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Can we afford to build an extremely large groundbased diffraction limited optical/IR telescope?

Can we afford to build an extremely large groundbased diffraction limited optical/IR telescope?. Jim Oschmann Francois Rigaut Mike Sheehan Larry Stepp Matt Mountain Gemini Observatory. Can we afford to build an extremely large groundbased diffraction limited optical/IR telescope?.

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Can we afford to build an extremely large groundbased diffraction limited optical/IR telescope?

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  1. Can we afford to build an extremely large groundbased diffraction limited optical/IR telescope? Jim Oschmann Francois Rigaut Mike Sheehan Larry Stepp Matt MountainGemini Observatory

  2. Can we afford to build an extremely large groundbased diffraction limited optical/IR telescope? Or can we afford ~ $1,000M Probably yes...

  3. Framework for a credible Extremely Large/Maximum Aperture Telescope Concept Science Case Gallagher et al, Strom et al An adaptive optics solution Rigaut et al A telescope concept Mountain et al A viable instrument model Ramsay Howat et al

  4. Spectroscopic Imaging at 10 milli-arcsecond resolution - using NGST as “finder scope” Simulated NGST K band image • Blue for z = 0 - 3 • Green for z = 3 - 5 • Red for z = 5 - 10  = 0.1 l 2K x 2K IFU 0.005” pixels 48 arcseconds

  5. Modeled characteristics of 20m and 50m telescope Assumed point source size (mas) 20M 1.2mm 1.6mm 2.2mm 3.8mm 4.9mm 12mm 20mm (mas) 20 20 26 41 58 142 240 50M 1.2mm 1.6mm 2.2mm 3.8mm 4.9mm 12mm 20mm (mas) 10 10 10 17 23 57 94 h 70% 70% 50% 50% 50% 50% 50% Assumed detector characteristics 1mm < l < 5.5mm 5.5mm < l < 25mm Id Nr qe Id Nr qe 0.02 e/s 4e 80% 10 e/s 30e 40% (Gillett & Mountain, 1998)

  6. Relative Gain of groundbased 20m and 50m telescopes compared to NGST Imaging Velocities ~30km/s Groundbased advantage NGST advantage

  7. An Adaptive Optics Solution

  8. No correction (AO off) MCAO on New Directions for Adaptive Optics ~ arcminute corrected FOV’s possible (Rigaut et al) • Numerical simulations • 5 guide stars & 5 Wavefront sensors • 2 mirrors • 8 turbulence layers • 40’’ Field of view • J band • Fully correctedPSF across full field of view Optical Performance - Strehl Ratio at 500nm across a 20” x 20” FOV (Ellerbroek,1994) Multiconjugate Adaptive Optics On Axis Edge FOV Corner FOV 0.942 0.953 0.955

  9. 4.2 x 109 18.5 mm pixels 1.2 m 6.7 X 107 Pixels 1.2 m Instrumentation -- the next constraint? (Ramsay Howatt et al) R = 8,000 across J, H & K 2K x 2K IFU 0.005” pixels l 10 arcsec Lets not assume diffraction limited instruments for 30m ~ 100m telescopes will be small

  10. A 400 year legacy of groundbased telescopes 0 The next step ? 50m telescope

  11. 0 Technology has made telescopes far more capable, and affordable

  12. Cassegrain Instrument #2 ~ 3m Adaptive Optics Unit 50m 2m diameter Cassegrain Instrument #1 F/20 Cassegrain focus F/1 parabola M1, 2m diameter M2 Optical Design • Requirements • 50m aperture • Science field of view 0.5 - 1.0 arcminutes • Useable field of view 1.0 - 2.0 arcminutes (for AO tomography) • Minimize number of elements (IR performance) • Aim for structural compactness • KISS

  13. 1 arcminute FOV (Science Field) 30 60 0 arcsec 0 arcsec. 30 arcsec. l/10 0 arcsec 30 arcsec 60 arcsec. Guide star FOV rms wavefront error 1 micron wavelength Optical Performance

  14. F/1 Segmented Parabola 50m Segment testing (no null lenses) ~25m Primary Mirror Approach The volume of glass in a 50-mm thick 8-meter segment is 2.5 cubic meters. This volume is equivalent to a stack of 1.5-meter diameter boules 1.4 meters high. Actively controlled polishing The sag of an 8-meter segment is only 80 mm Testing Ion Figuring Final Testing

  15. Primary Mirror Support • To reduce mass, reduce mirror substrate thickness ~ 50mm (1/4 of Gemini, ESO-VLT) • Individual segments still have to be supported against self weight Gravitational print through requires between 120 - 450 support points for a 20 cm thick meniscus

