TMT Optics Discussions with Industry - Thirty Meter Telescope

TMT 

Telescope Optics 

Eric Williams 

Discussions with Industry 

November 2009 

TMT.OPT.PRE.09.099.DRF03 1

Outline 

Telescope Optics Overview 

Primary Mirror 

Secondary and Tertiary Mirror Systems 

Optical Coating & Equipment 

Test Instruments 

– Prime Focus Camera 

– Global Metrology System 

CO 2 Snow Cleaning System 


Overview of Telescope Optics 


Telescope Optics 

M1 System 

M2 System 

M3 System 


TMT Optical Design: 

Ritchey Chrétien 

M1 Parameters 

– ø30m, f/1, Hyperboloid 

k = -1.000953 

– Paraxial RoC = 60.0m 

– Sag = 1.8m 

– Asphericity = 29.3mm (entire M1) 

M2 Parameters 

– ø3.1m, ~f/1, Convex hyperboloid, 

k = -1.31823 

– Paraxial RoC = -6.228m 

– Sag = ~196mm 

– Asphericity = 850 μm 

M3 Parameters 

– Flat 

– Elliptical, 2.5 X 3.5m 

– Articulated for instrument tracking and fast instrument switching 


Outline 








CO2 Snow Cleaning System 


M1 Array Parameters 

Segment pattern: 

– 30m circumscribed, well-filled aperture 

– 492 segments 

Six identical sectors of segments: 

– Take advantage of 6-fold symmetry 

– 82 types per sector, each with: 

Unique hex outline 

Unique optical prescription 

SECTOR-B 

Y M1 

1 

2 

3 

4 

5 

SECTOR-A 

55 

45 66 

36 56 

28 46 67 

37 57 

15 21 29 47 68 

38 58 77 

6 10 30 48 69 

23 39 59 78 

7 11 16 22 

17 31 49 70 

12 24 40 60 79 

8 18 32 50 71 

13 25 41 61 80 

9 19 33 51 72 

14 26 42 62 81 

20 34 52 73 

27 43 63 82 

35 53 74 

44 64 

54 75 

65 

76 

X M1 

SECTOR-D 

SECTOR-E 

Ø 30m 

View from Sky 


The TMT Primary Mirror (M1) 

Segment Parameters 

– Low Expansion glass or glass-ceramic 

– ~1.44m x 45mm thick 

Segments mounted to mirror cell 

– Each an off-axis asphere 

Maximum asphericity is 225μm 

– 2.5mm gaps, 0.5mm edge bevels 

Segment Support Assembly (SSA): 

– Provides passive support for 

3 in-plane DOF 

2 lateral directions and clocking 

– Permits active control of 

3 out-of-plane DOF by M1CS 

Piston/Tip/Tilt 

– Periodic shape correction by M1CS 

21 DOF Warping Harness 


Polished Mirror Assembly 

PMA is Produced by Polisher and Delivered to TMT 

– 7 sets of 82 types 


Polished Mirror Assembly 

Polished Mirror 

SSA 


M1 Segment Fabrication Requirements 

M1 Requirements and Specifications are posted on the TMT website: 

Surface figure requirements specified in terms of a Structure Function 

with an additional allowance for Low-Order shape errors 

Segments are to be mounted and final figured on the support system, 

zenith pointing, at mean observing temperature (2°C) 

Additionally, 

– Surface quality: Roughness

M1 Cell 


M1 Cell 


M1 Cell 


M1 Cell 


M1 Cell 


M1 Segment Installation 

Mounted 

Segment 

Segment Assembly 

Installed (MSA) in 

Array 

Subcell 

Mirror Cell 

(partial) 

Segment Position 

Actuators 


Segment Support Assembly (SSA) 

SSA Design is mature: 

– Under development for several years 

– Extensive trade-studies and FEA optimization performed 

– Prototypes have been built and tested 

Mechanical prototype complete (Aluminum Segment) 

Glass prototype in process 

– Detailed prototype drawings 

have been released 

In Final Design Phase: 

– Making minor improvements 

based on prototype testing 

– Design for manufacturability 

– Additional prototypes 

SSA design-description and drawings 

are on the TMT Website 


Axial Support System 

Two level, 27-point whiffletree system 

– All-Aluminum design (nearly) 

– Triangles nested for compactness 

– Small triangles made from extrusions 

– Large triangles made from castings 

82 SSA types due to segmentation 

– Variation in hex outlines must be 

compensated for. 

– Accomplished by “shifting pivots” 

Pivots are the joints in the whiffletree 

12 Pivot holes machined in unique 

locations to re-balance the whiffletree 

– SSAs outwardly identical 

– Internal difference is a few mm 

TMT.OPT.PRE.09.099.DRF03 19 

Whiffletrees ride on Moving Frame

Lateral Support System 

Lateral Support Design 

– Flat Central Diaphragm w/Outer Rim Slots 

– Material: Invar 36 

– Bonded directly to mirror 

Rim 

t=3mm 

Central 

hub 

t=9mm 

Moving Frame concept isolates diaphragm 

– Makes operating diaphragm deflections small 

– High strength material not required 

Flexure 

t=0.350mm 

Diaphragm 


Subcell Design 

Fixed Frame 

– Provides a stiff, stable interface between MSA and Mirror Cell 

– Welded 6061-T6 Aluminum (2 versions due to segmentation) 

– Interfaces 

Mirror cell (via AAPs) 

Segment (via tower registration features) 

Actuators (bolted and pinned joints at ends of Fixed Frame) 

Fixed Frame 

AAP (3x) 

Actuator 

Attachment (3x) 


Subcell Alignment 

Subcell Installation & Alignment: 

– M1 Array populated with 492 Fixed Frames 

– Mass Simulators installed 

Mass load mirror cell 

– Surveying targets attached to fixed Frames: 

Required surveying accuracy 0.100 mm 

Mass Simulator 

(~215kg Lead) 

Qty. = 492 

~105 tons 

Surveying 

Target, 3 ea. 


SSA Prototype Gravity Testing 


Segment Production Plans 

Production plan: See Statement of Work on TMT website 

– Contract start - October 2011 

– Facilitization, First Article PMA and Low Rate Production 

First article PMA – Mid 2013 (~18 mo. after contract start) 

Produce the first ~12 segments - Complete end 2013 

Develop and refine production processes 

Demonstrate scalable processes 

– Full Production (574 segments total) 

Production rate increase to support delivery schedule 

– First 120 segments to observatory: Mid 2016 

– Next 372 segments in equal lots – ~30/mo., over next 1 year 

– 82 spare segments follows – Complete late 2018 

– Segment Support Assemblies (qty. 574) – Complete as required 

– Production of 500 Subcells – Complete End 2015 


Outline 










M2 / M3 Requirements 

M2 and M3 requirements are posted on the TMT website: 

Surface figure requirements specified in terms of a Structure Function 

with an additional allowance for Low-Order shape errors 

Additionally, 

– Surface quality: Roughness

M2 System – Conceptual Design 

Telescope Top-end: 

– M2 Positioner Assembly (M2PA) 

– M2 Cell Assembly (M2CA) 

Telescope Top-end 

M2PA 


M2CA

M2 System – Conceptual Design 


M2 Conceptual Design 

M2 Positioner 

M2 System 

M2 Cell Assembly 

60 ACTIVE Axial 

Support Actuators 

Up to 24 Lateral Support 

Actuators 


Secondary Mirror (M2) 

M2 is a large convex aspheric mirror 

– 100mm thick solid meniscus 

Low expansion glass or glass ceramic 

Testing will be performed on the support system in 2 orientations: 

– Mirror face down and 

– Horizon pointing 

Sag 193 mm 

Clear Aperture ID 0.22 m 

Clear Aperture OD 3.024 m 


M3 System - Conceptual Design 


M3 System - Conceptual Design 

Tertiary Mirror 

3.5m x 2.5m (elliptical flat) 

M3CA 

Rotation 

Axis 

Tilt Axis 

M3PA 

3.5 m 

Cable 

Wrap 

(inside) 

Tertiary 

tower 

M1 

M3 Control 

TMT.OPT.PRE.09.099.DRF03 

Electronics 

32

Tertiary Mirror (M3) 

Mirror blank – similar to M2 

– Low expansion glass or glass ceramic 

– Mirror thickness: 100 mm 

Mirror support system 

– Also has an Active 60-point axial support 

– 24 point lateral support system 

Gravity components in both X & Y due 

to tracking 

Testing will be performed on the support 

system in 3 orientations: 

– mirror face up 

– two horizon pointing orientations 

Y M3 

vertical & X M3 

vertical 

Y M3 

60-point axial 

support pattern 

X M3 


Key M2 Positioner Requirements 

Motion Requirements 

DOF 

Piston 

Tip & Tilt 

X & Y De-center 

Range of Motion 

+/- 15mm 

+/- 2 mrad 

+/- 15mm 

Max. Error 

±2 μm 

± 30 arcsec 

± 100 μm 

Accuracy: 30mm RMS 

Smoothness of motion is critical (TBD) 

Control Bandwidth: > 0.1 Hz 


M3 Tracking Motion 

M3 must be able to steer the beam to 

any one of ~ 8 science instruments on the 

Nasmyth platforms 

For instruments not on the elevation 

axis, the M3 must track at variable 

rates in tilt and rotation as the 

telescope moves in elevation 

Tip/Tilt range: 50°+/- 8° 

Rotation range: +/-180° 

Elevation Axis 

MIRES 


M2 / M3 Development & 

Procurement Plan 

M2 and M3 Production begins October 2011 

TMT anticipates: 

– 12 month Preliminary Design Phase (PDP) 

– Final Design (12 months) 

– Fabrication, Integration & Testing (~ 4 years) 

– Shipment to the summit and AIV (2 months) 

M3 Schedule same 


Outline 










Key Optical Coating Requirements 

Coating Requirements Documents are available on the TMT website 

TMT requires high reflectance over wide spectral range: 

– 340nm-28μm (Required) 

– 310nm-28μm (Goal) 

Low Emissivity 

– [(M1+M2+M3) 0.90 

0.90 -> 0.95 

0.95 -> 0.97 

0.97 

Goal 

0.80 

0.90 

0.90 -> 0.95 

0.95 -> 0.98 

0.98 

0.98 


Coating Plants 

Design Requirements documents for Coating Plants are 

available on the TMT website. 

TMT will have two dedicated coating plants: 

– A large (3.5m capacity), batch-type system for recoating M2 & M3 

Conventional in design, used every 1-2 years 

– A production-capable system for routine recoating of M1 Segments 

called an In-line sputter system 

Load-locked to maintain a clean, high-vacuum process space 

Repeatable, high-yield process 

Includes up to 6 linear sputtering targets 

– permitting future process upgrades as the state-of-the-art in 

astronomical coatings is advanced 


Outline 










Prime Focus Camera 

Prime Focus Camera 

– Used during telescope assembly 

– Installed in telescope top-end 

before M2 mirror installation 

Mounts at Prime Focus 

– Used for segment alignment and 

diagnostics 

– Includes motorized alignment 

stages 

– Design Requirements Documents 

are pending 

Top End during AIV - On Sky Test 

PFC 

Dummy 

Mass 

Camera 


Global Metrology System (GMS) 

The GMS measures the positions of optics and instruments for diagnostic or 

alignment purposes intermittently during operation 

Telescope-mounted system 

Always “warm”, and on “stand-by’ 

– Can be used to re-align the telescope 

at any time 

Requirements: 

– Rapid and autonomous execution of 

programmed measurement routines 

– Measurement accuracy 50um (1σ) 

per axis 

Possibly based on laser trackers 

and permanently mounted retro-reflectors 

– Position calculation by distance 

measurements and trilateration 

Includes control and analysis software 

Design Requirements Documents are Pending 


Outline 










CO 2 Snow Cleaning 

Mirrors must be regularly cleaned between re-coatings 

– CO 2 Snow cleaning every 2 weeks 

Mechanical process: momentum transfer form snow flakes 

Telescope parked at the Horizon (so dust falls off after 

cleaning) 

