m 2.5m 1.97m -3.1m 3.8' 0" 0.0008" 0.029" 0.16" 0.33"F/21.9 22.7m 2.0m 2.53m -2.4m 3.8' 0" 0.0008" 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 designFor 20 meter aperture, Stop at primary, Tertiary 3m beyond primary vertex, quartiary 2.5m from optical axis, focus 4m from quartiaryUnvignetted FOV 20 minutes diameter

Telescope Parameters: Diff limited Image Quality (arcseconds) geometric-RMSPrimary F# Final F/# pri to sec Sec Dia Field Dia Field Curve field diam * 0' 1' dia 6' dia 14' dia 20' diaF/1 F/8.6 16.9m 3.2m 1m -3.3m 3.1' 0" 0.001" 0.042" 0.23" 0.47" F/11.4 17.6m 2.5m 1.32m -2.5m 3.1' 0" 0.001" 0.042" 0.23" 0.47"

F/14.7 18.1m 2.0m 1.7m -1.9m 3.1' 0" 0.001" 0.042" 0.23" 0.55"(**)F/1.25 F/8.6 20.6m 3.6m 1m -4.6m 3.3' 0" 0.001" 0.037" 0.20" 0.41"

F/13.4 22.1m 2.5m 1.5m -3.1m 3.3' 0" 0.001" 0.037" 0.20" 0.41"F/17.3 22.6m 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.

Optical Design Trade-Off’sOptical 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.

LOT Optical Configuration BaselineLOT 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

Observational ModesObservational Modes

Natural Seeing• Maximum 20’ field, 1.97 metres diameter with a 3.1 metre field curvature

• 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, 0.1” to 0.2” images

High Order AO• 20” field, H Band Strehl ~0.4

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

Elevation ViewElevation View

Primary Mirror Cell (Monocoque)Primary Mirror Cell (Monocoque)

Modeled Performance• Maximum deflections due

to gravity <2mm

Sectioned MonocoqueMirror cell

Mirror segment access

Integrated ModelIntegrated Model

DisturbancesWind

GravityThermal

TelescopeStructuralDynamics

Optics Model

Atmosphere

Edge SensorsWFS

Drive Encoders

[Q] Wavefront[F]

Star Field

Instrument+ Detector

Observation

SignalProcessing

ActuatorsSecondary MirrorTelescope Drives

Segments

ControlSystem

+

Sensor Noise

ActuatorError

+AO

Control

DM’s

DisturbancesWind

GravityThermal

TelescopeStructuralDynamics

Optics Model

Atmosphere

Edge SensorsWFS

Drive Encoders

[Q] Wavefront[F]

Star Field

Instrument+ Detector

Observation

SignalProcessing

ActuatorsSecondary MirrorTelescope Drives

Segments

ControlSystem

+

Sensor Noise

ActuatorError

+AO

Control

DM’s

Primary Mirror Candidate MaterialsPrimary Mirror Candidate Materials

Standard

• Zerodur - glass ceramic manufactured by Shott Glass Technologies• ULE - Titanium Silicate Glass by Corning (92.5% SiO2 and 7.5% TiO2)• 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)

Substrate Material ConsiderationsSubstrate Material Considerations

Material Property Units SIC

Bery

llium

(S-6

5H H

IP)

Carb

on F

ibre

Stee

l (10

15)

Alum

inum

606

1-T6

Inva

r

Copp

er

Zero

dur

ULE

Boro

silic

ate

(Pyr

ex)

Density Kg/m 3̂ 3140 1840 1550 7810 2710 8050 8900 2520 2200 2230Young's Modulus Gpa 420 303 130 193 71 141 117 92.9 67 64Poisson's Ratio 0.25 0.12 0.3 0.33 0.31 0.24 0.17 0.2Yield Strength Mpa 217 315 275 448 64Ultimate Tensile Strength Mpa 375 420 310 235 57CTE 10 -̂6/K 2.2 11.5 0.2 11.9 23.9 1.3 17 0.05 0.03 3.25Specific Heat Capacity Cp (J/Kg/K) 680 1925 840 486 896 515 385 821 778 726Thermal Conductivity W/mK 180 216 5 51.9 167 10.15 391 1.64 1.3 1.13

Substrate Material ConsiderationsSubstrate Material Considerations

Material Property Units SIC

Ber

ylliu

m (S

-65H

HIP

)

Car

bon

Fibr

e

Stee

l (10

15)

Alu

min

um 6

061-

T6

Inva

r

Cop

per

Zero

dur

ULE

Bor

osili

cate

(Pyr

ex)

Specific Stiffness p/E 7.48 6.07 11.92 40.47 38.17 57.09 76.07 27.13 32.84 34.84Resonant Frequency (E/p)^.5 0.37 0.41 0.29 0.16 0.16 0.13 0.11 0.19 0.17 0.17Thermal Diffusivity D (m^2/sx10^-6) 84.30 60.98 3.84 13.67 68.78 2.45 114.11 0.79 0.76 0.70Steady State Distortion alpha/k 0.01 0.05 0.04 0.23 0.14 0.13 0.04 0.03 0.02 2.88Transient Distortion alpha/D 0.03 0.19 0.05 0.87 0.35 0.53 0.15 0.06 0.04 4.66Polishable? Y Y* Y* ? Y* ? ? Y Y YDimensionally Stable? Y Y ?? Y Y ? ? Y Y Y

Specific Stiffness = ρ/EResonant Frequency = √(E/ρ)Thermal Diffusivity (D) = k/(Cp x ρ)Steady State Distortion = α / kTransient Distortion = α / D

Silicon Carbide StudySilicon 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

Segment Size and GapSegment Size and Gap

444x1m12x6m

114x2m6x8m

30x4m1x20m

Various segment sizes for constant gap (10 mm)

Hexagonal 20 cm wide support spider

Segment Size and GapSegment Size and Gap

Primary F/Ratio Versus Dome SizePrimary F/Ratio Versus Dome Size

Diameter 104MHeight 70M

Diameter 75MHeight 55M

Elevation axis 18M above grade

Diameter 72MHeight 48M

Diameter 51MHeight 38M

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

Optical Design & Fabrication Parameter SpaceOptical 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