the tmt laser guide star facilityao4elt.lesia.obspm.fr/sites/ao4elt/img/pdf/boyer.pdf · adjustable...

TMT.AOS.PRE.09.025.REL01AO4ELT - Paris, June 24, 2009

1

The TMT Laser Guide Star FacilityCorinne Boyer1, Brent Ellerbroek1, Luc Gilles1

1 TMT Observatory Corporation

AO4ELT - Paris, June 24, 2009


2

Presentation Outline

Laser Guide Star Facility (LGSF) requirements– Technology/design choices– Derived LGSF design approach– Requirement trade studies– Summary of main LGSF and Laser System requirements

LGSF design status


3

TMT High Level ScienceRequirements

TMT science goals require:– To achieve near diffraction-limited performance in the near- and

mid-IR, including:High sky coverage, which implies Laser Guide Star (LGS)AO for higher-order wavefront correctionDiffraction-limited wavefront compensation over large TMTaperture, which requires multiple sodium LGS to defeat thecone effect

– To maximize science utility at first light. This implies:To design the LGSF as an integral part of the TMTobservatoryTo select an approach with low risk and reasonable cost


4

LGSF Asterisms Requirements

Generate LGS asterismsrequired by TMT first light andnext generation AO systems:– Multi Conjugate AO (black): 1

LGS on-axis and 5 at a 35arcsec radius

– Mid IR AO asterism (red): 3LGS at a 70 arcsec radius

– Multi Object AO (blue): 3LGS at a 70 arcsec radiusand 5 at a 150 arcsec radius

– Ground Layer AO (green): 1LGS on-axis and 4 LGS at a510 arcsec radius

– Additional asterism if neededwith up to 9 LGS and radiusvarying from 5 to 510 arcsec


5

Technology/Design Choices

Technology choices:– Solid state lasers, with either Continuous Wave (CW) or mode locked

CW pulse format:Sum Frequency Generation (SFG) currently demonstrated at 50Wpower levelOther laser technologies (µs-pulsed lasers with dynamic refocusing,ms-pulsed lasers) not currently available at or near TMT power level

– Conventional optics for beam transport from lasers to launch telescopeHollow core fiber transport currently not an option for TMT pathlength and TMT peak power requirements

Design choices:– Laser beams projection from behind M2 to minimize LGS elongation.– Laser system installed within telescope azimuth structure to provide a

fixed gravity orientation.


6

Conservative LGSF DesignApproach

LGSF consists of 3 mainsystems:–Laser System within

telescope azimuth structure.–BTO/LLT System, to

transport beams to thetelescope top end andproject them from the LLT.

–Laser Safety System forprotection of people,observatory hardware,aircraft and neighboringtelescopes.

Design based uponexisting LGSF systems

Lasers


7

Requirement Trade Studies

Three requirement trade studies:– LGS power requirement– Laser Launch Telescope (LLT) aperture diameter– Budget for LGS pointing jitter on the sky


8

LGS Power Requirement

Derived from LGS WFS signal requirement of 2.8 106 PDEs/m2/sec– Corresponds to 900 PDEs/subap/frame at 800 Hz; WFE of 43 nm RMS– Derived from simulations including LGS elongation, WFS spot size, RTC

processing algorithms for gradient estimation and tomographyRequired LGS power then depends upon assumptions for– Sodium column density (4x1013 atoms/m2)– Laser/sodium coupling coefficient (130 photons m2/s/W/atom)– Uplink/downlink optical throughput and atmospheric transmittance

Baseline parameter assumptions yield a requirement of 25W per LGS– Photon return will vary with conditions, but factor of two reduction is

acceptable with a tolerable WFEDynamic refocusing with µs-pulsed lasers would reduce laser powerrequirements by a factor of 2-3 by eliminating LGS elongation:– Highly desirable for upgraded AO systems with 0.25x0.25m sub-

apertures (Power requirement otherwise increases by factor of 5-6)


