1. 1 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

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

3. 3 Framework for a credible Extremely Large/Maximum Aperture Telescope Concept Science Case An adaptive optics solution A telescope concept A viable instrument model Gallagher et al, Strom et al Rigaut et al Ramsay Howat et al Mountain et al 4 4 4 4

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

5. 5 Modeled characteristics of 20m and 50m telescope Assumed detector characteristics  m <  m 5.5  m <  m I d N r q e I d N r q e 0.02 e/s 4e 80% 10 e/s 30e 40% Assumed point source size (mas) 20M 1.2  m 1.6  m 2.2  m 3.8  m 4.9  m 12  m 20  m (mas) 20 20 26 41 58 142 240 50M 1.2  m 1.6  m 2.2  m 3.8  m 4.9  m 12  m 20  m (mas) 10 10 10 17 23 57 94  (Gillett & Mountain, 1998)

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

7. 7 An Adaptive Optics Solution

8. 8 No correction (AO off) (Rigaut et al) New Directions for Adaptive Optics ~ arcminute corrected FOV’s possible (Rigaut et al) Numerical simulationsNumerical 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 viewFully corrected PSF across full field of view MCAO on 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. 9 Instrumentation -- the next constraint? (Ramsay Howatt et al) 2K x 2K IFU 0.005” pixels 10 arcsec R = 8,000 across J, H & K 4.2 x 10 9 18.5  m pixels 1.2 m Lets not assume diffraction limited instruments for 30m ~ 100m telescopes will be small 6.7 X 10 7 Pixels

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

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

12. 12 Optical Design RequirementsRequirements –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 50m F/1 parabola M1, 2m diameter M2 2m diameter ~ 3m F/20 Cassegrain focus Adaptive Optics Unit Cassegrain Instrument #1 Cassegrain Instrument #2

13. 13 Optical Performance 0 arcsec30 arcsec 1 arcminute FOV (Science Field) 0 arcsec. 30 arcsec. 60 arcsec. Guide star FOV rms wavefront error 1 micron wavelength /10 0 arcsec 3060

14. 14 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. F/1 Segmented Parabola Segment testing (no null lenses) 50m Actively controlled polishing The sag of an 8-meter segment is only 80 mmTesting Ion Figuring Final Testing ~25m

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

16. 16 Primary Mirror Support - continued As self weight deflection  D 4 /t 2, ~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. 17 Does maintaining 10,000 actuators challenge the Quality Control Engineers? What Mean Time Between Failures (MTBF) does this require?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 hoursMTBF required is 380,000 hours Required service life of each actuators, assuming maintenance is 5 yearsRequired service life of each actuators, assuming maintenance is 5 years

18. 18 Challenges for the Structural Engineers... ChallengesChallenges 20mm mirror substrate still weighs ~ 110 kg/m 2 (c.f ~ 75 kg/m 2 for Gemini/Zeiss M2)20mm mirror substrate still weighs ~ 110 kg/m 2 (c.f ~ 75 kg/m 2 for Gemini/Zeiss M2) Mirror segments + cells could weigh 5.5 x 45 + 200 = 450 tonnesMirror segments + cells could weigh 5.5 x 45 + 200 = 450 tonnes Wind…………..Wind………….. 10 m/s across 50m a lot of energy at ~ 0.2 Hz10 m/s across 50m a lot of energy at ~ 0.2 Hz Telescope Optical Structure Requirements: 50m surface must be held ~ /10 against gravitational and wind loads 50m surface must be held ~ /10 against gravitational and wind loads Relative pointing and tracking ~ 3 arcseconds rms Relative pointing and tracking ~ 3 arcseconds rms Absolute pointing/tracking provided by Star-tracker Absolute pointing/tracking provided by Star-tracker Precision guiding/off-setting controlled by M4 and A&G/AO system 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 “Clean” top-end for IR emissivity, but rigid enough to launch 5 laser beacons

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

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

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

22. 22 Concept Summary Adaptive Optics Unit Cassegrain Instrument #1 Cassegrain Instrument #2 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 or rotation relative to 50m structure to 50m structure M2 easy to make/test - may need a little more rigidity…. rigidity….

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

24. 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. 25 Image of a 21 st Century Ground-Based Observatory -- 50m Class

26. 26

27. 27 How do we cost a 50m? (1999) $522 Contingency $100M

28. 28 How do we cost a 50m? Risk assessment Adaptive OpticsAdaptive 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 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 ?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?

29. 29 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

30. 30 “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 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 With apologies to The House at Pooh Corner