Well established process on 8 & 10m telescopes 

TMT Requires three CO 2 cleaning systems: 

– M2: A hand-held system (supply, wand, nozzles) 

– M3: An automated system that self-deploys, operates, and re-stows 

– M1: A large scale system, fully automated, and rapid (660 m 2 ) 

Must stow and not block field of view 

Must be inherently safe 

– cannot damage mirrors (even in a major earthquake) 


Subaru CO 2 Snow 

Cleaning Implementation 

CO 2 

Cleaning 

Boom 

Click 

Here 

8m primary 

mirror, 

seen nearly 

edge-on 


TMT M1 CO 2 System Concept 

(one of a few) 

Curved composite 

boom, equipped with 

CO 2 nozzles 

Pivot axis pointed at 

center of curvature, 

constant distance to 

the optical surface 

Support member fixed 

to top-end 

compression member, 

Counter-tensioned 

cable system actuates 

the boom 


Counterclockwise Rotation 


Center Position 


Clockwise Rotation 


Stowed Configuration 

System stows in shadow of top-end structure 

Click picture to show 


Acknowledgements 

The TMT Project gratefully acknowledges the support of the 

TMT partner institutions. They are the Association of 

Canadian Universities for Research in Astronomy (ACURA), 

the Association of Universities for Research in Astronomy 

(AURA), the California Institute of Technology and the 

University of California. This work was supported, as well, 

by the Canada Foundation for Innovation, the Gordon and 

Betty Moore Foundation, the National Optical Astronomy 

Observatory, which is operated by AURA under cooperative 

agreement with the National Science Foundation, the 

Ontario Ministry of Research and Innovation, and the 

National Research Council of Canada. 


Back-up Material 

Additional M1, M2 and M3 Requirements 

– Surface figure structure function 

Segment Support Assembly 

– FEA Overview 

– Design Details 

– Warping Harness 

– Integration 

Coating 

– Facility Layout 

– Gemini Coating Performance 

Keck Telescope Heritage 



Additional M1, M2 & M3 Requirements 

Mirror surface structure function 

M2 & M3 Positioner motion 


M1 System Engineering 

M1 design requirements flow-down from top-level ORD and OAD 

– M1 Design Requirements Document (DRD) TMT.OPT.DRD.07.007.REL01 

Segment Polishing Spec. 

TMT OPT SPE 07 002 CCR03 

– Polished Segment Drawing 280-TMT-01-01000 Rev C 

– Polished Mirror Assembly Drawing 280-TMT-01-11000_C_PMA 

Mirror Segment Blank Spec. 

– Telescope Performance Error Budgets 

TMT OPT SPE 07 001 CCR06 

These documents are on the TMT Website 


Segment Polishing and Metrology 

Requirements (1/3) 

The surface figure of finished 

segments is specified by an 

atmospheric structure function 

(tilt removed) 

– Relates the amplitude of surface error 

to the separation distance 

– Based on a Kolmogorov atmosphere 

having r 0 

= 1.0m 

Structure Function (D), nm 2 

Separation distance δ, m 

Surface of Optic 

Requirement 

D(δ) = 〈[z(x) – z(x + δ)] 2 〉 < A[10.60(δ/d) 5/3 – 13.75(δ/d) 2 + 3.42(δ/d) 3 ] + 2B 2 

x = a given position on the mirror surface 

5 

z(x) = surface figure error at location x 

2 

2 3 

⎛ 1 ⎞ ⎛ 500nm 

⎞ ⎛ d ⎞ 

A = leading coefficient = 2907 nm 2 

A = ⎜ ⎟ ⎜ ⎟ 

2 2 

⎜ 

⎟ 

⎝ ⎠ ⎝ π ⎠ ⎝ r0 

B = High frequency surface roughness = 2 nm 

⎠ 

δ = Separation between point pairs 

d = diameter of segment = 1.44 meters 

r 0 

= Quasi-Fried’s parameter TMT.OPT.PRE.09.099.DRF03 = 1.0 meters 

55



Segments are to be final figured on the support system 

– Tested zenith pointing, at mean observing temperature (2°C) 

