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Development and validation of vibration source requirements for TMT to ensure AO performanceHugh Thompson and Doug MacMartin

AO4ELT3 Conference, Florence, Italy26-31 May 2013TMT.SEN.PRE.13.040.REL01Presentation OutlineTMT.SEN.PRE.13.040.REL0123Rough scale of the problemMany current AO systems are limited by vibration

ALTAIR on Gemini sees vibration of ~10 mas rms after correction

Survey of similar problems at several telescopes:Caroline Kulcsr ; Gaetano Sivo ; Henri-Franois Raynaud ; Benot Neichel ; Franois Rigaut, et al."Vibrations in AO control: a short analysis of on-sky data around the world", Proc. SPIE 8447, Adaptive Optics Systems III, 84471C (September 13, 2012)

For TMT the entire on-axis NFIRAOS budgeted wavefront error of 187 nm corresponds to only ~ 5 mas of tip/tilt

TMT.SEN.PRE.13.040.REL013On Axis WFEDelivered wavefront187First order turbulence compensation117LGS control loop117DM fitting error75DM projection error46LGS WFS aliasing error42Tomography error30Servo lag4LGS WFS non-linearity19LGS WFS noise46TMT pupil function27Opto-mechanical implementation71Telescope pupil misregistration12Telescope and observatory OPD37M1 static shape26M2 & M3 static shape11Segment dynamic mis-alignment14Dome seeing16Mirror seing14Field dependent astigmatism0NFIRAOS51Residual instrument30AO compomnents errors & higher order effects66DM effects49LGS WFS & Na layer39Control algorithm21Simulation undersampling48NGS Mode WFE at 50% sky coverage58Residual tip/tilt jitter due to windshake16Residual telescope vibration10Residual telescope tracking jitter17Residual tip/tilt jitter due to turbulence32Residual plate scale mode due to turbulence35Residual plate scale mode due to windshake5Field dependent wavefront error20Contingency80How do we flow AO requirements down?

Segment dynamic displacement(due to vibration) 10nmTelescope image jitter (due to vibration) 10nm equivalent to 0.275 masPump impeller Balance Grade 6.3?TMT.SEN.PRE.13.040.REL015The questions in more detailWhat is the sensitivity of image quality to vibration?How does this vary with amplitude, frequency and location?What are the worst expected sources of vibration with respect to these sensitivities?What can be done to mitigate them?Do we need to increase AO error budget allocation to vibration?What standards/requirements do we have/will we develop to maintain acceptable vibration levels?How will we assess and verify vibration performance against predictions?

TMT.SEN.PRE.13.040.REL0156Finite Element ModelFEM of telescope structure includes nodes for each M1 segment, M2, M3 and each instrumentOptical sensitivity combined with nodal motions from FEM determines performance effects due to:image jitter M1 segment motion

TMT.SEN.PRE.13.040.REL016Additional model detailsAO rejection curves included (median conditions)15 Hz Type II controller for tip/tilt63 Hz DM bandwidthNo additional narrowband rejectionFrequency-resolved calculations are smoothedReasonable estimate of rms performance, not worst caseUsing simple ground transmission estimates (no soil and pier model)No direct transmission path measurements for comparison (either soil or on telescopes)Instruments modeled as lumped masseswrong above ~12 Hz7TMT.SEN.PRE.13.040.REL01Modelling GoalsDetermine allowable vibration source amplitudes

Assess:Relative influence of location of sourcesMain contributors to image jitter (M1, M2, M3, focal plane)Sensitivity to source input frequency8TMT.SEN.PRE.13.040.REL01Modelled SourcesUnit forces are input at 6 locationsPierAlso covers sources in facility building with an additional factor to account for attenuation through soilInstruments (NFIRAOS, MIRES) on Nasmyth platformsLaser Service Enclosure (LSE)Cable wraps (Az and El)

9TMT.SEN.PRE.13.040.REL01After smoothing, after AO rejection

Results combining M1 and image motion10Pier forcingNFIRAOS forcingIn both cases image motion is dominant above 10 HzTMT.SEN.PRE.13.040.REL01

