Development and validation of vibration source requirements for TMT to ensure AO performance

1. Development and validation of vibration source requirements for TMT to ensure AO performance Hugh Thompson and Doug MacMartin AO4ELT3 Conference, Florence, Italy26-31 May 2013

2. Presentation Outline • TMT AO error budget for vibration • Sensitivity of TMT structure to vibration • Examples of observatory vibration sources • Are

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 ; BenoîtNeichel ; FranҫoisRigaut, 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

4. How do we flow AO requirements down? Segment dynamic displacement (due to vibration) 10nm Telescope image jitter (due to vibration) 10nm equivalent to 0.275 mas ? Pump impeller Balance Grade 6.3

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?

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

7. 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

8. 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

9. 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)

10. Results combining M1 and image motion • After smoothing, after AO rejection Pier forcing NFIRAOS forcing • In both cases image motion is dominantabove 10 Hz

11. Check spatial correctability on M1 • M1 response at 30 Hz • AO spatial correctability isgood; correction isdominated by temporal bandwidth nm/N

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

13. 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

14. Compare actual sensitivity with fit to shaping filter for each source • FilterW(f): • f1=5 Hz • f2=20 Hz

15. Vibration Budget Specification on rms force after filtering by shaping filter (allows higher vibration at low or high frequency)

16. Source example ESO study of cryocoolers: “Low-vibration high-cooling power 2-stage cryocoolers for ground-based astronomical instrumentation” GerdJakob, Jean-Louis Lizon Proc. SPIE. 7733, Ground-based and Airborne Telescopes III 77333V (July 16, 2010) • 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

17. Source examplein 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

18. 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 • TMT is considering replacing water-glycol with phase-change refrigerant to reduce coolant mass flow (and forces) by a factor of 10

19. 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

20. Things to do • 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!

21. Conclusions • 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

22. Acknowledgements • 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.

23. You can build large structures without vibration problems • Mass helps • TMT dome = 2300 tons • Brunellesci’s dome = 37000 tons • The Duomo likely doesn’t have a vibration problem!