LARGE-OPTICS TECHNOLOGY LESSONS
FROM BALL’S NGST AND TPF PROGRAMS
STEVE KILSTON
BALL AEROSPACE & TECHNOLOGIES CORP.
HUBBLE’S SCIENCE LEGACY:
FUTURE OPTICAL/UV ASTRONOMY FROM SPACE
APRIL 5, 2002
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 1
Ball’s Two Large-Optics
Space Astronomy Programs
TPF
NGST
TRW-Ball
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 2
Main Topics
•
Key System Performance Capabilities
–
–
–
–
•
•
Wavelength Coverage
Angular Resolution
System Sensitivity and Dynamic Range
Usable FOV
TPF Visible-light Coronagraph Description
Relevant Technology Concerns
–
–
–
–
–
–
–
–
Instrument Technologies
Spacecraft Technologies
Launch and Deployment
Operational Reliability far from Earth
Robustness
Risks
Costs
Technology Development Roadmaps
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 3
System Performance Dependencies
PERFORMANCE
Wavelength
Coverage
Sensitivity
Min. Sun
Angle
Sky Coverage
Optical
FOV
Signal-to-Noise Ratio
Detector
Fill Factor
System
RMS Noise
Thermal
Stability
COST
Detector
Array Size
Slew
Time
System MTF
ACS -Related Jitter
Pixel
Subtense
Quantum
Efficiency
Orbit
Number of
Spacecraft
Detector
Size
System
Vibration
Ground
Segment
Costs
Space
Segment
Costs
Launch
Vehicle Sizes
Optical
Wavefront
Accuracy
Optical
Throughput
System Component
Costs
Launch
Costs
Angular Resolution
Focal
Length
Instrument
Cost
Data Rates &
Processing
Complexity
Spacecraft
Bus Cost
Target Flux
Spectral Band
Detector Noise
Momentof-Inertia
Integration
Time
On-Orbit
Lifetime
Stiffness &
Damping
Spacecraft Mass
Key:
Thermal System
ACS Size and Type
Aperture Diameter
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
Other
Ground
Segment
Factors
Propulsion System
Specified
Value
P. 4
Driving
Design
Factor
Comparison of NGST, TPF,
and Hubble-2 Performance
NGST
(~ 6-m aperture)
Wavelength Coverage
TPF
Hubble-2
(Visible-light, (8-m aperture,
4 x 10 m aper.) SUVO paper)
0.6 - 28 µm
0.3 - 1.0 µm
0.11 - 1.0 µm
Angular Resolution
30 mas
(λ/14 , 0.75 µm)
10 x 25 mas
(λ/140)
15 mas
(λ/14 , 0.5 µm)
System Sensitivity
MAB = 31
V = 34
MAB = 32
Usable FOV, arcmin
5x6
>2x2
12 x 12
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 5
Ball TPF Architecture Study Team
Astrophysics, Biomarkers, Optical
Science Lead,
Team Lead, StarLight Contractor,
Concepts, Wavefront Control,
Planets, Astrophysics, S/C, Optical Systems, Formation
Metrology, Controls, Orbits
Interferometry
Flying, Metrology, Mechanisms
Mid-Wave IR Imaging
and Instruments
Large Optical
Ground Systems
Design
Arch. Concepts, Beam and
Wavefront Control, Fourier
Transform Spectroscopy
Center for
Astrophysics
Planet Theory, Biomarkers
Biomarkers, Analysis,
Interferometry
Planet Atmospheres,
Chemical Evolution,
Biomarkers
Arch. Concepts,
Opt. Sys. Analysis
Astrophysics,Metrology,
Interferometry,
Formation Flying
Exoplanetary
System Detections
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
Large Optics,
Integration and
Test
IR Systems, Launch, Ops,
Propulsion, Detector Systems
S/C Controls,
Formation Flying
Gossamer and
Inflatable
Structures
P. 6
Some Leading Lights of the Ball TPF
Architecture Study Team
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 7
Ball TPF Visible-light Coronagraph -Major Features
•
•
•
•
•
Radial slices thru PSF
10
15
10
10
10
5
10
10 deg cone
20 deg cone
0 deg
Sky FOV (arcsec)
Light intensity (ph/s/cm
2
/um)
•
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Earth-like planet
0
0
2
4
Sky FOV (rad)
Image of star and
planet at focal plane
-6
6 x 10
-1
-0.5 0
0.5 1
Sky FOV (arcsec)
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
6
5.5
5
