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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
0000000
0000000
0000000
0000000
0000000
0000000
0000000
0000000
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
0000000
0000000
0000000
0000000
0000000
0000000
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0000000
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)
0000000
0000000
0000000
0000000
0000000
0000000
0000000
0000000
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.
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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
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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
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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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