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 Challenges in Large Ground Based telescopes: TMT

Mitchell Troya

Carl Nisslya, Joon Seoa aJet Propulsion Laboratory, California Institute of Technology,

Workshop on Technology for Direct Detection and Characterization of Exoplanets Keck Institute for Space Science (KISS) on the Caltech Campus

April 9-12, 2018

© 2018 California Institute of Technology. Government sponsorship acknowledged.

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Outline

•  OverviewofTMT•  Programma0cChallenges•  TechnicalChallenges

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Introduc0ontotheTMTDesign

TMTisasegmentedmirror op0cal-infraredtelescope witha30mfilledaperture

  TMTisaninterna0onalcollabora0veeffortbetweenCanada,China,India,Japan,US,andtheCaltechandUCastronomycommuni0es

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TMT Telescope Concept Overview

Pathoflightthroughtheaperture

ScienceInstrumentsMountedonNasmythPlaIorms

MountStructure

Flat2.5mx3.5mTer0aryMirror(M3)

30mHyperboloidalf/1PrimaryMirror(M1)

3.1mConvexHyperboloidalSecondaryMirror(M2)

Ritchey-Chré0enOp0calDesign

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TMT Primary Mirror (M1)

  492segments  1.44macrosscorners  3.5mmop0calgapsbetweensegments  1,473DegreesofRigidBodyFreedom  21warpingharness’spersegment,totalof8,856Dof.

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Timeline for Science Requirements and Instrument Selection

  ~2000:CaliforniaExtremelyLargeTelescope(CELT)Studystarted  2004:TMTReferencedesignestablished  ~2005:ScienceRequirementsDocument(SRD)released  2006:Instrumentfeasibilitystudies  2007:Last“significant”updatetoSRD  2008:Firstgenera0on/lightinstrumentsselected  2019:2ndgenera0oninstrumentstudies  ~2028:Firstlight  ~2030:Scienceopera0onsstart

  >25yearsbetweenfirstlightandini0alsciencerequirements/referencedesign

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Programmatic: Similarities to Space

  TMTandotherELTsarelargeprojectsapproachingorexceedingspacebasedprojectsintermsof: Cost$1-2Bdollar Complexity  Interna0onalinvolvement/collaboratorsandtheassociatedcomplexi0es Timelines

 TMT~25-30years  JWST~25years

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Programmatic: Differences from Space

  ELTprojectsaresignificantlymoreexpensiveandcomplexthenpreviousgroundbasedprojects Notusedtoformalsystemengineering Mul0plesciencegoalsthatcoverawiderange:

 Seeinglimited,diffrac0onlimited,high-contrast 0.3to~30microns.Rangeof100 FoV:~1arcsecto~15arcmin.Arangeof~1000

  Telescopedesignisnotop0mizedforhigh-contrastimagingorplanetdetec0on

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Planet Detection Requirements

  Exoplanetdetec0onfromtheground: Certainlyseenasnotachievable(evenwithELTs)whensciencerequirementswerefirstdeveloped

 S0llseenasmanyasnotachievable

 Nichescience Result:Requirementsdevelopmentandanalysisdoesnotreflectahighpriorityonexoplanetdetec0on Requirementsweresetinearlyphaseofproject

 Verylillewecandowillchangethedesign,requirementsandperformanceofthetelescopeintermsofexoplanetdetec0on

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TMT Science Contrast Requirements

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Technical Challenges

  Pupiland/orfieldrota0on Reflec0vityvaria0onsfromop0cs Obscura0onnotop0mizedforhigh-contrastimaging Op0calWavefrontErrors

 Alignment(segment0p/0lt/piston) Residualsegmentfigure Thermal Gravity Segmentedges

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Reflectivity variations from optics

  TheSRDspecifiesthattheM1segmentreflec0vity'sshouldbebelerthen99%atwavelengthslongerthen1.5microns  Thebaselinesegmentreplacescenariois~10segmentsevery2weeks.  Thisimpliesanaveragesegmentwillberecoatedevery~1year

  Ameansegmentreflec0vityof99%witha1%varia0onresultsinacontrastof~1.3E-7from3to10λ/D  Thisisasignificanterrortermaslargeastheimpactfromphaseerrors

  Solu0onswillberequired.Themostlikelyseemstobetousemul0pledeformablemirrorstocorrecttheamplitudeandphaseerrors

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Obscuration Not Optimized for High-Contrast Imaging

  3.6mcentralobscura0on  0.225mM2supportlegs

  M2supportswill“segment”thepupilwhenusingExtremeAO  Thiswilllikelyintroducingwavefrontreconstruc0onerrors

  AM2gapsbetweensegmentsare~0.016m  M1effec0vegaps3.5mm

  Crea0vediffrac0onsuppressionsystemsrequired

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M1 Residual Figure Errors Post 1202 AO control

 ResidualM1FiguringErrorisdominateerrorterm Gravityerrorsfromsegmentsupport(PSaxialandPSlateral)aresignificantatlargerzenithangles

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M1 Residual Figure Errors: Phase Maps Post 602 AO Control

  ~17nmRMSOPD 1stgenera0onAO(NFIRAOS)doesnotsignificantlyimproveerrors

  1202AOreduceserrorsto~12nmRMSOPD  Edgeeffectsfromcontrolaresignificant

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Contrast From All M1 Phase Errors (Results from 2006 TMT PFI Study)

  Phase errors are dominated by residual segment aberrations   Contrast is:

–  1.4 x 10-7 at 3λ/D –  5.6 X 10-8 from 3 to 10

to λ/D

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Segment Edge Artifacts (From a working telescope)

  Kecksegmentsappeartosufferfromsmallbutsignificantsurfacear0factsneartheedges(60-100mm)that:

  Placelimitsonphasingaccuracybycrea0ngachroma0ceffects  Directlyimpactimagequalityduetolightdiffractedatangleslargerthan±3.5arcsecondsfromtheedges.

