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Agenda GSMT Controls Workshop, 11 September, 2001

Agenda GSMT Controls Workshop, 11 September, 2001. 9:00 am      GSMT Overview – Brooke Gregory, Larry Stepp 9:30 am      Pointing Control for a Giant Segmented Mirror Telescope                 – Patrick Wallace

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Agenda GSMT Controls Workshop, 11 September, 2001

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  1. AgendaGSMT Controls Workshop,11 September, 2001 9:00 am      GSMT Overview – Brooke Gregory, Larry Stepp 9:30 am      Pointing Control for a Giant Segmented Mirror Telescope                 – Patrick Wallace 10:00 am      Implications of Wind Testing Results on the GSMT Control Systems -- David Smith 10:30 am      To be defined – Mark Whorton 11:00 am     MSFC's Heritage in Segmented Mirror Control Technology                 – John Rakoczy 11:30 am      Current concepts and status of GSMT control system                 – George Angeli 12:00 pm       Lunch 1:00 pm       Informal discussions 5:00 pm Adjourn

  2. GSMT Overview B. Gregory, L. Stepp 11 September 2001

  3. AURA NIO: Mission • In response to the AASC call for a Giant Segmented Mirror Telescope (GSMT) AURA formed a New Initiatives Office (NIO) • collaborative effort between NOAO and Gemini to explore design concepts for a GSMT • NIO mission “ to ensure broad astronomy community access to a 30m telescope contemporary in time with ALMA and NGST, by playing a key role in scientific and technical studies leading to the creation of a GSMT.”

  4. AURA New Initiatives Office Contracted Studies Opto-mechanics Myung Cho Controls George Angeli Adaptive Optics Ellerbroek/Rigaut (Gemini)

  5. Objectives: Next 2 years • Develop point design for GSMT & instruments • Develop key technical solutions • Adaptive optics • Active compensation of wind buffeting • Mirror segment fabrication • Investigate design-to-cost considerations • Carry out conceptual design activities that support and complement other ELT efforts • Develop a partnership to build GSMT

  6. AURA New Initiatives OfficeApproach to GSMT Design Parallel efforts: • Understand the scientific context for GSMT • Develop the key science requirements • Address challenges common to all ELTs • Wind-loading • Adaptive optics • Site • Develop a Point Design • Based on initial science goals • Key part is conceptual design of instruments

  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. Point Design: Scientific Motivations • Enable high-Strehl performance over several arc-minute fields • Stellar populations; galactic kinematics; chemistry • Provide a practical basis for wide-field, native seeing-limited instruments • Origin of large-scale structure • Enable high sensitivity mid-IR spectroscopy • Detection of forming planetary systems Telescope design should be driven by needs of instruments

  9. Point Design: Basic Design Concepts • Explore a radio telescope approach • Possible structural advantages • Possible advantages in accommodating large instruments • Use aspheric optics – good image after two reflections • Incorporate an adaptive M2 • Compensate for wind-buffeting • Reduce thermal background • Deliver enhanced-seeing images • Explore prime focus option • Attractive enabler for wide-field science • Cost-saving in instrument design

  10. Radio Telescope Structural Design • Fast primary focal ratio • Lightweight steel truss structure • Small secondary mirror • Secondary supported on tripod structure • Elevation axis behind primary mirror • Span between elevation bearings is less than diameter of primary mirror – allows direct load path

  11. Optical Design Optical design: Classical Cassegrain Primary diameter: 30 meters Primary focal ratio: f/1 Secondary diameter: 2 meters Secondary focal ratio: f/18.75

  12. 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. Point Design Structure Plan View of StructurePattern of segments Gemini

  14. Lower Elevation Structure

  15. Primary Mirror Segments • Size chosen for point design: • 1.15-m across flats -- 1.33-m corner to corner • 50 mm thickness • Number of segments: 618 • Maximum asphericity ~ 110 microns (equal to Keck)

  16. Segment Support • Point design axial support is 18-point whiffletree • FEA Gravity deflection 15 nm RMS

  17. Wind Loading • Primary challenge may be wind buffeting • More critical than for existing telescopes • Structural resonances closer to peak wind power • Wind may limit performance more than local seeing • Solutions include: • Site selection for low wind speed • Optimizing enclosure design • Dynamic compensation • Adaptive Optics • Active structural damping

  18. Initital Structural Analysis Horizon Pointing - Mode 1 = 2.16 Hz

  19. Structural Analysis • Total weight of elevation structure – 700 tonnes • Total moving weight – 1400 tonnes • Gravity deflections ~ 5-25 mm • Wind buffeting response ~ 10-100 microns Deflections are primarily rigid-body motions • Lowest resonant frequencies ~ 2 Hz

  20. Instruments • NIO team developing design concepts • 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 • Build on extant concepts where possible • Define major design challenges • Identify needed technologies

  21. Instrument Locations on Telescope • Fixed Gravity Cass • Direct-fed Nasmyth • Fiber-fed Nasmyth • Prime Focus • Co-moving Cass View showing Fixed Gravity Cass instrument

  22. MOMFOS with Prime Focus Corrector Conceptual design fits in a 3m dia by 5m long cylinder

  23. Instrument Locations on Telescope View showing Co-moving Cass instrument

  24. MCAO/AO foci and instruments Oschmann et al (2001) MCAO opticsmoves with telescope elevation axis 4m MCAO Imager at vertical Nasmyth Narrow field AO or narrow field seeing limited port

  25. MCAO System: Current Layout

  26. Instrument Locations on Telescope Fiber-fed MOMFOS MCAO-fed NIRDIF or MCAO Imager Cass-fed MIHDAS

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

  28. McKale Center – Univ of Arizona

  29. GSMT – at same scale

  30. Key Point-Design Features • Radio telescope structure • Advantages: • Direct load path to elevation bearings • Cass focus can be just behind M1 • 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

  31. 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

  32. 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

  33. 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) Goal: 8000 actuators 30cm spacing on M1

  34. Key Point-Design Features • Prime focus location for MOMFOS • Advantages: • Fast focal ratio leads to instrument of reasonable size • Adaptive prime focus corrector allows enhanced seeing performance • Disadvantages: • Issues of interchange with M2

  35. Key Enabling Techniques: Active and Adaptive Optics Active Systems: • M1 segment rigid body position • ~ 1 Hz • Piston, tip & tilt • M1 segment figure control – • Based on look-up table ~ 0.1 Hz • Low order -- Astigmatism, focus, trefoil, coma • M2 rigid body motion • ~ 5-10 Hz • Five axes – active focus & alignment, image stabilization • Active structural elements (?) • Active alignment • Active damping

  36. Key Enabling Techniques: Active and Adaptive Optics Adaptive Systems: • Adaptive secondary mirror • ~ 20-50 Hz • ~ 1000-10,000 actuators • Adaptive mirror in prime focus corrector • Multi-conjugate wide-field AO • ~ 3 DMs • Laser Guide Stars • High-order narrow-field conventional AO • ~ 10,000 – 50,000 actuators Active and Adaptive Optics will be integrated into GSMT Telescope and Instrument concepts from the start

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