4.5
4
3.5
3
2.5
2
•
•
•
•
•
•
•
Science
Directly detect Earth-like planets in
the habitable zones surrounding
~ 150 solar-type stars within 20pc
Observe Biomarkers from 0.4-1.0 µ m
General Astrophysics research
utilizing the largest telescope
mirror in space
Optics
10 x 4 Meter Monolithic Primary,
4-cm thick + backplane, 940 kg
Active Optics for Wavefront Control
Off-Axis Design, plus shaped-pupil
options – very low diffraction
Spacecraft
Launch in 2015
Earth Trailing Drift-Away or L2 orbit
Delta IV H – direct insertion launch
3-Axis Stabilized Spacecraft
Large Thermal Shield
Launch Mass ~ 6,000 kg
5-Yr Design Life (10 yrs of
Expendables)
P. 8
Main Technology Issues
Affecting Performance
•
Optical Architecture & Size -- for adequate performance & to fit in launcher
•
Active Optics -- wavefront quality for large, thin optics; mechanical or thermal
•
Wavefront Sensing -- to guide active optics and/or a deformable mirror
•
Coatings, Filters, & Passbands -- UV operation a challenge
•
Stability: Thermal & Mechanical -- for efficient use of AO; shields, damping
•
Testability -- large test chamber, demonstration of end-to-end performance
•
Contamination -- mild requirements for visible-light system, strict for UV
•
Detectors -- CCD successors, better UV QE; maybe λ info., photon counting
•
Radiation-hardening of Electronics -- vital for deep space operation
•
Observing Efficiency -- slew rates, settle times, field-of-regard
•
Orbit and Launcher -- driven by thermal stability, data link, servicing needs?
•
Mission Time Frame -- affects technology development possibilities
•
Dual-use Missions -- TPF as Hubble-2 affects design, does astrophysics
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 9
Large Space Optics Issues
•
Segmented vs. monolith -- monolith difficult, but minimizes diffraction
–
Phasing of segments, phase stability and maintenance
–
Segment latches for deploy/redeploy
•
Elliptical vs. circular -- elliptical easier to package, maximizes resolution
•
Off-axis vs. on-axis -- clear aperture needed for coronagraph
•
Deployable vs. rigid structures
•
Material -- Glass vs. composite vs. Be; elastic memory materials
–
Precision, lightweight (<25 kg/m 2) glass honeycomb developed by Kodak - baseline for
TPF coronagraph-- lowers mass, thus mission cost; sets best operating temperatures
• Optical quality excellent for large UV telescopes
•
•
–
Difficult to achieve optical quality soon w/ thin films (stretched membranes, shells)
–
Nonlinear behavior in zero-g environment
Laboratory development needs to begin now if monolith (long-lead item)
–
Start with 0.5 - 1.0 m flat mirror, move on to half-scale powered mirror (2 x 5 m)
–
Validation in space? Flight system test & optical alignment techniques
Passive vs. controlled surface and/or Deformable/Correction optics
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 10
Segmented Optics Construction
• Monolithic primary mirror built from segments drawing on AMSD technology
• AMSD mirror/actuator system areal density = 15 kg/m2 (TPF allocates 35 kg/m2)
• Core and faceplate segments fused together to form the monolithic PM blank
•
Existing Kodak facilities and capabilities will be utilized to produce segments
•
New Kodak facilities will be needed to process and test the full primary mirror
•
30 cm ULE blank
•
The TPF monolithic PM will have a
segmented front and back plane
–
–
–
–
–
–
Segmented core
Segmented back plate
19 mm deep core
Low Temp Fusion of segments
Proof of concept blank
Core strut thickness of 0.02” (0.5 mm)
demonstrated
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
TAKE PICTURES.
