Can we afford to build an extremely large groundbased diffraction limited optical/IR telescope?Jim OschmannFrancois RigautMike SheehanLarry SteppMatt Mountain Gemini Observatory

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

Probably yes...

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Framework for a credible Extremely Large/Maximum Aperture Telescope ConceptScience CaseAn adaptive opticssolutionA telescope conceptA viable instrumentmodelGallagher et al, Strom et alRigaut et alRamsay Howat et alMountain et al

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Spectroscopic Imaging at 10 milli-arcsecond resolutionSimulated NGST K band imageBlue for z = 0 - 3Green for z = 3 - 5Red for z = 5 - 10 = 0.148 arcsecondsl2K x 2K IFU0.005 pixels- using NGST as finder scope

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Modeled characteristics of 20m and 50m telescopeAssumed 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%

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%(Gillett & Mountain, 1998)

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Relative Gain of groundbased 20m and 50m telescopes compared to NGSTGroundbased advantageNGST advantageImagingVelocities ~30km/s

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An Adaptive Optics Solution

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AO performance on a 50m Telescope

Diffraction limited imaging constrained to small field of view

Chun, 1998

Chart3

0.590.740.850.950.970.991

0.460.580.680.770.790.860.88

0.290.370.440.520.560.690.73

0.080.10.140.20.240.430.52

0.020.030.040.080.110.280.4

00.010.010.030.050.20.3

000.010.010.030.160.25

00000.010.080.15

1.2 micron

1.6 micron

2.2 micron

3.8 micron

4.9 micron

12 micron

20 micron

Field Angle (arcsec)

Strehl

5k actuator AOS on 50-m (Median Seeing)

Sheet1

J02.555.110.215.320.425.551

0.390.310.190.050.01000

H03.67.214.4121.6128.8136.0272.03

0.590.460.30.080.020.0100

K05.2810.5621.1131.6742.2252.78105.56

0.760.610.390.130.030.010.010

L010.1720.3440.6861.0281.36101.7203.39

0.910.740.50.190.070.030.010

M013.827.5955.1982.78110.38137.97275.95

0.950.780.550.240.10.050.030.01

N040.4280.84161.67242.51323.35404.19808.37

0.990.850.680.430.280.20.160.08

20074.61149.22298.44447.66596.89746.111492.21

10.880.730.520.40.30.250.15

J02.555.110.215.320.425.551

0.780.610.380.10.020.0100

H03.67.214.4121.6128.8136.0272.03

0.870.680.440.120.030.0100

K05.2810.5621.1131.6742.2252.78105.56

0.930.740.480.160.040.010.010

L010.1720.3440.6861.0281.36101.7203.39

0.980.790.540.20.080.030.010

M013.827.5955.1982.78110.38137.97275.95

0.990.810.570.250.110.060.030.01

N040.4280.84161.67242.51323.35404.19808.37

10.860.690.430.280.20.160.08

20074.61149.22298.44447.66596.89746.111492.21

10.880.730.520.40.30.250.15

J02.555.110.215.320.425.551

0.130.10.060.020000

H03.67.214.4121.6128.8136.0272.03

0.320.250.160.040.01000

K05.2810.5621.1131.6742.2252.78105.56

0.540.440.280.090.020.0100

L010.1720.3440.6861.0281.36101.7203.39

0.820.660.450.170.070.020.010

M013.827.5955.1982.78110.38137.97275.95

0.880.730.510.220.10.050.030.01

N040.4280.84161.67242.51323.35404.19808.37

0.980.840.680.420.270.20.160.08

20074.61149.22298.44447.66596.89746.111492.21

0.990.870.720.520.40.30.250.15

J02.555.110.215.320.425.551

0.590.460.290.080.02000

H03.67.214.4121.6128.8136.0272.03

0.740.580.370.10.030.0100

K05.2810.5621.1131.6742.2252.78105.56

0.850.680.440.140.040.010.010

L010.1720.3440.6861.0281.36101.7203.39

0.950.770.520.20.080.030.010

M013.827.5955.1982.78110.38137.97275.95

0.970.790.560.240.110.050.030.01

N040.4280.84161.67242.51323.35404.19808.37

0.990.860.690.430.280.20.160.08

20074.61149.22298.44447.66596.89746.111492.21

10.880.730.520.40.30.250.15

Sheet1

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1.2 micron

1.6 micron

2.2 micron

3.8 micron

4.9 micron

12 micron

20 micron

Field Angle (arcsec)

Strehl

1k actuator AOS on 50-m (10% Seeing)

Sheet2

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1.2 micron

1.6 micron

2.2 micron

3.8 micron

4.9 micron

12 micron

20 micron

Field Angle (arcsec)

Strehl

5k actuator AOS on 50-m (10% Seeing)

Sheet3

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1.2 micron

1.6 micron

2.2 micron

3.8 micron

4.9 micron

12 micron

20 micron

Field Angle (arcsec)

Strehl

1k actuator AOS on 50-m (Median Seeing)

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1.2 micron

1.6 micron

2.2 micron

3.8 micron

4.9 micron

12 micron

20 micron

Field Angle (arcsec)

Strehl

5k actuator AOS on 50-m (Median Seeing)

Mark Chun (August 21, 1998) :

The above are predictions of the performance of a 1k or 5k actuator AOS on a 50-m telescope on Mauna Kea. The median conditions are taken to be r_o = 25 cm and theta_o = 2 arcsec while the 10% best seeing conditions are assumed to be r_o = 40 cm and theta_o = 2 arcsec (same as median (pessimistic assumption)). IN ALL CASES I've assumed that the total phase variance is equal to 1.5 * fitting error. This is probably okay for the bright NGS case.

