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 1
Can we afford to build an extremely large groundbased diffraction limited optical/IR telescope? Or can we afford ~ $1, 000 M Probably yes. . . 2
Framework for a credible Extremely Large/Maximum Aperture Telescope Concept Science Case 4 Gallagher et al, Strom et al An adaptive optics solution Mountain et al 4 Rigaut et al A telescope concept Ramsay Howat et al 4 A viable instrument model 4 3
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 2 K x 2 K IFU 0. 005” pixels 48 arcseconds 4
Modeled characteristics of 20 m and 50 m telescope Assumed point source size (mas) 20 M (mas) 1. 2 mm 1. 6 mm 2. 2 mm 3. 8 mm 4. 9 mm 12 mm 20 mm 50 M (mas) 1. 2 mm 1. 6 mm 2. 2 mm 3. 8 mm 4. 9 mm 12 mm 20 mm h 20 20 26 41 58 142 10 10 10 17 23 57 70% 50% 50% 240 94 50% Assumed detector characteristics 1 mm < l < 5. 5 mm < l < 25 mm Id 0. 02 e/s Nr qe Id Nr qe 4 e 80% 10 e/s 30 e 40% (Gillett & Mountain, 1998) 5
Relative Gain of groundbased 20 m and 50 m telescopes compared to NGST Groundbased advantage Velocities ~30 km/s NGST advantage Imaging 6
An Adaptive Optics Solution 7
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 corrected PSF across full field of view No correction (AO off) MCAO on Optical Performance - Strehl Ratio at 500 nm across a 20” x 20” FOV (Ellerbroek, 1994) Multiconjugate Adaptive Optics On Axis Edge FOV Corner FOV 0. 942 0. 953 0. 955 8
Instrumentation -- the next constraint? (Ramsay Howatt et al) R = 8, 000 across J, H & K 2 K x 2 K IFU 0. 005” pixels l Lets not assume diffraction limited instruments for 30 m ~ 100 m telescopes will be small 1. 2 m 10 arcsec 4. 2107 109 6. 7 X x Pixels 18. 5 mm pixels 1. 2 m 9
The next step ? 50 m telescope A 400 year legacy of groundbased telescopes 0 10
Technology has made telescopes far more capable, and affordable 0 11
– 50 m aperture ~ 3 m – Science field of view 0. 5 - 1. 0 50 m arcminutes – Useable field of view 1. 0 - 2. 0 arcminutes (for AO tomography) 2 m diameter – Minimize number of elements (IR performance) – Aim for structural compactness F/20 Cassegrain focus – KISS F/1 parabola M 1, 2 m diameter M 2 Cassegrain Instrument #2 Adaptive Optics Unit • Requirements Cassegrain Instrument #1 Optical Design 12
Optical Performance 1 arcminute FOV (Science Field) 30 0 arcsec. 60 30 arcsec. l/10 0 arcsec 30 arcsec 60 arcsec. Guide star FOV rms wavefront error 1 micron wavelength 13
Primary Mirror Approach F/1 Segmented Parabola 50 m 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 Segment testing (no null lenses) The sag of an 8 -meter segment is only 80 mm ~25 m Testing Ion Figuring Final Testing 14
Primary Mirror Support l l To reduce mass, reduce mirror substrate thickness ~ 50 mm (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 15
Primary Mirror Support continued • As self weight deflection a D 4/t 2, ~8 m diameter, 50 mm 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 50 m mirror 16
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 17
Challenges for the Structural Engineers. . . Telescope Optical Structure Requirements: • 50 m 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 M 4 and A&G/AO system • “Clean” top-end for IR emissivity, but rigid enough to launch 5 laser beacons • Challenges • 20 mm mirror substrate still weighs ~ 110 kg/m 2 (c. f ~ 75 kg/m 2 for Gemini/Zeiss M 2) • Mirror segments + cells could weigh 5. 5 x 45 + 200 = 450 tonnes • Wind…………. . • 10 m/s across 50 m a lot of energy at ~ 0. 2 Hz 18
Resonant Frequencies of Large Telescopes Frequency (Hz) Parabolic Reflector Antenna Systems Optics Systems (Laser/Infrared) Lowest Servo Resonant Frequency 2 Hz Telescope Aperture 50 m 19
Conceptual Design for an F/1 50 m Optical/IR Telescope 20
Optical/Mechanical concept Three levels of figure control: Mirror-to-cell actuators Integrated mirror/cell segment Large stroke actuators Mirror support truss with smart structure elements/active damping as needed • 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 21
Concept Summary Ca Ins sseg tru rai m n #1 ent Ada Op ptive C t Un ics I ass ns eg it tru rai m n #2 ent Optical support structure uses at least three levels of active control Collimated beam allows M 3 & M 4 to be tested independently and allows AO/instrument structure to be rigidly coupled to F/20 focus - insensitive to translation or rotation relative to 50 m structure M 2 easy to make/test - may need a little more rigidity…. 22
An Enclosure for 50 m -- “how big? ” 75 m 150 m 75 m 30 degrees 150 m • Restrict observing range to airmasses < 2. 0 • “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 23
Framework for a credible Extremely Large/Maximum Aperture Telescope Concept Science Case An adaptive optics solution A telescope concept A viable instrument model 24
Image of a 21 st Century Ground-Based Observatory -- 50 m Class 25
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How do we cost a 50 m? (1999) $522 • Contingency $100 M 27
How do we cost a 50 m? Risk assessment • Adaptive Optics – multiple-conjugate AO needs to be demonstrated – deformable mirror technology needs to expanded for 50 m ( x 10 - 20 more actuators • How do we make a “light-weight”, 4 - 8 m 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 cellsegment 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 10 m - 20 m “new technology telescope” · this is probably to only way to establish a credible cost for a 50 m - 100 m diffraction limited optical/IR groundbased telescope 29
“Supposing a couldn’t “Supposing wetree fell down a 50 or 100 m afford Pooh, when we were when we could Pooh, underneath it? ” have been doing “Supposing it didn’t, ” something more ‘said Pooh after careful useful `” thought. “Supposing we could, ” The House at Pooh Corner said Pooh after careful thought. With apologies to The House at Pooh Corner 30
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