This removes the 1-g print-thru and much of the thermal distortion 

Polisher allowed to subtract low order aberrations from final test 

– Permitted in order to reduce polishing cost and production risk 

Subtracted errors are due to polishing and metrology errors 

These errors are removable by segment warping harness 

Aberration 

Focus 

Astigmatism 

Coma 

Trefoil 

Allowable Surface Error 

nm RMS 

50 

100 

10 

20 




Segments must be good right to the edge – no edge roll-off 

– Required for PFI Image contrast & APS segment phasing accuracy 

Surface quality: Roughness

Key M2 and M3 

Mirror Cell Requirements 

Surface figure specified by Structure Function 

– Similar to approach used to specify segment figure requirement 

Described in DRD’s on TMT Website 

– Controls errors due to: 

Polishing and Metrology 

Support system manufacturing tolerances 

Thermal distortion: Blank material and support system 

Low-order allowance 

Active optic correction by look-up table 

– Correct figure using 60 axial 

support actuators: 

Aberration 

M2 

Fabrication errors 

Focus 

N/A 

Coating stress 

Astigmatism 200 

Thermal distortion 

Coma 

20 

Stiffness to reject wind 

Trefoil 

50 

Seismic requirements 

Allowable Surface Error, nm RMS 


M3 

100 

200 

20 

50

M2 Structure Function Requirements 

The surface figure of the M2 mirror is 

specified by an atmospheric structure 

function (tilt removed) 




having r 0 

= 3.20m 



25000 

20000 

15000 

10000 

5000 

Requirement 

0 

0 0.2 0.4 0.6 0.8 1 

Separation distance, δ/d 



5 


2 

2 

3 

⎛ 1 ⎞ ⎛ 500nm 

⎞ ⎛ 30m 

⎞ 


A = ⎜ ⎟ ⎜ ⎟ 


2 2 

⎜ 

⎟ 

⎝ ⎠ ⎝ π ⎠ ⎝ r0 

⎠ 


d = diameter of beam footprint = 3.046 meters 

r 0 


59

M3 Structure Function Requirements 

The surface figure of the M3 mirror is 

specified by an atmospheric structure 

function (tilt removed) 




having r 0 

= 4.0m 



35000 

30000 

25000 

20000 

15000 

10000 

5000 

Requirement 

0 

0 0.2 0.4 0.6 0.8 1 

Separation distance, δ/d 



5 

2 


3 


0 2 ⎛ 500nm 

⎞ ⎛ 30m 

⎞ 

A = cos(45 ) ⎜ ⎟ 


2 

⎜ 

⎟ 

⎝ π ⎠ ⎝ r0 

⎠ 


d = diameter of beam footprint = 1.33 meters 

r 0 


60


Segment Support Assembly 

FEA Overview 

SSA Design Details 

Warping Harness 

Integration 


Mechanical Simulation 

SSA development relied heavily on FEA 

Detailed FE models: 

– Included all load paths and 

significant masses 

Typical FEA Unit load case results 

Lateral (+1g X 

) 

RMS = 11.99 nm, P-V = 219 nm 

Purple = -110. nm, Red = +110. nm 

Y 

Lateral (+1g Y 

) 

RMS = 11.99 nm, P-V = 219 nm 


X 

Axial (-1g Z 

) 

RMS = 10.4 nm, P-V = 60 nm 


ΔT = +1°C 

RMS = 2.28 nm, P-V = 13.11 nm 

Purple = -8.55 nm, Red = +4.55 nm 

Credit: IMTEC 


Segment Support Performance 

Segment passive support print-thru: 

