Giant Segmented Mirror Telescope L M Stepp G

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

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 AURA New Initiatives Office

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 30 m telescope contemporary with NGST and ALMA. AURA New Initiatives Office

AURA New Initiatives Office Approach 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 AURA New Initiatives Office

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 AURA New Initiatives Office

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 AURA New Initiatives Office

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 AURA New Initiatives Office

Optical Performance Cassegrain Focus: Narrow Field Linear diameter of 2 -arcmin field is 0. 33 m Spot diagrams at center of field and at radius of one arc minute. The circles indicate the Airy disk diameter for = 2. 5 microns. AURA New Initiatives Office

Optical Performance Cassegrain Focus: Wide Field Linear diameter of 12 -arcmin field is 1. 96 m Spot diagrams at center of field and at radius of 6 arc minutes. The circle diameter is 0. 5 arcsec. AURA New Initiatives Office

Telescope Emissivity Source Telescope emissivity (%) Aluminum coatings Silver coatings M 1 coating 2. 0 1. 3 M 2 obscuration 0. 4 Segment joints 0. 9 M 2 support tripod 2. 7 Total 8. 0 6. 6 AURA New Initiatives Office

Structural Design Concept Based on Radio Telescope • Lightweight steel truss structure • M 2 supported on tripod • Elevation axis behind M 1 AURA New Initiatives Office

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 M 2 support AURA New Initiatives Office

Initial Point Design Structure Plan View of Structure Pattern of segments Gemini AURA New Initiatives Office

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 AURA New Initiatives Office

Current Structural Concept AURA New Initiatives Office

Instrument Locations Prime Focus AURA New Initiatives Office

Instrument Locations Co-moving Cassegrain Focus AURA New Initiatives Office

Instrument Locations Fixed-gravity Cassegrain Focus AURA New Initiatives Office

Instrument Locations MCAO-fed Nasmyth Focus AURA New Initiatives Office

Opto-mechanical Features • Segments grouped into rafts • • • 7 segments per raft 16 types of rafts 91 rafts total AURA New Initiatives Office

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 AURA New Initiatives Office

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 AURA New Initiatives Office

Stray light baffles (if required) • • M 1 baffle 13. 5 m long M 2 baffle 3 m diameter Central obscuration 3 m diameter Fully baffle 5 arcmin diameter field AURA New Initiatives Office

Adaptive Optics Systems • Adaptive mirror in prime focus corrector • Adaptive secondary mirror • Multi-conjugate wide-field AO • High-order narrow-field conventional AO AURA New Initiatives Office

Direct Cassegrain AO • Conventional AO – Single guide star • Uses adaptive M 2 – 2400 actuators – 20 -40 Hz • F/18. 75 image – At Cassegrain Focus • Serves as first stage in higher AO systems AURA New Initiatives Office

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

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 AURA New Initiatives Office

High-performance NGS AO Optical Design by Richard Buchroeder AURA New Initiatives Office

Prime Focus AO System • Corrects M 1 warping and ground-level turbulence – Achieves moderate improvement over 20 -arcmin FOV • Multiple NGS wavefront sensors • Adaptive mirror conjugate to M 1 – ~ 1000 actuators • Tip-tilt mirror AURA New Initiatives Office

Performance of Point Design AO Systems AURA New Initiatives Office

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 NIr. ES – Science: origins of planetary systems MCAO-fed near-IR imager – Science: stellar populations Diffraction-Limited Near-IR Coronagraph – Science: characterization of mature planets AURA New Initiatives Office

Summary of Instrument Concepts Instrument Wavelength Image Resolution Spectral Resolution FOV Multiplex MOMFOS 0. 4 - 1 m 1” 2000 - 20, 000 20 arcmin 700 NIRDIF 1 - 2. 5 m 0. 1” x 1” 5000 - 10, 000 2 arcmin 26 MIHDAS 16 - 20 m 0. 2” (DL) 100, 000 1 arcsec 1 NIr. ES 1 - 5 m 0. 03” (DL) 100, 000 0. 1 arcsec 1 MCAO Imager 1 - 2. 5 m 0. 03” (DL) Imager 1. 5 - 2 arcmin 1 MEIFU 0. 4 - 1 m 0. 1” x 0. 18” 500 - 1500 5 arcmin 5, 000 1 - 5 m 0. 03” (DL) Imager 2 arcsec 1 Coronagraph AURA New Initiatives Office

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 AURA New Initiatives Office

Multiple Controls Systems Overlap in Parameter Space LGS MCAO ~100 spatial & temporal avg AO (M 2) Zernike modes ~50 spatial & temporal avg spatial avg a. O (M 1) ~20 temporal avg ~10 spatial avg Secondary rigid body 2 spatial & temporal avg Main Axes 0. 001 0. 1 1 10 100 Bandwidth [Hz] AURA New Initiatives Office

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 AURA New Initiatives Office

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 AURA New Initiatives Office

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 m 3) cryogenic (~ 10 K) instruments Cost-effective segment fabrication Fabrication of adaptive secondary mirror AURA New Initiatives Office

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 AURA New Initiatives Office

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 AURA New Initiatives Office

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 AURA New Initiatives Office

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 AURA New Initiatives Office

Secondary Mirror Optical 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 AURA New Initiatives Office

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

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. AURA New Initiatives Office