  16. Primary Mirror Support - continued • As self weight deflection a D4/t2,~8m diameter, 50mm segment will need ~ 1800 support points • How many active support points do we need to correct deformations due to wind and thermal gradients? • Estimate 1 in 6, ~ 300/segment which implies > 10,000 actuators to actively support a 50m mirror

  17. Does maintaining 10,000 actuators challenge the Quality Control Engineers? • What Mean Time Between Failures (MTBF) does this require? • Assume 95% up-time, over 356 x 12 hour nights • Assume unacceptable performance will occur when 5% of actuators fail • Assume it takes 1 hour to replace actuator, and that we can service 8 actuators a day, over 250 maintenance days • Therefore we can replace/service 2,000 actuators/year • MTBF required is 380,000 hours • Required service life of each actuators, assuming maintenance is 5 years

  18. Challenges for the Structural Engineers ... • Telescope Optical Structure Requirements: • 50m surface must be held ~ l/10 against gravitational and wind loads • Relative pointing and tracking ~ 3 arcseconds rms • Absolute pointing/tracking provided by Star-tracker • Precision guiding/off-setting controlled by M4 and A&G/AO system • “Clean” top-end for IR emissivity, but rigid enough to launch 5 laser beacons • Challenges • 20mm mirror substrate still weighs ~ 110 kg/m2 (c.f ~ 75 kg/m2 for Gemini/Zeiss M2) • Mirror segments + cells could weigh 5.5 x 45 + 200 = 450 tonnes • Wind………….. • 10 m/s across 50m a lot of energy at ~ 0.2 Hz

  19. Resonant Frequencies of Large Telescopes Parabolic Reflector Antenna Systems Optics Systems (Laser/Infrared) Lowest Servo Resonant Frequency Frequency (Hz) 2Hz Telescope Aperture 50m

  20. Conceptual Design for an F/1 50m Optical/IR Telescope

  21. Mirror-to-cell actuators Integrated mirror/cell segment Large stroke actuators Mirror support truss with smart structure elements/active damping as needed Optical/Mechanical concept Three levels of figure control: • Each mirror segment • is controlled within an individual cell • Each cell is then controlled with respect to the primary mirror support structure • The support structure may have to use “smart structure” technology to maintain sufficient shape and/or damping for slewing/tracking

  22. Cassegrain Instrument #2 Adaptive Optics Unit Cassegrain Instrument #1 Concept Summary Optical support structure uses at least three levels of active control Collimated beam allows M3 & M4 to be tested independently and allows AO/instrument structure to be rigidly coupled to F/20 focus - insensitive to translation or rotation relative to 50m structure M2 easy to make/test - may need a little more rigidity….

  23. 75m 75m 150m 150m An Enclosure for 50m -- “how big?” • Restrict observing range to airmasses < 2.0 30 degrees • “Astro-dome” approach • Heretical proposition #1 - excavate • significantly lowers enclosure cost • further shields telescope from wind • reliant on AO to correct boundary layer • Heretical proposition #2 - perhaps the wind characteristics of a site are now more important than the seeing characteristics

  24. Framework for a credible Extremely Large/Maximum Aperture Telescope Concept Science Case An adaptive optics solution A telescope concept A viable instrument model

  25. Image of a 21st Century Ground-Based Observatory -- 50m Class

  26. How do we cost a 50m? (1999) $522 • Contingency $100M

  27. How do we cost a 50m?Risk assessment • Adaptive Optics • multiple-conjugate AO needs to be demonstrated • deformable mirror technology needs to expanded for 50m ( x 10 - 20 more actuators • How do we make a “light-weight”, 4 - 8m aspheric segment mounted in its own active cell and can we afford 45 - 180 of them? • How much dynamic range do we need to control cell-segment to cell-segment alignment ? • Will “smart”, and/or active damping systems have to be used telescope • evaluate by analysis and test. • Composites or Steel?

  28. Risk assessment - continued • Telescope Structure and wind loading • We need to characterize this loading in a way that is relatively easy to use in finite element analysis. This is easy, but mathematically intensive. Basically for each node that gets a wind force, a full vector of force cross spectra is generated, therefore the force matrix is a full matrix with an order equal to the number of forces (10’s of thousands). • Enclosure concept (do we need one)? • What concept can we afford both in terms of dollars/euros and environmental impact (note Heretical Proposition #2) • WE NEED A TECHNOLOGY TEST-BED • a 10m - 20m “new technology telescope” • this is probably to only way to establish a credible cost for a 50m - 100m diffraction limited optical/IR groundbased telescope

  29. “Supposing a tree fell down Pooh, when we were underneath it?” “Supposing it didn’t,” said Pooh after careful thought. The House at Pooh Corner “Supposing we couldn’t afford a 50 or 100m Pooh, when we could have been doing something more ‘useful `” “Supposing we could,” said Pooh after careful thought. With apologies to The House at Pooh Corner

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