9

LGS Return Signal: spread betweennominal and pessimistic cases

Csspecified

Csreduced

Csspecified

Csreduced

45089.4

0

0.00125

0.8

0.750.84

0.42

6525

4x1013

0.25

Nominal Throughput

48091.8

0

0.00125

0.8

0.750.70

0.34

13025

4x1013

0.25

24091.8

0

0.00125

0.8

0.750.7

0.34

6525

4x1013

0.25

PessimisticThroughput

90089.4

0

0.00125

0.8

0.750.84

0.42

13025

4x1013

0.25

Sodium layer range, kmz

End-to-end LGSF transmittanceTLGSF

Photodetection events per subaperture perframe

Npde

Zenith angle, degreesθ

WFS integration time, secτ

Equivalent transmittance due to imperfectbeam quality

TBQ

One-way atmospheric transmittance at zenithTATM

End-to-end telescope and AO systemefficiency, photoelectrons/photon

TAOS

Unsaturated coupling efficiencyPhotons*m2/s/W/ion

s

Average laser power, WPL

Column Density, ions/m2Cs

WFS subaperture area, m2A

DefinitionVariable


10

Impact of Reduced LGS WFS SignalLevel on AO Performance

LGS WFS noise: 43 nm RMS WFE at 900 PDEPerformance optimized by minimizing the combined servo lag and noiseerrors

7540091250

4360047500

--8000 (baseline)900

Incremental RMSWFE at optimalframe rate, nm

Roughly optimalAO frame rate, Hz

Incremental RMSWFE at 800 Hz, nm

Signal level at800 Hz, PDEs


11

Laser Launch Telescope ApertureDiameter and Beam Profile

Initial trade study of these parameters has been performed toquantify their impact upon laser power requirementsGaussian laser beam profile with a 1/e2 beam diameter 0.6 x LLTdiameter preferable to top-hat beamLaser power requirements are reduced only modestly when LLTdiameter is increased above 0.5m:– Less than 5% reduction for 1m diameter with elongated LGS– Less than 10% reduction for 1m diameter with dynamic refocusing

Baseline TMT LLT diameter is 0.5m– More studies planned to determine final value

Adjustable LLT focus set to the range of sodium layer provides asmall (<3%) improvement in LGS spot size:– Included in design, since it will aid installation and commissioning


12

LGS Pointing Jitter

Budget for LGS tip/tilt jitter is 50 mas (1-axis, 1-sigma) on the sky, based on:– Expected linear dynamic range of LGS WFS matched filter algorithm– And LLT diameter

Budget is dominated by atmospheric tip/tilt servo lag, not windshakeLGS pointing will be controlled by a Fast Steering Mirror (FSM) to null theerror as measured at the LGS WFS. Two locations have been studied:– Within the LGSF optical path– Just before the LGS WFS in each AO system– Bandwidth is limited by sampling rate of the LGS WFS in both cases and by

round-trip time-of-flight to sodium layer in the LGSF case.Correction is more efficient when the FSM is located within the AO system,but the implementation within LGSF is less expensiveBaseline within the LGSF

1871FSM in AO system2946FSM in LGSF

Tip/tilt residual jitter, mas RMS-3dB rejection BW, Hz


13

High observationefficiency

Rapid switching from one asterism to anotherAsterismsBlind pointing accuracy on the sky of ~1 arcsecLGS Pointing

IRMOS andNFIRAOS upgrade

MCAO wavefronterror budgetAllow to generateMIRAO and GLAOasterisms

Sciencerequirements

- Downtime: < 0.5%- Time to startup, shutdown

LGSF Operation

75%LGSF Throughput98% circularly polarizedLGS Polarization

- Additional lasers for IRMOS/MOAO asterism- Increased power for NFIRAOS upgrade

Laser SystemUpgrade

RequirementParameter

Tip/Tilt jitter < 50 mas (1-axis, 1-sigma)LGS Tip/Tilt Jitter

85% of energy in a 1.2 x diffraction-limited coreLGS Beam Quality25W per LGS (150W in 6 LGS)LGS Power

