Giant Segmented Mirror Telescope

1. Giant Segmented Mirror Telescope OSA Conference on Optical Fabrication and Testing May 3, 2002 L. M. Stepp, G. Z. Angeli, S. C. Barden, M. K. Cho, B. L. Ellerbroek , P. E. Gillett, B. Gregory, S. E. Strom

2. Extremely Large Telescopes • Astronomers are already planning telescopes larger than the 6-10-meter current generation • Larger ground-based telescope will be needed to: • Understand the origin and formation of • Large scale structure in the Universe • Galaxies • Stars • Planetary systems • Complement other planned observing facilities • NGST • ALMA • SKA

3. The USA Decadal Review • In May 2000, the US astronomy decadal review committee recommended the construction of a 30-meter Giant Segmented Mirror Telescope (GSMT) as its highest ground-based initiative • In response, AURA formed a New Initiatives Office (NIO) to support scientific and technical studies leading to creation of GSMT • NIO is a joint venture of the National Optical Astronomy Observatory (NOAO) and the Gemini Observatory • Goal is to ensure broad astronomy community access to a 30m telescope contemporary with NGST and ALMA.

4. AURA New Initiatives OfficeApproach to GSMT Design Three Parallel efforts: • Understand the scientific context for GSMT in NGST / ALMA era • Develop the key science requirements • Develop a Point Design • Based on initial science goals & instrument concepts • Address challenges common to all ELTs • Site testing and selection • Cost-effective segment fabrication • Characterization of wind loading • Hierarchical control systems • Adaptive optics • Cost control techniques

5. Science Goals Driving the Point Design Telescope design should provide: • High-Strehl performance over ~ arc-minute fields • Stellar populations; galactic kinematics; chemical abundances • High sensitivity mid-IR spectroscopy and high dynamic range imaging • Forming and mature planetary systems • Wide-field, native seeing-limited multi-object spectroscopy • Origin of large-scale structure in the universe

6. NIO Point Design Philosophy The design of a next-generation telescope is a systems challenge • Requires an integrated approach that takes advantage of the dynamic compensation available from AO systems The point design should: • Be responsive to the science goals • Help identify key technical issues • Help define factors important to the science requirements • Provide an opportunity to develop needed analytical methods The point design does not need to be: • Completely detailed • 100% consistent

7. Point Design Optical System Optical Design: • 30-m aperture • F/18.75 • Classical Cassegrain Primary Mirror: • Aspherical • Segmented • Fast focal ratio -- F/1 • Hexagonal segments • Segment size -- 1.33 m across corners Secondary Mirror: • Small -- 2-m diameter • Convex • Aperture stop • Adaptive

8. Optical PerformanceCassegrain Focus: Narrow Field Spot diagrams at center of field and at radius of one arc minute. The circles indicate the Airy disk diameter for  = 2.5 microns. Linear diameter of 2-arcmin field is 0.33 m

9. Optical PerformanceCassegrain Focus: Wide Field Spot diagrams at center of field and at radius of 6 arc minutes. The circle diameter is 0.5 arcsec. Linear diameter of 12-arcmin field is 1.96 m

10. Telescope Emissivity

11. Structural Design ConceptBased on Radio Telescope • Lightweight steel truss structure • M2 supported on tripod • Elevation axis behind M1

12. Initial Point Design Structure Concept developed by Joe Antebi of Simpson Gumpertz & Heger • Based on radio telescope • Space frame truss • Single counterweight • Cross bracing of M2 support

13. Initial Point Design Structure Plan View of StructurePattern of segments Gemini

14. Initial Structural Analysis • Total weight of elevation structure – 700 tonnes • Total moving weight – 1400 tonnes • Gravity deflections ~ 5-25 mm • Primarily rigid-body tilt of elevation structure • Lowest resonant frequencies ~ 2 Hz

15. Current Structural Concept

16. Instrument LocationsPrime Focus

17. Instrument LocationsCo-moving Cassegrain Focus

18. Instrument LocationsFixed-gravity Cassegrain Focus

19. Instrument LocationsMCAO-fed Nasmyth Focus

20. Opto-mechanical Features • Segments grouped into rafts • 7 segments per raft • 16 types of rafts • 91 rafts total

21. Summary of Segment Properties • Segment dimensions • 1.15-m across flats -- 1.33-m corner to corner • 50 mm thickness • Segment weight: 157 kg if Zerodur; 133 kg if ULE • Number of segments: 618 • Maximum departure from sphere 110 microns • Comparable to Keck

22. Segment Supports • Axial support: 18-point whiffletree • FEA Gravity deflection 15 nm RMS • Lateral support: 3 bipods -- line of action at mid-plane • FEA Gravity deflection 2.2 nm RMS

23. Stray light baffles (if required) • M1 baffle 13.5 m long • M2 baffle 3 m diameter • Central obscuration 3 m diameter • Fully baffle 5 arcmin diameter field

24. Adaptive Optics Systems • Adaptive mirror in prime focus corrector • Adaptive secondary mirror • Multi-conjugate wide-field AO • High-order narrow-field conventional AO

25. Direct Cassegrain AO • Conventional AO • Single guide star • Uses adaptive M2 • 2400 actuators • 20-40 Hz • F/18.75 image • At Cassegrain Focus • Serves as first stage in higher AO systems

