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- Front Matter: Volume 6687
- JWST
- Interferometry I
- Interferometry II
- Systems and Concepts
- Telescopes and Mirrors I
- Telescopes and Mirrors II
- Wavefront Sensing and Control (WFSC)
- TPF External Occulter I
- TPF External Occulter II
- Formation Flying
Front Matter: Volume 6687
Front Matter: Volume 6687
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This PDF file contains the front matter associated with SPIE Proceedings Volume 6687, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.
JWST
Technology demonstration of large stable cryogenic composite structures for JWST
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The need for JWST's metering structure to be stable over time while at cryogenic temperatures is derived from its
scientific objectives. The operational scenario planned for JWST provides for the optical system to be adjusted on
regular intervals based upon image quality measurements. There can only be a limited amount of optical
degradation between the optical system adjustments in order to meet the scientific objectives. As the JWST primary
mirror is segmented, the structure supporting the mirror segments must be very stable to preclude degradation of the
optical quality. The design, development and, ultimately, the verification of that supporting structure's stability rely
on the availability of analysis tools that are credibly capable of accurately estimating the response of a large
structure in cryogenic environments to the nanometer level. Validating the accuracy of the analysis tools was a
significant technology demonstration accomplishment. As the culmination of a series of development efforts, a
thermal stability test was performed on the Backplane Stability Test Article (BSTA), demonstrating TRL-6 status
for the design, analysis, and testing of Large Precision Cryogenic Structures. This paper describes the incremental
development efforts and the test results that were generated as part of the BSTA testing and the associated TRL-6
demonstration.
Development of electronic speckle pattern interferometry for testing JWST composite structures
Show abstract
The stability requirements for the James Webb Space Telescope (JWST) optical metering structure are driven by the
science objectives of the mission. This structure, JWST Optical Telescope Element (OTE) primary mirror backplane, has
to be stable over time at cryogenic temperatures. Successful development of the large, lightweight, deployable,
cryogenic metering structure requires verification of structural deformations to nanometer level accuracy in
representative test articles at cryogenic temperature. An instantaneous acquisition phase shifting speckle interferometer
was designed and built to support the development of JWST Optical Telescope Element (OTE) primary mirror
backplane. This paper discusses characterization of the Electronic Speckle Pattern Interferometer (SPS-DSPI) developed
for JWST to verify its capabilities to measure structural deformations in large composite structures at cryogenic
temperature. Interferometer performance during the Backplane Stability Test Article (BSTA) test that completed the
TRL-6 (Technology Readiness Level-6) demonstration of Large Precision Cryogenic Structures will also be discussed.
Measurement of structural damping in the backplane of the James Webb Space Telescope
Show abstract
This document describes the Cryogenic Damping Test (CDT) of the James Webb Space
Telescope (JWST) Backplane Structural Test Article (BSTA). Contained in this report
are descriptions of test configuration, highlights of data, review of methods used to
extract modal parameters, and presentation of results and conclusions.
End-to-end commissioning demonstration of the James Webb Space Telescope
Show abstract
The one-meter Testbed Telescope (TBT) has been developed at Ball Aerospace to facilitate the
design and implementation of the wavefront sensing and control (WFSC) capabilities of the
James Webb Space Telescope (JWST). We have recently conducted an "end-to-end"
demonstration of the flight commissioning process on the TBT. This demonstration started with
the Primary Mirror (PM) segments and the Secondary Mirror (SM) in random positions,
traceable to the worst-case flight deployment conditions. The commissioning process detected
and corrected the deployment errors, resulting in diffraction-limited performance across the
entire science FOV. This paper will describe the commissioning demonstration and the WFSC
algorithms used at each step in the process.
Using multifield measurements to eliminate alignment degeneracies in the JWST testbed telescope
Show abstract
The primary mirror of the James Webb Space Telescope (JWST) consists of 18 segments and is 6.6 meters in diameter.