FURTHER.
P. 11
TPF Primary Mirror
Technology Development Plan
OBJECTIVES
Proof-of-Principle
Produce 1/2
Meter Flat
•Develop processing technique using a flat lightweight optic with
segmented face sheets.
•Demonstrate low spatial frequency surface control with actuators
Segment
Demonstration
Produce 1+ meter
powered segment
• Repeat above objectives on larger
lightweight optic with power.
Sub-scale Development
Design, and build 1/4
scale Primary Mirror (PM)
TAKE PICTURES.
FURTHER.
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
• Achieve TPF requirements
on subscale PM
Full-scale Development
Design and build full scale
Primary Mirror
P. 12
Active Optics
• One stage of correction for NGST, two for a TPF coronagraph
– Actuators between primary mirror back surface and backplane, ~ 10/m2
• Stroke - tens of µm; setting resolution ~ 7 nm
– Deformable mirror (DM) for mid spatial frequencies, ~ 250 x 100 elements
• Stroke - ~ 100 nm, compensates for primary mirror actuator inaccuracy
• Setting resolution (iterative setting avoids absolute accuracy reqts.)
– Xinetics DMs have achieved resolution of 25 pm
• Temporal and thermal stability requirements
– Xinetics DMs stable to 100 pm over weeks
< 10 mK temperature stability needed
• Need accelerated life tests to prove space durability
• Other types and suppliers of DMs are entering the arena
DM
• Comparison to ground-based Adaptive Optics
– Much lower temporal frequency required in space -- DM fixed for hours
– Wide field of view possible in space -- no isoplanatic angle
– Atmospheric dispersion in visible/UV limits spectral bandwidth
• Potential use for correcting amplitude non-uniformity
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 13
Mirror Coatings and Spectral Passbands
•
Tradeoff of throughput vs. spectral range (especially UV)
– Silver gives high reflectivity, but no UV performance
– Aluminum allows operation in the UV
– Reflectivity ~ 92-96% per mirror across the visible band
– ~ 50-60% throughput after 10 mirrors
– TPF coronagraph design uses Al for primary and secondary mirror, Ag for
subsequent mirrors
• Pickoff mirror could feed UV instruments (no access to DM)
• Example of dual-use mission
•
Durability and technical challenges
•
System spectral measurement accuracy involve filter and detector
performances and stabilities
– Good calibration devices and processes are needed
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 14
Wheel Isolation Greatly Reduces LOS
Jitter at High Frequencies
2
With isolator included
1
10
1 mas
allocation
1
10
Large, slow
RW set
10
4 Hz
0
RMS jitter (mas)
RMS jitter (mas)
10
0
10
-1
10
-2
10
Small/fast
RW set
-3
10
0
10
1
10
2
Choose
small/ fast
RW
10
Reaction wheel speed (RPS)
•
•
•
•
-1
10
10
-2
1.5 Hz
isolation
-3
10
0
10
1
10
2
10
Reaction wheel speed (RPS)
Line of sight jitter is below the 1 mas RMS requirement over most speeds.
Small/fast wheel produces less jitter than the larger wheel
HST-class wheel with 1.5 Hz isolation will meet requirements except at a few
resonances.
These results are open-loop – they don’t engage fine steering mirror pointing loop
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 15
Dynamic mirror distortion due to wheel
harmonics is expected to be tolerable
• Without isolator:
Movie of primary mirror deformation
no RWA isolation
• Worst-case primary
mirror deformation
(41 Hz)
Movie of star/planet PSF no
RWA isolation
planet
• Gives spatial/ temporal
RMS = 8.4nm
(~17 nm WF error)
• With 1.5 Hz isolator:
• Worst-case primary
deformation (4.6 Hz)
Movie of primary mirror deformation
with RWA isolation
planet
• Spatial/temporal RMS
0.05 nm (~ 0.1 nm
WF error)
• Doubling wheel speed
expected to suppress
by > 10x
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
Note : Tip/tilt component removed
- deformation only.