  TheseeffectsarelikelycausedbyIBFresidualswithaspa0alperiodof1-3cmand10-20nmamplitude.

  MeasurementsoftheKecksegmentswithaninterferometerhaverecentlybeenexecutedbyTMTandweareintheprocessofanalyzingthedata

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Scattered Light From Edges Single Segment

  Imagesarediffrac0onpalernsformedbylightfromsinglesegmentspassingthroughthephasingcameraop0cswiththephasingmask  Onthelesagoodsegmentandontherightoneoftheworstsegments(SP14/SN09).

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Scattered Light From Edges A Systematic Evaluation

  Photometryfromasegmentedgeovera6cmsemi-circlecanbemeasuredusingtheabovesubaperturemaskand0l0ngsegmentsoutofthestack  Thetworedcircleshighlightsubaperturesonsegments(SP)20and36thatclearlyhavelowerfluxthanthose(circledinwhite)onSP6and15

subaperturesare12cmindiameter

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Scattered Light From Edges A Systematic Evaluation

  25%ofsegmentshaveedgeswithasignificantreduc0on(>20%)inintensitywithin±3.5arcseconds

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Preliminary Results From Measurements of Keck segments with an Interferometer

  ThepredictedTMTresidualAO(120CL)M1surfaceerrorsare6nmRMSsurface  TheproposedTMTrequirementforthesespa0alfrequenciesis5nmRMSsurface  Ar0factsfromIBFsupportpadsareexcludedfromtheRMSsurfaceerrorcalcula0ons

  RMSsurfaceerrorsoverthe15cminterferometricphasemeasurement  Zernikeorders1and2removed

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Segment Edge Summary

  StressMirrorPolishing(SMP)wasdesignedtoNOTintroduceedgeeffects  IonBeamPolishing(IBF)postSMPhowever,canintroduceedgeeffectsatthese1-3cmspa0alfrequencies

 Othermirrorpolishingtechniquessuchasthoseusedforsegmentsforspacetelescopeswillalsolikelyintroduceedgeeffects  IftheTMTsegmentsaresimilartotheKecksegmentsitwouldreducetheH-bandStrehlby~5%andhaveasignificantimpactoncontrast

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Summary and Conclusions Related to Planet Detections

  Therearemanysimilari0esinthechallengesgroundandspacetelescopesface

  AtTMTitwillbedifficulttochangethetelescoperequirementsbasedonthoseforplanetdetec0on

  Instrument/ScienceteamsneedtoworkwithTMTtounderstandhowthetelescopedesignwillimpactperformance  Thespecificscienceinstrumentdesigns(wavelength,diffrac0onsystem)andsciencecaseneedtobeevaluated

  TheTMTPFIstudyshowedthat  Thetelescopealignmenterrorsarenotasignificantsourceoferror  Residualsegmentaberra0onsareasignificantconcern

  Segment“edge”effectsneedtobeunderstoodandevaluated

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Acknowledgements

This research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, and was sponsored by the California Institute of Technology and the National Aeronautics and Space Administration. The TMT Project gratefully acknowledges the support of the TMT collaborating institutions. They are the Association of Canadian Universities for Research in Astronomy (ACURA), the California Institute of Technology, the University of California, the National Astronomical Observatory of Japan, the National Astronomical Observatories of China and their consortium partners, and the Department of Science and Technology of India and their supported institutes. 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.

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Backups

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TMT Science Contrast Requirements “Achievable contrast with coronagraph”

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1st Generation TMT Instruments

  IRIS-InfraRedImagingSpectrometer  IRMS-InfraRedMul0-SlitSpectrometer(MOSFIRE-TMT) WFOS-Wide-FieldOp0calSpectrometer

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Wavefront Error Table

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2006 Feasibility Design study for a “Planet Formation Instrument for TMT”

  Inves0gatedtheimpactoftelescopeaberra0onsoncontrast  Relevantconclusionsfromthatstudy:

–  Thetelescopewillnotlimitcontrastatthe10-8level–  Therela0velysmallsegmentgapsdonotlimitcontrast,butthelarger

obscura0onsfromM2andit’ssupportsarechallenging–  Segment-to-segmentreflec0vityvaria0onsareanissue

 Willrequireamplitudecontrolusinga2ndDM–  Segmentphasingandtelescopealignmentingeneralisnotadriverinthe

performance 5sigmacontrastat3λ/D:~2*10-8

–  Residualsegmentaberra0onsareakeydriverintheperformance 5sigmacontrastat3λ/D:~2*10-7

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Segment Aberrations Before and After AO

 RMS: 17.3 nm  P-V: 242 nm

 RMS: 9.1 nm  P-V: 199 nm

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Contrast From Segment Alignment Errors

  Segment piston and residual tip/tilt errors are about equal in magnitude

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Various Segment Aberrations

RMS:40nmP-V:352nm

RMS: 26 nm P-V: 272 nm

RMS: 11 nm P-V: 153 nm

RMS: 17 nm P-V: 242 nm

RMS: 4 nm P-V: 38 nm

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AO Corrected Segment Aberrations

RMS:21nmP-V:354nm

RMS: 14 nm P-V: 257 nm

RMS: 6 nm P-V: 126 nm

RMS: 9 nm P-V: 199 nm

RMS: 2 nm P-V: 36 nm

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Contrast Versus Segment Aberrations Assumptions

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