Systems Issues:
Optical Design & Fabrication
GSMT Working Group on Optical Design & Fabrication
David Anderson
Richard Buchroeder
Earl Pearson
Tom Sebring
Larry Stepp -- Chair
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
Any telescope larger than ~ 8 meters will have a
segmented primary mirror.
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For mirrors larger than ~ 8 meters, costs increase rapidly for:
 blank fabrication
 polishing
 transportation
 coating
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
Three large segmented-mirror telescopes already exist:
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Keck I
Keck II
Hobby Eberly
Several others are in work or have been proposed:
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Gran Telescopio Canarias (GTC)
Large Aperture Multi-Object Spectroscopic Telescope (LAMOST)
Mexican Infrared-Optical Telescope (TIM)
Southern African Large Telescope (SALT)
These projects serve as the starting point for the design of
any extremely large telescope
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
Key Systems Issues
Primary mirror segment size
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Segment size will affect the telescope structural design
Cost dictates segments smaller than about 2.5 meters
 Blank cost per square meter
 Low-cost optical finishing technologies
 planetary polishing
 replicating
 ion figuring
 Transportation
 Coating facilities
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
Key Systems Issues
Primary mirror segment size
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However, smaller segments also have drawbacks:
 Increased number of rigid points required on structure
 Increased number of actuators -- cost & reliability concerns
 Increased control system computational requirements
 Increased edge sensing error propagation
The optimum is likely to be in the range of 1-2 meters.
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
Key Systems Issues
Spherical or aspherical segments?
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Low-cost finishing technologies favor spherical segments
 planetary polishing
 replication (cost driven by number of masters)
Fixed-mirror telescope designs often use spherical primaries
For steerable telescopes, the optical design can provide better
performance with fewer elements with an aspherical primary
The low-cost production and testing of aspherical
segments is a key area for development.
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
Paraboloidal Segments
30-meter telescope
Segment Size (meters)
Focal Ratio
0.5
1.0
1.5
1.0
C4 = 369 m
C6 = 22 m
astig = 4.3 Mpa
coma = 2.9 Mpa
C4 = 56 m
C6 = 4 m
astig = 0.7 Mpa
coma = 0.5 Mpa
C4 = 18 m
C6 = 1 m
astig = 0.2 Mpa
coma = 0.2 Mpa
1.5
C4 = 810 m
C6 = 72 m
astig = 4.2 Mpa
coma = 4.3 Mpa
C4 = 123 m
C6 = 12 m
astig = 0.6 Mpa
coma = 0.7 Mpa
C4 = 38 m
C6 = 4 m
astig = 0.2 Mpa
coma = 0.2 Mpa
2.0
C4 = 1405 m
C6 = 170 m
astig = 4.1 Mpa
coma = 5.6 Mpa
C4 = 213 m
C6 = 28 m
astig = 0.6 Mpa
coma = 0.9 Mpa
C4 = 66 m
C6 = 9 m
astig = 0.2 Mpa
coma = 0.3 Mpa
Asphericity calculated for worst case (outer edge) segments
Stress calculated for a 50mm thick segment (varies linearly with thickness)
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
Paraboloidal Segments
50-meter telescope
Segment Size (meters)
Focal Ratio
0.5
1.0
1.5
1.0
C4 = 225 m
C6 = 8 m
astig = 2.6 Mpa
coma = 1.0 Mpa
C4 = 35 m
C6 = 1 m
astig = 0.4 Mpa
coma = 0.2 Mpa
C4 = 11 m
C6 = 0.4 m
astig = 0.1 Mpa
coma = 0.1 Mpa
1.5
C4 = 500 m
C6 = 26 m
astig = 2.6 Mpa
coma = 1.6 Mpa
C4 = 77 m
C6 = 4 m
astig = 0.4 Mpa
coma = 0.3 Mpa
C4 = 24 m
C6 = 1 m
astig = 0.1 Mpa
coma = 0.1 Mpa
2.0
C4 = 826 m
C6 = 62 m
astig = 2.5 Mpa
coma = 2.1 Mpa
C4 = 134 m
C6 = 10 m
astig = 0.4 Mpa
coma = 0.3 Mpa
C4 = 42 m
C6 = 3 m
astig = 0.1 Mpa
coma = 0.1 Mpa
Asphericity calculated for worst case (outer edge) segments
Stress calculated for a 50mm thick segment (varies linearly with thickness)
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
Key Systems Issues
Choice of segment configuration: petals vs hexagons
Petals
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All petals in each ring are
identical
Harder to polish (not close to
round shape)
Edge sensor positions vary
from one segment to another
LMS, 3/07/00
Hexagons
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Only six copies of each segment
type
Closer to circular shape --easier
to polish
Edge sensor positions are the
same for each segment
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
Key Systems Issues
The optical design will be driven by the requirements of
the science instruments
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Focal ratio
Field of view (& physical size of focal plane)
Image quality
Curvature of field
Control of distortion
Control of stray light
Photometric stability
Location, size and number of instruments
Strawman instrument designs are needed as early as
possible to guide the optical design work.