P. 16
Summary of Initial Design of Thermal
Control Subsystem
Thermal Design Provides Accurate Control over
Primary Mirror Temperature variations
Spatial variations (over 24-hour cycle) Controlled to
within 0.03 ºC
Without full Sunshield, Primary Mirror Spatial
variations Exceed 0.12 ºC
Active Heater Control of Optical Bench is Required Passive Design Would Result in Primary Mirror
Temperatures of -140 ºC
Primary Mirror Operating Temperatures of 0 ºC are
Favored: Simplifies Manufacturing Testing &
Calibration. Also at 0ºC CTE (Coefficient of
Thermal Expansion) of ULE, Zerodur are Minimized
Active Heating Provides Precise Control over Primary
Mirror Temperature Gradients - Bench Heater
Power Requirement ~ 700 W
ZERODUR CTE Curve
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 17
Observatory Ground Test Overview
(Integrated System Test)
•
•
•
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
Several Facilities to choose from
– AEDC Mark I φ12.8m x 25m
– GRC SPF φ30.5m x 37.2m
– Johnson A φ16.8m x 27.4m
Test Set up Evaluated for Plum
Brook Space Power Facility
– Largest Facility
– Very Low vibration level
– NASA controlled facility
Test the Entire Observatory
– Only the Sunshield & Array
Removed for Testing
– Vertical orientation eliminates
moments into primary aperture
– Vacuum Test at on-orbit
thermal environment
P. 18
Delta IV (4050-H19) Heavy Launch
Configuration
Baseline 4 x 10 Primary
Mirror
•
Robust Launch Margins
•
Direct Injection to
Heliocentric Earth Trailing
Drift Away Orbit
Several Packaging Approaches
still need to be explored
Currently not taking full
advantage of Delta IV
capabilities (35% Launch
Margin)
Need to work trades to
maximize primary mirror with
adequate launch margin (20 to
25% pre-phase A margins
should be acceptable)
•
•
•
–
–
Launch Mass ~ 6,000 kg
Launch Capability of 9,255 kg
to C3 = 0.3 km 2/sec 2
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
Alternative
4 x 13 Primary
Mirror
P. 19
TPF Visible-Light Coronagraph -Technology Readiness and Prospects
NASA TECHNOLOGY READINESS LEVELS
Technology
Current
TRL
Future work
Estimated
Completion
Date
Resulting
TRL
Large, lightweight
optics
3
Lab demo of scale
models
2005
6
Wavefront sensing with
science camera
3
TPF techology
testbed demos
2005
6
Deformable Mirrors
5
Lifetime tests in lab
2003
6
Thermal Control
4
NGST validation
2008
7
2
Lab demo of full
scale masks
2003
6
6
NGST validation
2008
8
Binary Pupil Masks with
edge adjustment (CG)
Integrated model of full
optical system
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 20
Risk Matrix for UV/isible Coronagraph
Technology
Risk Level
Large Optics
Moderate
Wavefront Stability
Moderate
Amplitude Uniformity
Moderate
Wavefront Sensing
Low
Deformable Mirrors
Low
Binary Pupil Masks (option)
Low
Graded FPA Masks (option)
Moderate
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 21
Conclusions and Recommendations for
UV/Optical Hubble 2
•
A large-mirror telescope with wavefront quality excellent for UV can
be built and tested, and is feasible for launch in the 2015 timeframe
–
–
–
–
Technology development should be adequately supported
Active-optics primary actuators could enable high-quality segmented mirror
DM could set wavefront to accuracy needed for coronagraph (λ/10,000)
Needs high mechanical stability throughout an observation
• Maintains wavefront accuracy
• Therefore thermal & vibrational control
– Elliptical monolith primary would fit in launch fairing
•
Thermal contol to millikelvin levels may be the key technology need
– Thermal shield plus computerized network of temp. sensors and heaters
– Need to model and demonstrate level achievable
•
Will be expensive (up to $1.5 B), but can host several instruments
– High spatial and spectral resolution, good FOV and UV/optical- coverage
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 22
TPF Technology Studies Roadmap
Coronagraph Technology Areas
Proof-of-Conc. Studies
Large, lightweight
optics
Lab Demonstrations
Wavefront Sensing
Integrated Modeling
Pupil and FP Masks
Wavefront Stability
Deformable Mirrors
Amplit.Uniformity,
Thermal and
Vibrational Effects
Common
Technologies
Phenomenology
TPF Test Benches
- Common Scene
Lab Demonstrations
Cryogenic Actuators
Cryogenic Nulling
Angle
Tracking/Metrology
Formation Flying
Control
Science Developments
Kepler
Keck Interferometry
Interferometry Technology Areas
Coronagraph and
Interferometer
Comparison Tests
LBT Interferometry
Integrated
Modeling
Wavefront
Stability
Amplitude
Uniformity,
Thermal and
Vibrational Effects
Precursor or Parallel
Technology Developments
Eclipse or similar missions
NGST
ESA Studies
StarLight
TPF Architecture
Selection
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
SIM
P. 23
Dual-use Missions
•
TPF/SUVO is the main example (if TPF is a visible-light coronagraph)
– TPF is charged with doing astrophysics 30%-50% of the time
– Both need large, high-accuracy mirrors
• TPF in visible needs primary mirror better than λ/30 at UV
– High pointing accuracy & stable thermal environment permit long exposures
– A pickoff mirror after the Al-coated primary and secondary mirrors can
divert the light beam into UV instruments
•
Even a 4-m TPF/SUVO could detect Earths at 8 pc, and many Jupiters
•
Operational reliability without servicing for deep-space missions could
lead to more dual-use missions (which presumably get higher funding)
– Bigger funding levels could increase reliability (not faster, better, cheaper)
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 24
Operational Lifetime
•
Need to figure out optimum lifetime (science likes long life; costs much)
•
Operational reliability without servicing for deep-space missions could
lead to more dual-use missions (which presumably get higher funding)
– HST had 240 serviceable components, needed to replace only 33 in 12 years
– Bigger funding levels could increase reliability (not faster, better, cheaper)
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 25
Wavefront Sensing (WFS)
•
WFS method depends on required accuracy
– Shack-Hartmann method limited to ~λ/100 or coarser
• Adequate for ground-based adaptive optics
– Phase-diversity methods can potentially reach λ/10,000
• Needed for high contrast coronagraphy
• Algorithms require demonstration at high accuracy levels
• No additional hardware required - uses science camera
– TPF/SUVO combination could use this + mirror actuators for UV observing
•
Integration time as a function of accuracy
– Limits sensitivity if WFS time is limited (e.g. ground-based observing)
– Limits observing efficiency otherwise
•
Update frequency depends upon system stability
– Achievable stability for TPF coronagraph has not yet been fully quantified
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 26
Contamination
•
“Easy” in visible/IR - hard in UV (affected by few nm thickness)
- Affects options: closed vs. open structure, intermittent heating
MOLECULAR: multi-molecular drops ~1-10 nm high
PARTICLES: dominant particle sizes ~0.1–1.0 µm
MICROMETEORITES: impacts on the telescope system will be common
Molecular deposit quantities
At launch
~ 0.5 - 1 nm
Steady-state on orbit
~ 0.5 - 1.5 nm
Molecular contaminant rates
—
< 0.004 nm / day
Level 100 - 200
Level 100 - 200
Quiescent periods
Nil
Nil
During meteor impact
—
TBD
Particle distributions
Particle redistribution rates
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 27
Detector Technology -Summary of Characteristics and Types
----------------------------------------------------------------------------------------------------CHARACTERISTICS
CCD Si Hybrid MCP
APD
STJ Microbolo InSb
----------------------------------------------------------------------------------------------------Wavelength coverage ( µm) 0.1-1
0.1-1
0.01-1
0.1-1 0.01-5
any
0.4 - 5
Pixel pitch ( µm)
6-25
17-30
?
?
?
?
25 -50
QE (%, UV has lower QE)
>80
>80
>90
>90
?
?
85
Read noise (electrons)
3*
3*
0
0
?
?
15
Dark current (counts/s/pix) 0.01
?
?
?
?
?
0.01
Overlight damage
N
N
Y
N
N
N
N
CR degradation
low
low
low
?
?
?
low
Format size (2001)
4K x 4K 1K x 1K 1K x 1K
?
10x10 100x100 4K x 4K
Energy resolution
N*
N*
N(?)
N(?)
Y
Y
N
Time stamping
N*
N*
Y
Y
Y
Y
N
Operating temperature (K) 150
30
150
150
0.1
0.1
77
Power supply
5V
5V
1 KV
5V
10 mV
1 mV
15 V
Technology readiness
high moderate high
low
low
low
high
----------------------------------------------------------------------------------------------------* Modified CCDs and hybrid devices that support non-destructive read modes can in principle have
less than 1 e/read effective read noise and can therefore be used to detect individual photons
Modified CCDs can time-tag photon arrival to within a frame read-time
Since there is no read-noise penalty, low dark-current devices can be mated to integral field
spectrographs to determine the energy of each photon
Backup chart
P. 28
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
Stability: Thermal and Mechanical
•
Balance between passive and active is a system level trade
– Depends on relevant timescales
•
Performance requirements for TPF coronagraph appear to require both
passive and active thermal controls
–
–
–
–
–
•
Large sunshields, insulation, and ULE glass needed for passive control
Distributed heating elements for active control
Open vs. closed loop control
Open Loop - thermal modeling can predict required heater settings
Closed Loop - network of temperature sensors leads to real time feedback
Effects of localized vibration sources (reaction wheels, thrusters) can
be suppressed with isolators
– Disturbances are high-frequency, allowing strong suppression
•
•
Microsnap (spontaneous release of strain) is poorly quantified and
difficult to isolate, since distributed throughout the structure
Integrated modeling required for optimal system design
– Quantifies performance advantages of thermal & mechanical design options
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 29
Thermal effects on wavefront error
•
First order finite-element thermal model of primary mirror
– With full sunshield
– Divided mirror into 500 elements
– Examined effects of large azimuth slews (no change in star)
separated by 8 hour integrations
•
Results
– Front-to-back temperature difference changed by 1.2 mK over 8 hours
• Coherent across full mirror, with rms ripple of ~0.1 mK
– We calculate 0.03 Å peak-peak ripple on front surface due to glass only
• Assumed ULE glass with CTE of 10 -8/K
•
The mirror backing structure has not been included yet
– Will add a large low-frequency contribution to deformation
– Design has not been optimized to control thermal deformations
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 30
Abstract
Ball Aerospace and Technologies Corp. and its partners have been
considering many large space-astronomy design concepts, especially
through our work on the Next Generation Space Telescope (NGST) and
Terrestrial Planet Finder (TPF) programs. During those efforts our teams
strove to be open-minded and creative about design approaches and
suitable technology elements. Our evaluations of the candidate concepts
were based on performance, costs, and technology risks and required
development. We assessed many performance factors relevant to an
optical-UV successor to the Hubble Space Telescope (HST), such as
system sensitivity, angular resolution, and usable fields of view. Our teams
investigated key technology issues such as attitude control, thermal and
vibrational disturbances, ease of launch and deployment, and operational
reliability beyond Earth orbit (if that's required). In the case of TPF we
considered several large-optics designs operating at optical wavelength
bands, with wavefront quality more than adequate for UV operation too.
We'll discuss here some of our work's potential implications for choosing
an HST optical-UV successor's technologies.
S. KILSTON -- LARGE-OPTICS TECHNOLOGY LESSONS
P. 31