A sequence of commissioning steps is carried out at a single field point to align the segments. At that single field point,
though, the segmented primary mirror can compensate for aberrations caused by misalignments of the remaining
mirrors. The misalignments can be detected in the wavefronts of off-axis field points. The Multifield (MF) step in the
commissioning process surveys five field points and uses a simple matrix multiplication to calculate corrected positions
for the secondary and primary mirrors. A demonstration of the Multifield process was carried out on the JWST Testbed
Telescope (TBT). The results show that the Multifield algorithm is capable of reducing the field dependency of the TBT
to about 20 nm RMS, relative to the TBT design nominal field dependency.
TRL-6 for JWST wavefront sensing and control
Show abstract
NASA's Technology Readiness Level (TRL)-6 is documented for the James Webb Space Telescope (JWST) Wavefront
Sensing and Control (WFSC) subsystem. The WFSC subsystem is needed to align the Optical Telescope Element
(OTE) after all deployments have occurred, and achieves that requirement through a robust commissioning sequence
consisting of unique commissioning algorithms, all of which are part of the WFSC algorithm suite. This paper identifies
the technology need, algorithm heritage, describes the finished TRL-6 design platform, and summarizes the TRL-6 test
results and compliance. Additionally, the performance requirements needed to satisfy JWST science goals as well as the
criterion that relate to the TRL-6 Testbed Telescope (TBT) performance requirements are discussed.
Microshutter array system for James Webb Space Telescope
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We have developed microshutter array systems at NASA Goddard Space Flight Center for use as multi-object
aperture arrays for a Near-Infrared Spectrometer (NIRSpec) instrument. The instrument will be carried on the
James Webb Space Telescope (JWST), the next generation of space telescope, after the Hubble Space
Telescope retires. The microshutter arrays (MSAs) are designed for the selective transmission of light from
objected galaxies in space with high efficiency and high contrast. Arrays are close-packed silicon nitride
membranes with a pixel size close to 100x200 μm. Individual shutters are patterned with a torsion flexure
permitting shutters to open 90 degrees with minimized stress concentration. In order to enhance optical
contrast, light shields are made on each shutter to prevent light leak. Shutters are actuated magnetically,
latched and addressed electrostatically. The shutter arrays are fabricated using MEMS bulk-micromachining
and packaged utilizing a novel single-sided indium flip-chip bonding technology. The MSA flight system
consists of a mosaic of 2 x 2 format of four fully addressable 365 x 171 arrays. The system will be placed in
the JWST optical path at the focal plane of NIRSpec detectors. MSAs that we fabricated passed a series of
qualification tests for flight capabilities. We are in the process of making final flight-qualified MSA systems
for the JWST mission.
Interferometry I
System engineering the Space Infrared Interferometric Telescope (SPIRIT)
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The Space Infrared Interferometric Telescope (SPIRIT) was designed to accomplish three scientific objectives: (1) learn
how planetary systems form from protostellar disks and how they acquire their inhomogeneous chemical composition;
(2) characterize the family of extrasolar planetary systems by imaging the structure in debris disks to understand how
and where planets of different types form; and (3) learn how high-redshift galaxies formed and merged to form the
present-day population of galaxies. SPIRIT will accomplish these objectives through infrared observations with a two
aperture interferometric instrument. This paper gives an overview of SPIRIT design and operation, and how the three
design cycle concept study was completed. The error budget for several key performance values allocates tolerances to
all contributing factors, and a performance model of the spacecraft plus instrument system demonstrates meeting those
allocations with margin.
The Space Infrared Interferometric Telescope (SPIRIT): optical system design considerations
Show abstract
The Space Infrared Interferometric Telescope (SPIRIT) was designed to accomplish three scientific objectives: (1) learn
how planetary systems form from protostellar disks and how they acquire their inhomogeneous chemical composition;
(2) characterize the family of extrasolar planetary systems by imaging the structure in debris disks to understand how
and where planets of different types form; and (3) learn how high-redshift galaxies formed and merged to form the
present-day population of galaxies. SPIRIT will accomplish these objectives through infrared observations with a two
aperture interferometric instrument. This paper gives an overview into the optical system design, including the design
form, the metrology systems used for control, stray light, and optical testing.
