Giant Segmented Mirror Telescope

1. OSA Conference on Optical Fabrication and Testing May 3, 2002 Giant Segmented Mirror Telescope

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.

9. Optical Performance Cassegrain Focus: Wide Field Spot diagrams at center of field and at radius of 6 arc minutes. The circle diameter is 0.5 arcsec.

10. Telescope Emissivity

11. Structural Design Concept Based 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

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

25. Direct Cassegrain AO

26. MCAO

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

29. Prime Focus AO System Corrects M1 warping and ground-level turbulence Achieves moderate improvement over 20-arcmin FOV

30. Performance of Point Design AO Systems

31. 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

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)

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

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