AURA New Initiatives Office

Larry Stepp and Brooke Gregory The GSMT Point Design

AURA New Initiatives Office Advances of the Past Decade

AURA New Initiatives Office What Lies Ahead? l 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) l What type of facility will provide the needed capabilities a decade hence?

AURA New Initiatives Office 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.

AURA New Initiatives Office AURA New Initiatives Office Approach 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

AURA New Initiatives Office What is a “Point Design”? l 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 l A point design is not: –A trade study that evaluates all possible options –A design that anyone is proposing to build

AURA New Initiatives Office 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

AURA New Initiatives Office 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

AURA New Initiatives Office Optical Design M1 diameter: 30 meters M1 focal ratio: f/1 M2 diameter: 2 meters M2 focal ratio: f/18.75 Optical design: Classical Cassegrain

AURA New Initiatives Office Radio Telescope Structural Design l Lightweight steel truss structure l Fast primary focal ratio l Small secondary mirror l M2 supported on tripod structure l Elevation axis behind M1 –Span between elevation bearings is less than M1 diameter –Allows direct load path

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 M2 support

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

AURA New Initiatives Office Primary Mirror Segments Segment dimensions –1.15-m across flats 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

AURA New Initiatives Office Initial Structural Analysis Horizon Pointing - Mode 1 = 2.16 Hz

AURA New Initiatives Office 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.

AURA New Initiatives Office Gemini 8-meter Telescope

AURA New Initiatives Office Sensor Locations Pressure sensors Ultrasonic anemometer

AURA New Initiatives Office Simultaneous Animations (c00030oo) Wind Pressure (N/m 2 ) Mirror Deformation (microns) Wind Speed at 5 Locations (m/sec)

AURA New Initiatives Office Average Pressure Structure Function (C00030oo) D(d) = d 0.41 Amplitude strongly dependent on vent gate setting

AURA New Initiatives Office Extrapolation to 30 Meters Pressure variation on 30-m mirror about twice 8-m Prms = 0.04d^0.5 RMS Pressure Difference (pascals) Sensor Separation (meters)

AURA New Initiatives Office Response of structure to wind

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

AURA New Initiatives Office Controllable Elements Adaptive Systems: High-order narrow-field conventional AO –~ 10,000 – 50,000 actuators Multi-conjugate wide-field AO –~ 3 DMs –Laser Guide Stars Adaptive secondary mirror –~ Hz –~ ,000 actuators Adaptive mirror in prime focus corrector

AURA New Initiatives Office Zernike modes Bandwidth [Hz] ~100 ~50 ~20 ~10 2 Controls Approach: Hierarchical Subsystems aO (M1) AO (M2) Main Axes LGS MCAO Secondary rigid body temporal avg spatial & temporal avg spatial avg

AURA New Initiatives Office Active and Adaptive Optics will be integrated into Telescope and Instrument concepts from the start.

AURA New Initiatives Office 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.

AURA New Initiatives Office Instrument Locations on Telescope Prime Focus Fiber-fed Nasmyth Direct-fed Nasmyth Co-moving Cass

AURA New Initiatives Office Instrument Locations on Telescope Prime Focus Fiber-fed Nasmyth Direct-fed Nasmyth Fixed Gravity Cass Co-moving Cass

AURA New Initiatives Office MCAO System

AURA New Initiatives Office MCAO System

AURA New Initiatives Office MCAO System

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

AURA New Initiatives Office Mayall, Gemini and GSMT Enclosures at same scale Mayall Gemini GSMT

AURA New Initiatives Office McKale Center – Univ of Arizona

AURA New Initiatives Office GSMT – at same scale

AURA New Initiatives Office Some Possible GSMT Enclosure Designs

AURA New Initiatives Office 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

AURA New Initiatives Office Summary: Key Point-Design Features Paraboloidal primary –Advantages: Good image quality over 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

AURA New Initiatives Office 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

AURA New Initiatives Office 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)

AURA New Initiatives Office 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

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

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

AURA New Initiatives Office Information on AURA NIO activities is available at: