The TMT L aser G uide S tar F acility ( LGSF )

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Kai Wei

Institute of Optics and Electronics (IOE),CAS

August 30,2010

The TMT Laser Guide Star Facility

(LGSF)

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Presentation Outline

• LGSF Requirements and Updates• The changes of the LGSF designs• What we have done for the LGSF?• The working plan for LGSF

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LGSF Requirements and Updates -Overall Description

• The LGSF is composed of 3 main sub-systems: – The Laser System (LAS), which includes the lasers, the Laser

Service Enclosure (LSE) and all associated electronics. – The Beam Transfer Optics (BTO) and Laser Launch Telescope

(LLT) System– The Laser Safety System (LSS), which is composed itself by

several sub-systems dedicated to: • Protecting people and observatory systems from laser light,

• Protecting aircraft from laser illumination,

• Protecting neighboring telescopes from laser beams within their field of view

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LGSF Requirements and Updates -Overall Description

• System Functions: – Project the early light NFIRAOS asterism.

– Project other asterisms as required by the AO modes

– Switch rapidly between asterisms.

– Use conventional optics for the Beam Transfer Optics and launch the AO asterisms from a Laser Launch Telescope located behind the TMT secondary mirror.

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LGSF Requirements and Updates -Specific Requirements

• Interface Requirements • General Constrains Requirements (Lifetime, Standards)• Environmental Requirements • Functional and Performance Requirements• System Attributes (Reliability, Maintainability, Security)• Access and Handling (installation, removal)

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• Asterism generation requirement

– NFIRAOS asterism: consists of 6 LGS, 5 equally spaced on a circle of radius of 35 arcsec and one additional on-axis guide star. (black)

– MIRAO asterism: consists of 3 LGS equally spaced on a circle of radius of 70 arcsec. (red)

– MOAO asterism: consists of 8 LGS, 3 equally spaced on a circle of radius of 70 arcsec and 5 equally spaced on a circle of radius of 150 arcsec. (blue)

– GLAO asterism: consists of 5 LGS, 4 equally spaced on a circle of radius of 510 arcsec and one additional on-axis guide star. (green) switch between asterisms within 2 minutes

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• Asterism generation requirement– as the telescope tracks a science field for exposure times of up to 60 minutes, with

1-axis, 1-sigma tip/tilt error for each laser beam of no more than 0.050 arcsec

• Telescope flexure compensation– The LGSF shall be capable of correcting for the effects of flexure of the telescope to

p end structure by up to ±15 mm in translation， ± 2.5 mrad in tilt and ± 4 mm in axial motion over the operating zenith angles between 1 and 65 degrees and at any observing temperature.

• Beam quality and polarization– 1.2 times the diffraction limited of 1/e2 beam diameter, for a near field 1/e2 diameter o

f 0.3m at the LLT. – A Beam Cleanup AO system may be included in the design, to correct the aberration

s on the out-going laser beams. The Beam Cleanup AO system will include one slow WFS and possibly up to one DM per laser beam.

– The LGSF system shall generate Laser Guide Stars which are 98% circularly polariz

ed.

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Four Kinds of the LGSF configuration

• LGSF Initial configuration• LGSF Baseline configuration• LGSF Elevation Journal configuration• LGSF Side Launch configuration

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1.Laser switchyard:an optical bench with motor-controlled beamsplitters and mirrors which accept the 50W input beams from the operating lasers and direct them to the proper outputs at the desired power. One can generate either two 25W beams or three 17W beams

LGSF Initial configuration -Overall Description

2.Beam Transport Opticstransport the nine output beams from the Laser Switchyard out of the LSE and up the telescope truss to the BTOOB behind the secondary mirror.Two of the mirrors in each beam position within the BTO path are controllable in tip/tilt to maintain both centering and pointing of the beams at the input to the BTOOB, correcting for the inevitable flexure of the telescope structure with altitude.

3.The diagnostic system directs a small fraction (0.5%) of each of the laser beams through a beamsplitter into two camera systems. one focused at a relatively close distance, the other at infinity. The near-field camera is used to evaluate the intensity profile and quality of the laser beam within the LGSF. The far-field camera views the projected LGS at diffraction-limited resolution to evaluate its image quality.

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LGSF Initial configuration -Diagnostic system

• Measure the locations of the laser beams at the input to the BTOOB to ensure that the centering and pointing mirrors in the BTO are properly compensating for telescope flexure.

• Measure the profile of the laser beams for comparison with the specified profile.

• Assure that the LGS beacons are properly aligned with the telescope pointing.

• Evaluate the image quality of the LLT by imaging a star.

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LGSF Initial configuration -Asterism Generator

The two mirrors at the periphery of the Asterism Generator will be controllable in tip/tilt to maintain centering of the beams on the LLT pupil and pointing of the LGS beacons on the sky in compensation for any flexure within the BTOOB and pointing error of the LLT itself resulting from flexure of the telescope structure with attitude.

The first mirror, on the back side of the Asterism Generator plate, is a high-bandwidth tip/tilt fast steering mirror (FSM) to compensate for jitter in the position of the LGS as measured by the associated WFSThe budget for fast tip/tilt correction assigns a value of 50 mas to the 1-axis, 1σ laser pointing jitter

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LGSF Initial configuration -Centering and Pointing mirrors

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LGSF Baseline configuration

The beams are transferred across to the –X elevation journal along the telescope elevation axis via two active steering mirror arrays and a fold array. The active arrays are used to follow the rotation of the telescope elevation structure as the zenith angle changes and to correct for any misalignments of the telescope top end due to thermal and flexure effects.

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LGSF Baseline configuration

Main advantage: Lasers located within the telescope azimuth structure to provide fixed-g orientation, allow a large laser footprint, limit vibrations into telescope and reduce wind obstruction.

Main disadvantage:The long optical path and in particular the deployable/retractable section of the optical path.

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LGSF Elevation Journal configuration

• Lasers attached to the -X elevation journal • Laser beams transported from the elevation journal up to the

launch• telescope located behind the secondary• Reduced optical path and in particular no deployable/retractable

section.• But lasers operating in variable gravity orientation, with tighter

limits on mass and volume.

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• Smaller output aperture 0.4m instead of 0.5m • Large asterism (17 arcmin) requirement deleted

– Telescope field of view: +/-2.5 arcmin

• No Up link AO upgrade path required• Calibration requirement with visible stars deleted• New Laser Guide Star acquisition system added

Revised designs for launch telescope developed for new requirements:Modified off axis reflective design Refractive design

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LGSF Side Launch configuration

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• Lasers and launch telescopes in several locations around M1:– 2 LGS per launch telescope (up to 4 locations) - SL2 configuration

• AO performance are slightly improved compared with center launch configuration– Required laser power reduced for equal wavefront error due to noise– Fratricide effect also minimized/eliminated

• Beam transfer optics and launch telescope simplified:– Very simple and short beam transfer optics– Launch telescope requirements somewhat simplified:

• Smaller output aperture needed: 0.4m instead of 0.5m• Field of view: +/-1.6 arcmin

• On another hand:– 4 launch telescopes required instead of 1– Doubled LGS elongation requires larger LGS WFS– De-rotation mechanisms needed in LGS WFS optical path to follow elongat

ion

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What we have done for LGSF

• Beam transfer Optics System:– the Optical Path

– the LGSF Top End• the Diagnostic

System• Asterism Generator• Launch Telescope

Assembly (LTA)• Acquisition System

• 3 main sub-systems:– Laser System

– Beam transfer optics system

– Laser safety system

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LGSF Elevation Journal Configuration

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Comparison of the two optical designs for the LTA

the confocal paraboloid design the refractive design

Two off-axis paraboloid mirror, Focus adjustment, K-miiror system

Objective lens nearly 16kg, Focus adjustment, K-miiror system

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Radius of field-angle(arc sec)

Confocal paraboloid design Refractive design

Strehl Ratio @ 589nm Strehl Ratio @ 589nm

210 km 90 km 210 km 90 km

0 0.99 0.99 0.94 0.94

35 0.97 0.98 0.95 0.95

70 0.97 0.97 0.97 0.97

150 0.94 0.95 0.98 0.98

Image quality for the two optical designsThe differences between the image quality of the two optical design is inconspicuous

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Issue Confocal Paraboloid Design Refractive Design

Elements

10 (2 input folds, 3 K mirrors, 3 refractive elements to

correct off-axis curvature, secondary, primary,

window); 14 surfaces

8 (2 input folds,3 K mirrors, 2 refractive elements,

objective); 10 surfaces

Element fabrication and test

Paraboloid straightforward to test.

