TMT.SEN.PRE REL01 Development and validation of vibration source requirements for TMT to ensure AO performance Hugh Thompson and Doug MacMartin AO4ELT3 Conference, Florence, Italy May 2013

TMT.SEN.PRE REL01 Presentation Outline

TMT.SEN.PRE REL01 3 Rough scale of the problem Many 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 Kulcsár ; Gaetano Sivo ; Henri-François Raynaud ; Benoît Neichel ; Fran ҫ ois 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 REL01 On Axis WFE Delivered wavefront187 First order turbulence compensation 117 LGS control loop 117 DM fitting error 75 DM projection error 46 LGS WFS aliasing error 42 Tomography error 30 Servo lag 4 LGS WFS non-linearity 19 LGS WFS noise 46 TMT pupil function 27 Opto-mechanical implementation 71 Telescope pupil misregistration 12 Telescope and observatory OPD 37 M1 static shape 26 M2 & M3 static shape 11 Segment dynamic mis-alignment 14 Dome seeing 16 Mirror seing 14 Field dependent astigmatism 0 NFIRAOS 51 Residual instrument 30 AO compomnents errors & higher order effects 66 DM effects 49 LGS WFS & Na layer 39 Control algorithm 21 Simulation undersampling 48 NGS Mode WFE at 50% sky coverage 58 Residual tip/tilt jitter due to windshake 16 Residual telescope vibration 10 Residual telescope tracking jitter 17 Residual tip/tilt jitter due to turbulence 32 Residual plate scale mode due to turbulence 35 Residual plate scale mode due to windshake 5 Field dependent wavefront error 20 Contingency 80 How do we flow AO requirements down? Segment dynamic displacement (due to vibration) 10nm Telescope image jitter (due to vibration) 10nm equivalent to mas Pump impeller Balance Grade 6.3 ?

TMT.SEN.PRE REL01 5 The questions in more detail What 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 REL01 6 Finite Element Model FEM of telescope structure includes nodes for each M1 segment, M2, M3 and each instrument Optical sensitivity combined with nodal motions from FEM determines performance effects due to: –image jitter –M1 segment motion

TMT.SEN.PRE REL01 Additional model details AO rejection curves included (median conditions) –15 Hz Type II controller for tip/tilt –63 Hz DM bandwidth –No additional narrowband rejection Frequency-resolved calculations are smoothed –Reasonable estimate of rms performance, not worst case Using 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 masses –wrong above ~12 Hz 7

TMT.SEN.PRE REL01 Modelling Goals Determine allowable vibration source amplitudes Assess: –Relative influence of location of sources –Main contributors to image jitter (M1, M2, M3, focal plane) –Sensitivity to source input frequency 8

TMT.SEN.PRE REL01 Modelled Sources “Unit” forces are input at 6 locations –Pier Also covers sources in facility building with an additional factor to account for attenuation through soil –Instruments (NFIRAOS, MIRES) on Nasmyth platforms –Laser Service Enclosure (LSE) –Cable wraps (Az and El) 9

TMT.SEN.PRE REL01 After smoothing, after AO rejection Results combining M1 and image motion 10 Pier forcingNFIRAOS forcing In both cases image motion is dominant above 10 Hz

TMT.SEN.PRE DRF01 Check spatial correctability on M1 11 M1 response at 30 Hz AO spatial correctability is good; correction is dominated by temporal bandwidth nm/N

TMT.SEN.PRE DRF01 Combined M1 and image motion for all sources 12 AO rejection Mass effect Telescope Pier 10x

TMT.SEN.PRE REL01 Model Results Summary All modeled telescope sources are roughly comparable in effect –Pier forcing a factor of 10 less impact –Locations in facility building likely reduce sources by an additional factor of 5-10 relative to pier Performance most sensitive to forces 5-20 Hz M1 soft actuators reduce M1 response at 30 Hz by factor of 10 Motion of M2 largest contributor to image motion above 10 Hz Residual dominated by image motion, not M1 above 10 Hz –Means that feed-forward of M2 motion may be effective –Narrowband rejection of tones may also help Internal flexibility of instruments not accounted for 13

TMT.SEN.PRE REL01 Compare actual sensitivity with fit to shaping filter for each source Filter W(f): –f 1 =5 Hz –f 2 =20 Hz 14

TMT.SEN.PRE REL01 Vibration Budget Sensitivity (nm per N) Fraction of budget Allowable force (N) Pier0.4335%20 Instruments3.750%3 LSE1.95%2 Cable wraps1.35% each2.5 each 15 Specification on rms force after filtering by shaping filter (allows higher vibration at low or high frequency)

TMT.SEN.PRE REL01 16 Source example Forces ~1N at 1- 2 Hz Frequency is low but higher harmonics can be problematic Large numbers required for TMT has led us to turbine expander cooling with no low- frequency reciprocating motion ESO study of cryocoolers: “Low-vibration high-cooling power 2-stage cryocoolers for ground-based astronomical instrumentation” Gerd Jakob, Jean-Louis Lizon Proc. SPIE. 7733, Ground-based and Airborne Telescopes III 77333V (July 16, 2010)

TMT.SEN.PRE REL01 17 Source example in the summit facilities 4-pole induction motors on 60 Hz AC generates ~29 Hz but newer VFD equipment moves frequencies with system demand –Do 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 1 –Results 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 required

TMT.SEN.PRE REL01 Pipe vibration Konstantinos Vogiatzis has made some initial models of turbulent flow in coolant pipes Forces are low in straight runs, but elbows produce significant broad-band forces 18 TMT is considering replacing water-glycol with phase-change refrigerant to reduce coolant mass flow (and forces) by a factor of 10

TMT.SEN.PRE REL01 Impact of increasing the error budget allocation to vibration An increase from 14 nm to 30 nm would not dramatically reduce observing efficiency –Roughly 3% impact in J band

TMT.SEN.PRE REL01 Things to do 20 On-going work needed to: –Develop the allowable vibration source budget allocated to subsystems –Improve estimate of propagation through soil (for enclosure and summit facility sources) –Improve all source estimates –Hopefully through force measurements made at a telescope near you!

TMT.SEN.PRE REL01 Conclusions 21 Vibration sources on the telescope must be limited to a few Newtons Vibration sources in the facility must be limited to a few hundred Newtons Possibly need to increase AO error budget allocation to vibration Further mitigation may be possible via –M2 feed-forward –Narrow-band rejection algorithms Conventional cryocoolers are not acceptable for TMT Keep summit facility source frequencies at 60 Hz when possible –Reduced sensitivities –Allows effective use of ~ 5 Hz isolators

TMT.SEN.PRE REL01 Acknowledgements 22 The TMT Project gratefully acknowledges the support of the TMT partner institutions –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 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.

TMT.SEN.PRE REL01 Mass helps –TMT dome = 2300 tons –Brunellesci’s dome = tons –The Duomo likely doesn’t have a vibration problem! You can build large structures without vibration problems