Mechanical design of the Space Infrared Interferometric Telescope (SPIRIT)
Show abstract
The Space Infrared Interferometric Telescope (SPIRIT), a candidate NASA Origins Probe mission, is a cryogenic 6-36m
variable-baseline imaging interferometer operating at 25 - 400 μm. SPIRIT utilizes dual, meter-class, telescopes which
translate along opposed deployable booms. The collimated beams from the telescopes are combined in a central
instrument module operating at 4K and lower. Mission-enabling mechanisms include the large, optical delay line scan
mechanism, the afocal collector telescope trolley drives, and the boom deployment mechanisms. This paper provides an
overview of the mechanical aspects of the conceptual design created to meet the challenging instrument requirements.
The SPIRIT thermal system
Show abstract
The Space Infrared Interferometric Telescope (SPIRIT) is envisioned to be a pair of one meter diameter primary
light collectors on either side of a beam combiner, all cooled to 4 K or lower. During an observation, the
collectors are required to move toward and away from the beam combiner to obtain information at various
baselines to simulate a filled aperture. The thermal design of this mission as presented in this paper provides each
light collector and the beam combiner with separate cryogenic systems. This allows the boom that attaches the
combiner and collectors, the motors and many of the mechanisms to operate at room temperature, thus simplifying
ground testing and reducing mission cost and complexity. Furthermore, the cryogenic systems consist of passive
radiators and mechanical coolers - a cryogen-free approach. This paper gives a description of the requirements
and resulting design for this architecture and some of the benefits and difficulties of this approach. A subscale
thermal vacuum test of one of the collector thermal systems was performed. The thermal model and test agreed
very well showing the viability of the thermal design and subscale cryo-thermal test approach.
Cryogenic far-infrared detectors for the Space Infrared Interferometric Telescope (SPIRIT)
Show abstract
SPIRIT is a spatial and spectral interferometer with an operating wavelength range 25 μm - 400 μm. As a double-Fourier interferometer, SPIRIT features sub-arcsecond spatial resolution and R≡λ/Δλ=3000 spectral resolution over a 1 arcmin field of view. Its three primary scientific objectives are to: (1) Learn how planetary systems form from protostellar disks, and how they acquire their chemical organization; (2) Characterize the family of extrasolar planetary systems by imaging the structure in debris disks to understand how and where planets form, and why some planets are ice giants and others are rocky; and (3) Learn how high-redshift galaxies formed and merged to form the present-day population of galaxies. The detector subsystem provides a set of far-infrared detector arrays in the SPIRIT instrument. These arrays are used for science purposes by detecting the faint interferometric signal. The resulting technology requirement is for a set of eight arrays operating at wavelengths of 25 μm - 400 μm, divided into two arrays (one for each interferometer output port) per octave of wavelength. At the short wavelength end, the arrays are 14×14 pixels, shrinking to 2×2 at the longest band. The per-pixel sensitivity requirement of 10-19 W/√Hz, coupled with speed of τeffective ~150 μs, make these relatively small arrays challenging. The operating temperature necessary to provide this sensitivity is around 50 mK. Over the majority of the SPIRIT wavelength range and sensitivity requirement, there are no commercial vendors of such detector arrays, and thus they will require a separate NASA-supported development.
Interferometry II
The wide-field imaging interferometry testbed: enabling techniques for high angular resolution astronomy
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The Wide-Field Imaging Interferometry Testbed (WIIT) was designed to develop techniques for wide-field of view imaging
interferometry, using "double-Fourier" methods. These techniques will be important for a wide range of future space-based
interferometry missions. We have provided simple demonstrations of the methodology already, and continuing
development of the testbed will lead to higher data rates, improved data quality, and refined algorithms for image reconstruction.