4th order aspheric on convex surface may be harder to test.

AlignmentOff-axis mirrors will be more

challenging to align.Approx 4 times less sensitive than

reflective.

Thermal stability

Should be relatively insensitive if the mount material is the

same as the mirror substrates.

Certain to be more sensitive to temperature changes, might

need active focus adjustment.

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Comparison of the two optical designs for the LTA - Thermal analysis for the refractive design

Temperature ( )℃

Focus Adjustments(microns)

Design value @ 0℃ 0

-1.0 -28

1.0 27

-5.0 -139

-2.7 -74

-0.3 -1

2.0 54

4.3 117

6.7 180

9 241

Changes from -5 to 9℃ ℃ 380

Changes quantity per 0.5℃ 13

Observing Performance Conditions :-Ambient air temperatures range: -5 ℃to +9 ℃

the WFE RMS of the LTA while the temperature changes 1 degree

Ambient air temperatures range: -5 to + 9 ℃ ℃

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Comparison of the two optical designs for the LTA - the 4th aspheric on convex surface

4th order parameter tolerance

Before-compensator

Sr (field angle ) CompensatorShift (mm)

After-compensator

Sr (field angle )

0 150 0 150

-4.0E-13 0.5121 0.2754 0.137 0.9054 0.8602

+6.0E-13 0.1629 0.3053 -0.203 0.9065 0.86

The acceptable manufacture error

At the maximum cutting position the manufacture error should be controlled less than 3.35 microns

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Comparison of the two optical designs for the LTA - The objective lens deformation

Deformation of the objective lens convex surface while the zenith angle 1°:The biggest deformation is the top and the edge of the surface, and the PV=106.7nm

Deformation of the objective lens concave surface while the zenith angle 1°:The biggest deformation is the top and the edge of the surface, and the PV=97.8nm

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Risk and Choice for the LTA

Subject Refractive DesignConfocal Paraboloid

DesignComments

Image qualityStrehl at 589nm slightly

inferior to reflective design

Image quality appear to be related to fabrication specifications and mounting, not with design

Fabrication and Test

4th order aspheric surface is difficult to test

Fabricating of the two mirrors may also need to be careful

Maybe the most important advantage of the confocal paraboloid design is that it has been implemented at the Gemini Observatory, and if the same design is considered the TMT LTA will benefit from the experience with the fabricating and mounting.

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Risk and Choice for the LTA

Subject Refractive DesignConfocal Paraboloid

DesignComments

Alignment and mounting

Approx 4 times less sensitive than reflective

Approx 4 times more sensitive than refractive

Both of these are very important during the operation process of TMTMechanical

flexureInsensitive Sensitive

Thermal stabilitySensitive but easy to

compensateInsensitive

Cover window Not need anymoreRequire an additional

optical elementnot complicated

Mass and moment

Objective weight~16kg. Fold mirrors ~3.2kg. Weight mostly at middle-top

Primary, secondary weight ~19.6kg. Weight mostly at bottom.