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
Key Systems Issues
Other optical issues
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Atmospheric Dispersion Compensation
Segmentation effects in the point spread function
 Coalignment effects
 Satellite images
 Diffraction spikes
Emissivity
 Boundaries between segments
 Large number of optical surfaces
 Contamination control
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
Key Systems Issues
Complementarity of active and adaptive optics
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Both active and adaptive optical systems will be needed
 Active optics (low bandwidth)
 Primary mirror segment position control
 Primary mirror segment figure correction
 Position control for secondary and tertiary mirrors
 Figure control for secondary and tertiary mirrors
 Adaptive optics (high bandwidth)
 Image stabilization (large tip-tilt mirror)
 Atmospheric compensation
 Correction of local seeing effects
 Fast correction of mirror figure errors
These two systems must complement each other.
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
Key Systems Issues
Integration of adaptive optics components
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In the Telescope
 Traditional AO applications have placed the adaptive
components far down the system
 In Gemini-ALTAIR the first deformable mirror is M6
 This helps keep the components small
 Recent concepts propose adaptive M4, M3, M2 or even M1
 Require locations conjugate to different heights, including zero
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In the Instruments
 To achieve performance goals, individual instruments may
need to incorporate:
 wavefront sensors
 tip-tilt mirrors
 deformable mirrors
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
Key Systems Issues
Control of wind buffeting
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The telescope may have structural resonances down to ~ 1 Hz
 The active segment alignment system must have a relatively
low bandwidth to avoid exciting structural resonances
Wind buffeting will cause relatively large structural deformations at
frequencies the active optics system may not be able to control
 The wind could excite resonances in the structure that have
large dynamic amplification factors
 Vortex shedding may introduce other oscillations
Wind buffeting can be reduced by a fully protective enclosure, but
this involves tradeoffs in enclosure cost and local seeing effects
Wind buffeting will increase the demands on the adaptive
optics system.
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
CY2000 Studies
Develop strawman designs of key science instruments
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Diffraction-limited narrow field of view
Diffraction-limited wider field of view
Seeing-limited wide field of view
Studies can be performed at NOAO and at Universities
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
CY2000 Studies
Measure wind loading on an 8-meter telescope
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Two coordinated studies are planned:
 Wind pressure on Gemini primary mirror surface
 Measure the spatial and temporal variation of wind
pressure on the Gemini M1 as a function of wind direction,
elevation angle and size of vent openings
 Principal investigator Dr. Myung Cho of Univ. of Arizona
 Response of the Gemini telescope structure to wind loading
 Measure the dynamic response of the Gemini telescope
structure to the wind pressures measured in the
coordinated study
 Principal investigator Dr. David Smith of Univ. of Mass.
These studies will provide key information for design of
the telescope structure and adaptive optics system.
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
CY2000 Studies
Develop technology for fabrication of aspheric segments
on a planetary polisher
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First phase, in CY2000, will be to prepare a paper study evaluating
feasibility and defining the technical approach
This will be followed in CY2001 by prototype fabrication studies
Possible contractors include Brashear, Carl Zeiss, Eastman Kodak,
Raytheon, REOSC, Tinsley and Zygo
This is a key investigation that could lead to a five-fold
reduction in the cost of polishing aspherical segments.
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO
Optical Design & Fabrication
CY2000 Studies
Investigate parallel work by other projects
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NOAO and Gemini staff will investigate work being done for other
projects, including OWL, CELT and NGST.
 Optical designs
 Optical fabrication
 Lightweight segment designs
 Segment control systems
 Actuator designs
Where possible, we will coordinate our studies with other
projects and we will investigate the possibility of costsharing arrangements.
LMS, 3/07/00
GSMT Systems Task Group Meeting
Boulder, CO