26. 3 DMs ADC Off-axis Parabola Fold Mirrors Beam Splitter Off-axis Parabola WFS beams Tip-tilt Mirror F/38 Focus Fold Mirrors MCAO Optical Design by Richard Buchroeder

27. MCAO • System parameters • 3 DMs at conjugate ranges of 0, 5, and 10 km • 5 sodium laser guide stars at center & corners of 1' square • 3 natural guide stars • Diameter of DMs 0.5 m • Final focal ratio: f/38 • FOV: 2 arcmin

28. High-performance NGS AO Optical Design by Richard Buchroeder

29. Prime Focus AO System • Corrects M1 warping and ground-level turbulence • Achieves moderate improvement over 20-arcmin FOV • Multiple NGS wavefront sensors • Adaptive mirror conjugate to M1 • ~ 1000 actuators • Tip-tilt mirror

30. Performance of Point Design AO Systems

31. Initial Instrument Concepts for GSMT • Design concepts driven by science objectives • Multi-Object, Multi-Fiber, Optical Spectrograph MOMFOS • Science: 3-D map of the early universe • Near IR Deployable Integral Field Spectrograph NIRDIF • Science: deconstructing young galaxies and pre-galactic fragments • Mid-IR, High Dispersion, AO Spectrograph MIHDAS • Science: origins of planetary systems • Near IR, AO Echelle Spectrograph NIrES • Science: origins of planetary systems • MCAO-fed near-IR imager • Science: stellar populations • Diffraction-Limited Near-IR Coronagraph • Science: characterization of mature planets

32. Summary of Instrument Concepts

33. ELT Control Systems Face Tough Challenges • Enemies of image quality gain strength as the telescope aperture grows: • Gravity • Predictable, telescope orientation varies slowly • Temperature gradients • Slowly varying • Atmospheric turbulence • Dynamic, can be modeled statistically • Wind buffeting • Dynamic, hard to predict • GSMT’s large size and low resonant frequency make wind buffeting a key issue • For a given Strehl ratio, required RMS wavefront is same as for smaller telescope

34. ~100 ~50 ~20 ~10 2 LGS MCAO spatial & temporal avg Zernike modes AO (M2) spatial & temporal avg aO (M1) spatial avg temporal avg spatial avg Secondary rigid body spatial & temporal avg Main Axes 0.001 0.01 0.1 1 10 100 Bandwidth [Hz] Multiple Controls Systems Systems Overlap in Parameter Space

35. Control Philosophy • Goal is to decouple control loops by separating them in • Space • Spatial frequency • Temporal frequency • Allows decentralization of control laws • Decoupling simplifies control system • Design • Implementation • Troubleshooting

36. Site Evaluation Studies • Survey of candidate sites by remote sensing (satellite data) • Wind flow and atmospheric turbulence modeling • Characterization of turbulent layers • Sodium layer measurements • On-site Measurements

37. Technical Challenges for an ELT • Active and adaptive compensation for wind buffeting • Adaptive correction of atmospheric turbulence • Segment co-alignment and phasing • Tip-tilt control of secondary mirror • Large (10-20 m3) cryogenic (~ 10 K) instruments • Cost-effective segment fabrication • Fabrication of adaptive secondary mirror

38. Segment Fabrication Challenges • Aspheric departures > 200 microns P-V • Mechanical dimensions accurate to ~ 0.1 mm • Bevel size <1 mm • Surface figure accuracy ~ 20 nm RMS • Production rate of ~ 200 segments / year • Large number of different: • segment shapes • orientations • asphericities

39. Optical Testing Challenges • Aspheric departures > 200 microns P-V • With respect to the optical test equipment: • Segment position must be known to ~ 0.3 mm • Segment clocking must be known to ~ 0.1 mrad • Figure measurement accuracy ~ 5 nm RMS • Radius of curvature repeatability ~ 0.5 mm in 60 m • Production rate of ~ 200 segments / year • Large number of different: • segment shapes • orientations • asphericities

40. We view segment fabrication as primarily a mass-production challenge • Cost • Schedule • Quality control NIO is collaborating with other ELT design groups, and consulting with commercial polishing firms, to develop cost-effective segment fabrication methods

41. Secondary Mirror Fabrication Challenges • 2-meter deformable facesheet ~ 3 mm thick • Bevel size <1 mm • Surface figure accuracy ~ 20 nm RMS with active correction • Figure must be good to the outer edge • Conformal backing structure of thermally-stable material • Must accommodate AO actuators • Must be stiff enough to allow fast tip-tilt & focus

42. Secondary MirrorOptical Testing Challenges • Convex aspheric surface • Figure measurement accuracy ~ 5 nm RMS • Facesheet extremely flexible • In-process testing should match acceptance test • Metrology mount with ~ 2400 actuators

43. Information on AURA NIO activities is available at: www.aura-nio.noao.edu

44. NOAO is operated by the Association of Universities for Research in Astronomy (AURA), Inc. under cooperative agreement with the National Science Foundation. Gemini is an international partnership managed by the Association of Universities for Research in Astronomy under a cooperative agreement with the National Science Foundation. Partner countries include the United States, United Kingdom, Canada, Chile, Australia, Argentina, and Brazil.