Not include mechanical element mass

Cost estimates lower May be expensive

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LTA Throughput Estimate

Element Number surface Surface

Throughput

Throughput

Objective lens

1 2 0.99 0.980

Collimator lenses

2 4 0.99 0.961

Fold Mirror 2 2 0.995 0.990

K Mirror 3 3 0.995 0.985

0.918

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LTA total error 0.241Objective lens manufacture WFE(p-v) 0.139L1 manufacturing tolerance Nominal data Tolerance Compensator p-v wavefront errorL1 convex surface radius 440. 822mm 0. 55mm~-0. 15mm 1. 37527mm~-0. 37499mm 0. 008L1 convex surface conic -0. 484869 0. 00015~-0. 00018 -0. 00813~0. 00976 0. 1L1 convex surface 4th parameter 1. 88495E-11 2. 5E-13~-2. 5E-13 -0. 00845~0. 00845 0. 09L1 concave surface radius 3476. 4188 2. 5mm~-3. 2mm -0. 09214~0. 11815 0. 03L1 middle thickness 70 ±0. 03mm -0. 02555~0. 02555 0L1 index errors 1. 5168 -0. 0001~+0. 00008 0. 08339~-0. 22811 0. 015LTA alignment 0.010LTA mirror parameters Nominal data Tolerance Compensator p-v wavefront errorL1 decenter ±0. 1mm ±0. 1mm 0L1 tilt ±180μ rad ±3414. 6μ rad 0. 01Surface irregularity(p-v) 0.196LTA mirror parameters Nominal data tolerance Number of elements p-v wavefront errorL1(surface and support deformation) 0. 14 1 0. 065Folding mirrors 0. 04 5 0. 179Lenses 0. 08 2 0. 048

Error budget of the LTA

The surface irregularities and wavefront errors are given in P-V at 589.3 nm. The RSS calculations use a factor of 2 for reflective surfaces and 0.43 for transmitting surfaces.

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Schedule for the LTA

ID WBS Code Beginning Time Ending TimeExpected Month

2014年2013年2012年

09月 07月04月 09月 12月02月05月 04月10月12月 11月 01月08月11月 10月03月12月06月10月 11月08月06月02月01月 07月 05月03月

1 74w2014-2-282012-10-1WBS 1.1

2 26w2013-3-292012-10-1WBS 1.2

3 26w2013-3-292012-10-1WBS 1.3

4 35w2013-5-312012-10-1WBS 1.4

5 43.6w2013-7-312012-10-1WBS 1.5

6 26w2014-8-292014-3-3WBS 1.6

82.6w2014-9-302013-3-1WBS 2

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13.2w2014-12-312014-10-1WBS 3.2


2012年 2013年 2014年

10月 11月 12月 01月 02月 03月 04月 05月 06月 07月 08月 09月 10月 11月 12月 01月 02月

1 4.6w2012-10-312012-10-1WBS 1.1.1

2 65.6w2014-1-312012-10-31WBS 1.1.2

3 41.6w2014-2-142013-5-1WBS 1.1.3

4 42.2w2014-2-282013-5-10WBS 1.1.4

5 74w2014-2-282012-10-1WBS 1.1.5


2013年 2014年

03月 04月 05月 06月 07月 08月 09月 10月 11月 12月 01月 02月 03月 04月 05月 06月 07月

1

2

3

8.6w2013-4-302013-3-1WBS 2.1

69.6w2014-7-302013-4-1WBS 2.2

8.6w2014-9-302014-8-1WBS 2.3

08月 09月

– 1.1 Objective Lens

– 1.2 Two Fold Mirrors

– 1.3 Focus Adjustment

– 1.4 K-mirrors System

– 1.5 Mechanical Flexure Compensator

– 1.6 Assembly Mounting

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Samples of our fabrication

a folding mirror surface test result d=23mmPV = 24.2nm(0.041λ)with a goal of 0.03λ is reliable

a folding mirror surface test result d=70mmPV = 29.2nm(0.05λ)with a goal of 0.03λ is reliable

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Samples of our fabrication

FSM surface test result d=80mmPV = 40.2nm(0.068λ)with a goal of 0.04λat about 30mm diameter mirror is reliable

Type

Diameter

/mm

Stroke Resonance

frequency

1 20mm ±4′ 1000Hz

2 30mm ±3.6′ 200Hz

3 50mm ±1.5′ 930Hz

4 78mm ±2.5′ 300Hz

5 60mm ±20′ 260Hz

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The working plan for LGSF

• A cost/performance trade study to increase the LTA field of view from 5 to 17 arc minutes

• An update to the existing cost estimate for the current LGSF design

• Update the conceptual design (and its cost estimate)

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Thank you!

The TMT L aser G uide S tar F acility ( LGSF )

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