LGSF Requirements Summary


14

- Downtime: < 0.85% (Yields ~1400 hours MTBF)- Time to startup, shutdown

Laser OperationHouse 9 lasers in TMT Laser System location (~12x4.8x2.6m)Volume

5mm ± 0.3mmPointing ≤ 25µrad and lateral shift ≤ 0.5mm

Output Beam Diameter

Tune on/off D2 lineWavelength Control

3% [15%] short [long] termPower Stability> 98% linearly polarizedPolarization

RequirementParameter

95% of energy in a 1.1 x diffraction-limited coreBeam Quality

130 photons-m2/s/W/ionSodium Layer Coupling

6 and 1 spare x 25W CW or mode locked CW solid state lasersfor first light AO system

Laser Power

Laser Requirements Summary


15

Conservative LGSF DesignApproach

LGSF consists of 3 mainsystems:–Laser System within

telescope azimuth structure.–BTO/LLT System, to

transport beams to thetelescope top end andproject them from the LLT.

–Laser Safety System forprotection of people,observatory hardware,aircraft and neighboringtelescopes.

Design based uponexisting LGSF systems

Lasers


16

LGSF Design Development

2006: Conceptual Design– NOAO study in collaboration with TMT AO group

2008: Top End redesign– NOAO study– Purpose: to incorporate telescope flexure compensation, and

adapt design for the new TMT Ritchey-Chrétien telescopeoptics

2008-2009: Laser System redesign– TMT AO group– Re-location to telescope azimuth structure– 3 x 50W lasers replaced with 6 x 25W lasers plus one spare for

first light (Upgrade system: 9 x 25W lasers)– Motivation: Provide lasers a fixed gravity orientation, and

reduce laser development risk


17

LGSF Optical Layout

Laser System:initially 6 25W lasers+ 1 spare, upgradableto 9 25 W lasers

BTO Optical Path-Azimuth optical path-Deployable optical path-Elevation optical path

LGSF Top End- Diagnostic bench- Asterism Generator- LLT


18

LGSF Optical Path

3-stage optical path from lasersystem to telescope top end:

– Azimuth optical pathBeams arranged in compact 3x3pattern

– Deployable/Retractable optical pathLocated along elevation axisRetractable to allow observationwith on axis instrumentsRotates to allow the telescope tomove in elevation

– Elevation optical path3 relay lenses

Total 78 m length and 8 mirrors perbeam (5 folds, 3 actuated)


19

LGSF Top End

Diagnostic optical benchmonitors beam alignment, powerand qualityAsterism generator maps 3x3pattern of beams into desiredasterismRotating K mirror maintains fixedasterism at the LGS wavefrontsensor focal plane0.5 m Laser launch telescopeprojects the beams onto sky

– Off axis reflective telescope– Pivot mount for telescope top

end flexure compensationTotal: 11 mirrors per beam (4actuated for pointing and fasttip/tilt)

K Mirror

AsterismGenerator

LLT

DiagnosticsOptical Bench


20

LGSF Conclusion

Mature conceptual design, which meets the TMT sciencerequirements:– Conservative and low risk approach based on demonstrated

technologies:Solid state CW lasersConventional optics for the beam transport

– Based upon similar concepts already successfully implemented atseveral observatories

– System complexity manageable with distributed control architectureNeed to develop the design of the deployable/retractable opticalpath and/or study alternative approaches.Preliminary design phase will start with the TMT construction phasein October 2011:– Schedule allows for the review of the current architecture and design as

technologies advance


21

The authors gratefully acknowledge the support of the TMT partner institutionsThey are:

– the Association of Canadian Universities for Research in Astronomy (ACURA)– the California Institute of Technology– and the University of California

This work was supported as well by:– the Gordon and Betty Moore Foundation– the Canada Foundation for Innovation– the Ontario Ministry of Research and Innovation– the National Research Council of Canada– the Natural Sciences and Engineering Research Council of Canada– the British Columbia Knowledge Development Fund– the Association of Universities for Research in Astronomy (AURA)– and the U.S. National Science Foundation.

Acknowledgements

the tmt laser guide star facilityao4elt.lesia.obspm.fr/sites/ao4elt/img/pdf/boyer.pdf · adjustable...

Documents