At present, the testbed effort includes five lines of development; automation of the testbed, operation in an
improved environment, acquisition of large high-quality datasets, development of image reconstruction algorithms, and
analytical modeling of the testbed. We discuss the progress made towards the first four of these goals; the analytical
modeling is discussed in a separate paper within this conference.
Direct UV/optical imaging of stellar surfaces: the Stellar Imager Vision Mission
Show abstract
The Stellar Imager (SI) is a UV/optical, space-based interferometer designed to enable 0.1 milli-arcsecond (mas) spectral
imaging of stellar surfaces and, via asteroseismology, stellar interiors and of the Universe in general. SI's science focuses
on the role of magnetism in the Universe, particularly on magnetic activity on the surfaces of stars like the Sun. SI's
prime goal is to enable long-term forecasting of solar activity and the space weather that it drives, in support of the
Living with a Star program in the Exploration Era. SI will also revolutionize our understanding of the formation of
planetary systems, of the habitability and climatology of distant planets, and of many magneto-hydrodynamically
controlled processes in the Universe. SI is a "Flagship and Landmark Discovery Mission" in the 2005 Sun Solar System
Connection (SSSC) Roadmap and a candidate for a "Pathways to Life Observatory" in the Exploration of the Universe
Division (EUD) Roadmap (May, 2005). We discuss herein the science goals of the SI Mission, a mission architecture
that could meet those goals, and the technologies needed to enable this mission. Additional information on SI can be
found at: http://hires.gsfc.nasa.gov/si/.
Wavefront sensing and closed-loop control for the Fizeau interferometry testbed
Show abstract
Stellar Imager (SI) is a proposed NASA space-based UV imaging interferometer to resolve the stellar disks of nearby
stars. SI would consist of 20 - 30 separate spacecraft flying in formation at the Earth-Sun L2 libration point. Onboard
wavefront sensing and control is required to maintain alignment during science observations and after array
reconfigurations. The Fizeau Interferometry Testbed (FIT), developed at the NASA/Goddard Space Flight Center, is
being used to study wavefront sensing and control methodologies for Stellar Imager and other large, sparse aperture
telescope systems. FIT initially consists of 7 articulated spherical mirrors in a Golay pattern, and is currently undergoing
expansion to 18 elements. FIT currently uses in-focus whitelight sparse aperture PSFs and a direct solve phase retrieval
algorithm to sense and control its wavefront. Ultimately it will use extended scene wavelength, with a sequential
diversity algorithm that modulates a subset of aperture pistons to jointly estimate the wavefront and the reconstructed
image from extended scenes. The recovered wavefront is decomposed into the eigenmodes of the control matrix and
actuators are moved to minimize the wavefront piston, tip and tilt in closed-loop. We discuss the testbed, wavefront
control methodology and ongoing work to increase its bandwidth from 1 per 11 seconds to a few 10's of Hertz and show
ongoing results.
Fresnel interferometric arrays for space-based imaging: testbed results
Show abstract
This paper presents the results of a Fresnel Interferometric Array testbed. This new concept of imager involves
diffraction focussing by a thin foil, in which many thousands of punched subapertures form a pattern related
to a Fresnel zone plate. This kind of array is intended for use in space, as a way to realizing lightweight large
apertures for high angular resolution and high dynamic range observations. The chromaticity due to diffraction
focussing is corrected by a small diffractive achromatizer placed close to the focal plane of the array.
The laboratory test results presented here are obtained with an 8 centimeter side orthogonal array, yielding
a 23 meter focal length at 600 nm wavelength. The primary array and the focal optics have been designed and
assembled in our lab. This system forms an achromatic image. Test targets of various shapes, sizes, dynamic
ranges and intensities have been imaged. We present the first images, the achieved dynamic range, and the
angular resolution.
