challenges in large ground based telescopes: tmt · 1 challenges in large ground based telescopes:...
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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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