The GSMT Point Design

2. The GSMT Point Design Larry Stepp and Brooke Gregory

3. Advances of the Past Decade

4. What Lies Ahead? • Astronomers already see the need for more powerful O/IR telescopes, to: • Extend the reach of current ground-based O/IR facilities • Complement space-based telescopes (e.g. NGST) • Complement next generation radio facilities (ALMA; SKA) • What type of facility will provide the needed capabilities a decade hence?

5. Decadal Review • In May 2000, the astronomy decadal review committee recommended, as its highest priority ground-based initiative, the construction of a 30-meter Giant Segmented Mirror Telescope (GSMT) • In response, AURA formed a New Initiatives Office (NIO) to support scientific and technical studies leading to the creation of a GSMT • Goal of ensuring broad astronomy community access to a 30m telescope contemporary with NGST.

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

7. What is a “Point Design”? • A point design is a learning exercise that: • Explores a single, plausible design • Helps identify key technical issues • Helps define factors important to the science requirements • Provides an opportunity to develop necessary analytical methods • A point design is not: • A trade study that evaluates all possible options • A design that anyone is proposing to build

8. GSMT Point Design: Scientific Motivations • Provide a practical basis for wide-field, native seeing-limited instruments • Origin of large-scale structure in the universe • Enable high-Strehl performance over ~ arc-minute fields • Stellar populations; galactic kinematics; chemical abundances • Enable high sensitivity mid-IR spectroscopy • Detection of stars & planetary systems in formation

9. Key Point Design Features • Fast aspheric primary • Stigmatic image after two reflections • Radio telescope-type design • Structural advantages • Accommodates large instruments • Adaptive secondary • Wind-buffeting compensation • Atmospheric correction in IR, with low emissivity • First stage in higher-order adaptive systems • Prime focus instrument • Convenient plate scale for seeing-limited observations • Enables wide-field science

10. Optical Design Optical design: Classical Cassegrain M1 diameter: 30 meters M1 focal ratio: f/1 M2 diameter: 2 meters M2 focal ratio: f/18.75

11. Radio Telescope Structural Design • Lightweight steel truss structure • Fast primary focal ratio • Small secondary mirror • M2 supported on tripod structure • Elevation axis behind M1 • Span between elevation bearings is less than M1 diameter • Allows direct load path

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. Primary Mirror Segments • Segment dimensions • 1.15-m across flats -- 1.33-m corner to corner • 50 mm thickness • Number of segments: 618 • Maximum departure from sphere 110 microns • Comparable to Keck • Axial support is 18-point whiffletree • FEA Gravity deflection 15 nm RMS

15. Initial Structural Analysis Horizon Pointing - Mode 1 = 2.16 Hz

16. 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 Large size and low resonant frequency make wind buffeting a key issue.

17. Gemini 8-meter Telescope

18. Ultrasonic anemometer Ultrasonic anemometer Pressure sensors Sensor Locations

19. Simultaneous Animations(c00030oo) Wind Pressure (N/m2) Mirror Deformation (microns) Wind Speed at 5 Locations (m/sec)

20. Response of structure to wind

21. Controllable Elements Active Systems: • M2 rigid body motion • ~ 5-10 Hz • Five axes • M1 segment rigid body position • ~ 1 Hz • Piston, tip & tilt • M1 segment figure control • Based on look-up table ~ 0.1 Hz • Astigmatism, focus, trefoil, coma • Active structural elements • Active alignment • Active damping

22. Controllable Elements Adaptive Systems: • Adaptive mirror in prime focus corrector • Adaptive secondary mirror • ~ 20-50 Hz • ~ 1000-10,000 actuators • Multi-conjugate wide-field AO • ~ 3 DMs • Laser Guide Stars • High-order narrow-field conventional AO • ~ 10,000 – 50,000 actuators

23. Controls Approach: Hierarchical Subsystems ~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]

24. Active and Adaptive Optics will be integrated into Telescope and Instrument concepts from the start.

25. Instruments • NIO team currently developing design concepts for 4 instruments: • Multi-Object, Multi-Fiber, Optical Spectrograph – MOMFOS • Near IR Deployable Integral Field Spectrograph – NIRDIF • MCAO-fed near-IR imager • Mid-IR, High Dispersion, AO Spectrograph – MIHDAS Paper by Sam Barden et al immediately after this one.

26. Instrument Locations on Telescope Prime Focus Fiber-fed Nasmyth Co-moving Cass Direct-fed Nasmyth

27. Instrument Locations on Telescope Prime Focus Fiber-fed Nasmyth Co-moving Cass Direct-fed Nasmyth Fixed Gravity Cass

28. MCAO System

29. MCAO System

30. MCAO System

31. Paper on: Adaptive optics requirements, concepts and performance estimates for Extremely Large telescopes by Brent Ellerbroek and Francois Rigaut at 1:10.

32. Mayall, Gemini and GSMT Enclosuresat same scale Mayall GSMT Gemini

33. McKale Center – Univ of Arizona

34. GSMT – at same scale

35. Some Possible GSMT Enclosure Designs

36. Summary: Key Point-Design Features • F/1 primary mirror • Advantages: • Reduces size of enclosure • Reduces flexure of optical support structure • Reduces counterweights required • Disadvantages: • Increased sensitivity to misalignment • Increased asphericity of segments

37. Summary: Key Point-Design Features • Paraboloidal primary • Advantages: • Good image quality over 10-15 arcmin field with only two reflections • Lower emissivity for mid-IR • Compatible with laser guide stars • Disadvantages: • Higher segment fabrication cost • Increased sensitivity to segment alignment

38. Summary: Key Point-Design Features • Radio telescope structure • Advantages: • Direct load path to elevation bearings • Can have short back focal distance • Allows small secondary mirror – can be adaptive • Allows MCAO system ahead of Nasmyth focus • Allows many gravity-invariant instrument locations • Disadvantage: • Requires counterweight • Sweeps out larger volume in enclosure

39. Summary: Key Point-Design Features • 2m diameter adaptive secondary mirror • Advantages: • Correction of low-order M1 modes • Enhanced native seeing • Good performance in mid-IR • First stage in high-order AO system • Disadvantages: • Increased difficulty (i.e., cost)

40. Summary: Key Point-Design Features • Prime focus location for MOMFOS • Advantages: • Fast focal ratio leads to reasonably-sized instrument • Adaptive prime focus corrector allows enhanced seeing performance • Disadvantages: • Issues of interchange with M2 • Requires fibers instead of slits

41. Plans for Next 15 Months • Involve community in defining GSMT scientific context • Continue structural analysis • Construct hierarchical system control model • Simulate system performance in presence of disturbances • Extend AO development efforts • Continue site testing • Develop cost-reduction strategies • Segment fabrication • Telescope structure • Adaptive optics • Instrument technologies • Enclosures

42. Acknowledgements Many people have contributed to this work, including: • George Angeli • Joe Antebi and Frank Kan of SG&H • Sam Barden • Dick Buchroeder • Myung Cho • Brent Ellerbroek • Paul Gillett • Brooke Gregory • Charles Harmer • Ming Liang • Matt Mountain • Joan Najita • Jim Oschmann • Jennifer Purcell • Francois Rigaut • Rick Robles • Mike Sheehan • David Smith of MERLAB • Steve Strom Plus many NOAO & Gemini scientists working on the GSMT science case

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