Check spatial correctability on M111M1 response at 30 Hz

AO spatial correctability isgood; correction isdominated by temporal bandwidth

nm/NTMT.SEN.PRE.13.025.DRF01Combined M1 and image motion for all sources12

AO rejectionMass effectTelescopePier10xTMT.SEN.PRE.13.025.DRF01Model Results SummaryAll modeled telescope sources are roughly comparable in effectPier forcing a factor of 10 less impactLocations in facility building likely reduce sources by an additional factor of 5-10 relative to pierPerformance most sensitive to forces 5-20 HzM1 soft actuators reduce M1 response at 30 Hz by factor of 10Motion of M2 largest contributor to image motion above 10 HzResidual dominated by image motion, not M1 above 10 HzMeans that feed-forward of M2 motion may be effectiveNarrowband rejection of tones may also helpInternal flexibility of instruments not accounted for13TMT.SEN.PRE.13.040.REL01Compare actual sensitivity with fit to shaping filter for each sourceFilter W(f):f1=5 Hzf2=20 Hz14

TMT.SEN.PRE.13.040.REL01Vibration BudgetSensitivity (nm per N)Fraction of budgetAllowable force (N)Pier0.4335%20Instruments3.750%3LSE1.95%2Cable wraps1.35% each2.5 each15Specification on rms force after filtering by shaping filter (allows higher vibration at low or high frequency)TMT.SEN.PRE.13.040.REL0116Source exampleForces ~1N at 1- 2 Hz Frequency is low but higher harmonics can be problematicLarge numbers required for TMT has led us to turbine expander cooling with no low-frequency reciprocating motionESO study of cryocoolers:Low-vibration high-cooling power 2-stage cryocoolers for ground-based astronomical instrumentationGerd Jakob, Jean-Louis LizonProc. SPIE. 7733, Ground-based and Airborne Telescopes III 77333V (July 16, 2010)

TMT.SEN.PRE.13.040.REL011617Source examplein the summit facilities4-pole induction motors on 60 Hz AC generates ~29 Hz but newer VFD equipment moves frequencies with system demandDo we want this?Need tight imbalance requirements and single or multi-stage isolation

Large fluid cooler used to exhaust all TMT waste heat has 8 fans of Balance Quality Grade 1Results in 10 N of force per rotor or worst-case in-phase imbalance of all 8 rotors equal to 80 N

At 59 Hz even 1 kN should be acceptable but careful tracking of all equipment is requiredTMT.SEN.PRE.13.040.REL0117Pipe vibrationKonstantinos Vogiatzis has made some initial models of turbulent flow in coolant pipesForces are low in straight runs, but elbows produce significant broad-band forces18

TMT is considering replacing water-glycol with phase-change refrigerant to reduce coolant mass flow (and forces) by a factor of 10TMT.SEN.PRE.13.040.REL01Impact of increasing the error budget allocation to vibration

An increase from 14 nm to 30 nm would not dramatically reduce observing efficiencyRoughly 3% impact in J band

TMT.SEN.PRE.13.040.REL01Things to do20On-going work needed to:Develop the allowable vibration source budget allocated to subsystemsImprove estimate of propagation through soil (for enclosure and summit facility sources)Improve all source estimatesHopefully through force measurements made at a telescope near you!

TMT.SEN.PRE.13.040.REL01Conclusions21Vibration sources on the telescope must be limited to a few NewtonsVibration sources in the facility must be limited to a few hundred NewtonsPossibly need to increase AO error budget allocation to vibrationFurther mitigation may be possible viaM2 feed-forwardNarrow-band rejection algorithmsConventional cryocoolers are not acceptable for TMTKeep summit facility source frequencies at 60 Hz when possibleReduced sensitivitiesAllows effective use of ~ 5 Hz isolators

TMT.SEN.PRE.13.040.REL01Acknowledgements22The TMT Project gratefully acknowledges the support of the TMT partner institutionsthe Association of Canadian Universities for Research in Astronomy (ACURA),the California Institute of Technologythe University of Californiathe National Astronomical Observatory of Japanthe National Astronomical Observatories and their consortium partnersAnd the Department of Science and Technology of India and their supported institutes.This work was supported as well bythe Gordon and Betty Moore Foundationthe Canada Foundation for Innovationthe Ontario Ministry of Research and Innovationthe National Research Council of Canadathe Natural Sciences and Engineering Research Council of Canadathe British Columbia Knowledge Development Fundthe Association of Universities for Research in Astronomy (AURA)and the U.S. National Science Foundation.TMT.SEN.PRE.13.040.REL01

Mass helpsTMT dome = 2300 tonsBrunellescis dome = 37000 tonsThe Duomo likely doesnt have a vibration problem!

You can build large structures without vibration problemsTMT.SEN.PRE.13.040.REL01