The models differ slightly from the previous plot I gave you in the way I handled the servo error. The approach is now more consistent. Note that this only really changed the on-axis SR for the longer wavelengths where I was being too pessimistic about the servo lag.

The Challenge - Multiple Laser Beacons

* * * * *

*

SRFA ~ 0.75 requires NBeacons

1.2mm 75 1.6mm 40 2.2mm 20 3.8mm 5 4.9mm 3 12.0mm

What is Tomography ?(Rigaut, this workshop)

90 km

90 km

1. Cone effect

2. Multiple guide star and tomography

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What is multiconjugate?(Rigaut, this workshop)

DM 1

DM 2

Turbulent layer 1

Turbulent layer 2

Off axis ray corrected

On axis ray corrected

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New Directions for Adaptive Optics ~ arcminute corrected FOVs possible (Rigaut et al)Numerical simulations5 guide stars & 5 Wavefront sensors2 mirrors8 turbulence layers40 Field of viewJ bandFully corrected PSF across full field of viewOptical Performance - Strehl Ratio at 500nmacross a 20 x 20 FOV(Ellerbroek,1994)

Multiconjugate Adaptive OpticsOn Axis Edge FOV Corner FOV

0.942 0.953 0.955

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Instrumentation -- the next constraint? (Ramsay Howatt et al)l2K x 2K IFU0.005 pixels10 arcsecR = 8,000 across J, H & KLets not assume diffractionlimited instruments for 30m ~ 100m telescopes will be small

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The next step ?50m telescope

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Technology has made telescopes far more capable, and affordable

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Optical DesignRequirements50m apertureScience field of view 0.5 - 1.0 arcminutesUseable field of view 1.0 - 2.0 arcminutes (for AO tomography)Minimize number of elements (IR performance)Aim for structural compactnessKISS

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Optical Performance

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Primary Mirror ApproachThe 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 polishingThe sag of an 8-meter segment is only 80 mm

Testing

Ion Figuring

Final Testing

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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 weightGravitational print through requires between 120 - 450support points for a 20 cm thick meniscus

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Primary Mirror Support - continuedAs self weight deflection a D4/t2, ~8m diameter, 50mm segment will need ~ 1800 support pointsHow 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

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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 nightsAssume unacceptable performance will occur when 5% of actuators failAssume it takes 1 hour to replace actuator, and that we can service 8 actuators a day, over 250 maintenance daysTherefore we can replace/service 2,000 actuators/yearMTBF required is 380,000 hoursRequired service life of each actuators, assuming maintenance is 5 years

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Challenges for the Structural Engineers ...Challenges20mm 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 HzTelescope 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

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Resonant Frequencies of Large TelescopesFrequency (Hz)Telescope Aperture50m2HzParabolic ReflectorAntenna SystemsOptics Systems (Laser/Infrared)Lowest Servo Resonant Frequency

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

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Optical/Mechanical conceptThree levels of figurecontrol: 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

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Concept SummaryOptical support structure uses at least three levels of active controlCollimated beam allowsM3 & M4 to be testedindependently andallows AO/instrumentstructure to be rigidlycoupled to F/20 focus- insensitive to translation or rotation relative to 50m structureM2 easy to make/test- may need a little more rigidity.

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An Enclosure for 50m -- how big?Restrict observing range to airmasses < 2.030 degreesAstro-dome approachHeretical proposition #1 - excavatesignificantly lowers enclosure costfurther shields telescope from windreliant on AO to correct boundary layer Heretical proposition #2 - perhaps the wind characteristics of a site are now more important than the seeing characteristics

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Framework for a credible Extremely Large/Maximum Aperture Telescope ConceptScience CaseAn adaptive opticssolutionA telescope conceptA viable instrumentmodel

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

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

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What can it cost?

Primary mirror assembly$622MTelescope structure & components$190M Secondary mirror assembly$11MMauna Kea Site $78MEnclosures $70MControls, software & communications $26MFacility instrumentation (A&G, AO) $35MCoating & cleaning facilities $9M Handling equipment $5M Project office $40M Total $1,086M

50m Telescope costs (1997$)

Scaled costs

Constrained costs

S (Keck + Gemini + ESO-VLT + Subaru) = $1,560M

How do we cost a 50m?Risk assessmentAdaptive Opticsmultiple-conjugate AO needs to be demonstrateddeformable mirror technology needs to expanded for 50m ( x 10 - 20 more actuatorsHow 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?

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Risk assessment - continuedTelescope Structure and wind loadingWe 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 (10s 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-BEDa 10m - 20m new technology telescopethis is probably to only way to establish a credible cost for a 50m - 100m diffraction limited optical/IR groundbased telescope

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Supposing a tree felldown Pooh, when we were underneath it?

Supposing it didnt,said Pooh after carefulthought. The House at Pooh CornerSupposing we couldntafford a 50 or 100mPooh, when we couldhave been doing something more useful `

Supposing we could,said Pooh after carefulthought. With apologies toThe House at Pooh Corner

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