Print-thru Error 

Axial Support 

AO Surface 

Figure Error 

Segment Assembly 

Polished & Coated Segment 

Mounted Primary Segment Segment Support Assembly (PSA) (MSA) 

Edge Sensors 

and cables 

Segment Position 

Actuators 

Credit: IMTEC 

Subcell 


Installed on Mirror Cell 

Actuator flexure 

Actuator 

Mirror Cell 

Note: Does not represent 

assembly sequence 

Credit: IMTEC 

Adjustable Alignment 

Positioner (AAP) 


Flexures Bonded to Segment 

Segment 

Central Diaphragm 

(bonded to segment) 

Edge Sensor 

12 ea. 

Alignment Arrow 

Points to center of M1 



Credit: IMTEC 

Axial flexure assemblies 

27 ea. bonded to segment 


Small Whiffletree Triangles Attached 



Small whiffletree triangle 

- 3 inner 

- 6 outer 

Credit: IMTEC 


Large Whiffletree Triangles Attached 

Large whiffletree triangle 

Credit: IMTEC 

Axial Support: Two level, 27-point whiffletree system 

– All-Aluminum design (nearly) 

– Triangles and sheet flexures Aluminum 

– Rod Flexures Stainless Steel 

Triangles nested for compactness 


Sheet Flexures Added 

Sheet flexure, 6ea 

In-plane connection between Whiffletree 

Triangles and Moving Frame 



Credit: IMTEC 


Moving Frame Attached 

Sheet flexure, 6ea 

In-plane connection between Whiffletree 

Triangles and Moving Frame 



Moving frame 

Credit: IMTEC 


Warping Harness Added 

Warping harness leaf-spring 

Warping harness actuator 



Credit: IMTEC 


Tower & Locks Installed 

Tower Assembly 

with Repeatable Interface 

Electrical Connector Bulkhead Panel 



Credit: IMTEC 


Fixed Frame Included 



Credit: IMTEC 

Fixed Frame 


WH Approach & Architecture 

Warping Harness Purpose 

– Allow automated periodic correction of low order surface distortions 

Fundamental Approach 

– Re-shapes the mirror by bending it in a controlled manner using whiffletree 

Architecture – 21 actuators per segment 

– Stepper motor drives lead screw that pushes on an instrumented leaf spring 

Small Whiffletree Triangle 

Small Whiffletree Triangle 

Large Triangle 

Axial Support Flexure 

Nut 

Screw 

WT Joint Flexure 

(sheet flexure not shown) 

Strain 

Gauge 

Leaf-spring 

Stepper Motor 


Warping Harness Design 

WH Actuator 

Assy (shown 

on test fixture) 

Actuator 

(typ.) 

Leaf Spring 

WH Drive Screw 

Assy / Nut 

Actuator Layout 

- 21 Actuators integrated 

into axial support system 


Section of Mirror Cell Top Chord 

Credit: IMTEC 


Group of Segments 

View of Seven Adjacent Segments – Top View 

– Note: M1 Array is continuous, not in pods or rafts of seven segments 

Credit: IMTEC 


Group of Segments 

View of Seven Adjacent Segments – Bottom View 

Credit: IMTEC 



Coating Facility Layout 

Gemini Coating Performance 


Coating Facility Layout 

The Coating Facility has been designed to efficiently support routine 

coating operations 

Telescope Enclosure 

Mirror 

Maintenance 

Room 

Segment Storage 

Area for 82 spares 

Stripping & 

Cleaning 

M1 Segment 

Clean room (10k) 

Process front end 

M2/M3 

M2 

M3 

M1 

ILSS 

ILSS 

Equipment & 

Storage 

Coating Plant 

Credit: M3 Engineering 


Baseline Coating Process 

Performance of the Gemini Coating relative to TMT Specifications 

100 

95 

Reflectance per Surface, % 

90 

85 

80 

75 

TMT Requirement 

Gemini Protected Silver 

70 

300 500 700 900 1100 1300 

Wavelength [nm]

TMT Optics Discussions with Industry - Thirty Meter Telescope

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