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 ~ $1,000M
Probably yes...
2
Framework for a credible
Extremely Large/Maximum
Aperture Telescope Concept
Science Case
 Gallagher et al, Strom et al
An adaptive optics
solution
Mountain et al

Rigaut et al
A telescope concept
Ramsay Howat et al


A viable instrument
model
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
2K x 2K
IFU
0.005” pixels
48 arcseconds
4
Modeled characteristics of 20m and 50m telescope
Assumed point source size (mas)
20M
(mas)
1.2mm 1.6mm 2.2mm 3.8mm 4.9mm 12mm 20mm
50M
(mas)
1.2mm 1.6mm 2.2mm 3.8mm 4.9mm 12mm 20mm
h
20
20
26
41
58
142
10
10
10
17
23
57
70%
70%
50%
50%
50%
50%
240
94
50%
Assumed detector characteristics
1mm < l < 5.5mm
Id
0.02 e/s
5.5mm < l < 25mm
Nr
qe
Id
4e
80%
10 e/s
(Gillett & Mountain, 1998)
Nr
qe
30e
40%
5
Relative Gain of groundbased 20m and 50m
telescopes compared to NGST
Imaging
Velocities ~30km/s
10
100
100
50M R=5
50m R=10,000
20m R=5
10
S/N Gain
20m R=10,000
10
10
1
0.01
0.01
1E-3
1E-3
10
W avelength ( m m )
1
0.1
0.1
1
10
1
1
0.1
10
100
Groundbased
advantage
100
1
NGST advantage
1
0.1
0.01
0.01
1E-3
1E-3
1
10
W avelength ( m m )
6
An Adaptive Optics Solution
AO p erfo rm ance o n a 50m
Telesco p e
Str ehl
5k actuator AOS on 50-m (Median Seeing)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1.2 micron
1.6 micron
2.2 micron
3.8 micron
4.9 micron
12 micron
20 micron
0
10
20
30
40
50
60
Field
Angle (arcsec)
Ø Diffraction limited
imaging
constrained to small field of view
Chun, 1998
7
An Adaptive Optics Solution
The Challenge - Multiple Laser Beacons
- still a lot of technologies to develop
*
*
*
*
*
SRFA ~ 0.75 requires NBeacons
1.2mm
1.6mm
2.2mm
3.8mm
4.9mm
12.0mm
20.0mm
75
40
20
5
3
<=1
<=1
8
An Adaptive Optics Solution
9
An Adaptive Optics Solution
What is multiconjugate?
(Rigaut, this workshop)
Turbulent laye r 1
DM 1
Turbulent laye r 2
DM 2
Off axis ray corrected
On axis ray corrected
19
10
New Directions for Adaptive Optics
~ arcminute corrected FOV’s possible (Rigaut et al)
• Numerical
simulations
No correction (AO off)
– 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
11
New Directions for Adaptive Optics
~ arcminute corrected FOV’s possible (Rigaut et al)
• Numerical
simulations
No
correction
MCAO
on (AO off)
– 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
12
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
MCAO
on (AO off)
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
13
Instrumentation -- the next constraint?
(Ramsay Howatt et al)
R = 8,000 across J, H & K
2K x 2K
IFU
0.005” pixels
4.2 x 109
1.2 m
10 arcsec
l
18.5 mm pixels
1.2 m
14
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
6.7 X 107 Pixels
Lets not assume diffraction
limited instruments for
30m ~ 100m telescopes will be small
15
The next step ?
50m telescope
Cumulative Area (m 2 )
A 400 year legacy of groundbased telescopes
1400
900
400
0
-100
1600
1700
1800
Year
1900
2000
16
Cumulative Area (m 2 )
Technology has made telescopes
far more capable, and affordable
1400
900
400
0
-100
1600
1700
1800
Year
1900
2000
17
Technology has made telescopes
far more capable, and affordable
70000
400 years of inflation
60000
50000
UK CPI
40000
30000
20000
10000
0
1600
1700
1800
1900
2000
Year
18
Technology has made telescopes
far more capable, and affordable
The Cost per Square Meter of Telescope Collecting Area
0
10
-1
Relative Cost/m
2
10
-2
10
-3
10
x 1000
-4
10
-5
10
-6
10
1600
1700
1800
1900
2000
Year
19
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
20
Optical Design
50m
2m diameter
F/1 parabola M1, 2m diameter M2
21
Optical Design
~ 3m
F/20 Cassegrain focus
22
F/20 Cassegrain focus
Adaptive
Optics
Unit
~ 3m
Cassegrain
Instrument
#1
Cassegrain
Instrument
#2
Optical Design
23
Optical Performance
1 arcminute FOV (Science Field)
0 arcsec
30 arcsec
24
Optical Performance
0 arcsec.
30 arcsec.
60 arcsec.
Guide star FOV
25
Optical Performance
0 arcsec
30
60
l/10
rms wavefront error
1 micron wavelength
26
Primary Mirror Approach
27
Primary Mirror Approach
F/1 Segmented Parabola
50m
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.
Segment testing (no null lenses)
~25m
28
Primary Mirror Approach
Actively controlled polishing
The sag of an 8-meter segment is only 80 mm
Testing
Ion Figuring
29
Final Testing
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
30
Primary Mirror Support
31
Primary Mirror Support
Gravitational print through requires between 120 - 450
support points for a 20 cm thick meniscus
32
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?
33
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
34
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
35
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
36
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
37
Resonant Frequencies of Large Telescopes
38
Resonant Frequencies of Large Telescopes
Frequency (Hz)
Parabolic Reflector
Antenna Systems
Optics Systems (Laser/Infrared)
Lowest Servo Resonant Frequency
2Hz
Telescope Aperture
50m
39
Conceptual Design for an F/1 50m
Optical/IR Telescope
40
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
41
Concept Summary
Optical support structure
uses at least three levels
of active control
42
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
43
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….
44
An Enclosure for 50m -- “how big?”
75m
150m
75m
30 degrees
150m
• Restrict observing range to airmasses < 2.0
• “Astro-dome” approach
45
An Enclosure for 50m -- “how big?”
75m
150m
75m
30 degrees
150m
• 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
46
An Enclosure for 50m -- “how big?”
75m
150m
75m
30 degrees
150m
• 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
47
Framework for a credible Extremely Large/Maximum
Aperture Telescope Concept
Science Case
An adaptive optics
solution
A telescope concept
A viable instrument
model
48
Image of a 21st Century Ground-Based Observatory
-- 50m Class
49
50
How do we cost a 50m?
“What can it cost?”
(1999)
50m Telescope
costs (1997$))
 Coating & cleaning facilities
 Handling equipment
 Project office
$622M $522
$190M
Scaled costs
$11M
$78M
$70M
$26M
$35M Constrained
costs
$9M
$5M
$40M
• Contingency
$100M







Primary mirror assembly
Telescope structure & components
Secondary mirror assembly
Mauna Kea Site
Enclosures
Controls, software & communications
Facility instrumentation (A&G, AO)
Total
$1,086M
S (Keck + Gemini + ESO-VLT + Subaru) = $1,560M
51
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 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?
52
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
53
“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
54
“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
55