An optical model of the wide-field imaging interferometry testbed
Show abstract
This paper describes computational results obtained with a high-fidelity optical model of the Wide-Field Imaging
Interferometry Testbed (WIIT). The WIIT model includes imperfections inherent in the hardware testbed, such as
deviations of the mirrors from their ideal shapes. Model interferograms (brightness in a detector pixel as a function of
optical delay) are presented here for several representative test scenes "observed" with multiple interferometric
baselines. The results match theoretical expectations and can be compared with real WIIT measurements to identify and
characterize instrumental and environmental artifacts in our laboratory data, and to aid in the interpretation of those data.
Beam combination for Stellar Imager and its application to full-aperture imaging
Show abstract
Stellar Imager (SI) will be a Space-Based telescope consisting of 20 to 30 separated apertures. It is designed
for UV/Optical imaging of stellar surfaces and asteroseismology. This report describes details of an alternative
optical design for the beam combiner, dubbed the Spatial Frequency Remapper (SFR). It sacrifices the large
field of view of the Fizeau combiner. In return, spectral resolution is obtained with a diffraction grating rather
than an array of energy-resolving detectors. The SFR design works in principle and has been implemented with
MIRC at CHARA for a small number of apertures. Here, we show the number of optical surfaces can be reduced
and the concept scales gracefully to the large number of apertures needed for Stellar Imager.
We also describe a potential application of this spatial frequency remapping to improved imaging with filled-aperture
systems. For filled-aperture imaging, the SFR becomes the core of an improved aperture masking
system. To date, aperture-masking has produced the best images with ground-based telescopes but at the
expense of low sensitivity due to short exposures and the discarding of most of the light collected by the telescope.
This design eliminates the light-loss problem previously claimed to be inherent in all aperture-masking designs.
We also argue that at least in principle, the short-integration time limit can also be overcome. With these
improvements, it becomes an ideal camera for TPF-C; since it can form speckle-free images in the presence of
wavefront errors, it should significantly relax the stability requirements of the current designs.
Systems and Concepts
Ares V launch capability enables future space telescopes
Show abstract
NASA's Ares V cargo launch vehicle offers the potential to completely change the paradigm of future space science
mission architectures. A major finding of the NASA Advanced Telescope and Observatory Capability Roadmap Study
was that current launch vehicle mass and volume constraints severely limit future space science missions. And thus, that
significant technology development is required to package increasingly larger collecting apertures into existing launch
shrouds. The Ares V greatly relaxes these constraints. For example, while a Delta IV has the ability to launch
approximate a 4.5 meter diameter payload with a mass of 13,000 kg to L2, the Ares V is projected to have the ability to
launch an 8 to 12 meter diameter payload with a mass of 60,000 kg to L2 and 130,000 kg to Low Earth Orbit. This
paper summarizes the Ares V payload launch capability and introduces how it might enable new classes of future space
telescopes such as 6 to 8 meter class monolithic primary mirror observatories, 15 meter class segmented telescopes, 6 to
8 meter class x-ray telescopes or high-energy particle calorimeters.
Large infrared telescopes in the exploration era: SAFIR
Show abstract
The Single Aperture Far Infrared (SAFIR) observatory - a concept design for a 10m-class spaceborne far- infrared and
submillimeter telescope, has been proposed for development, and given high priority by agency strategic planners.
SAFIR will target star formation in the early universe, the chemistry of our interstellar medium, and the chemical
processes that lead to planet formation. SAFIR is a telescope that, with passive cooling at Earth-Sun L2, achieves
temperatures that allow background-limited broad-band operation in the far infrared. This observatory is baselined as
being autonomous in deployment and operation, but consideration has been given to understanding the enabling
opportunities presented by Exploration architecture. As this architecture has become better defined, these opportunities
have become easier to understand.We present conceptual strategies that would use modestly enhanced Exploration
architecture to service and maintain SAFIR, allowing extended duration, lower risk and hardware cost, and performance
enhancements linked to the steep development curve for sensor technology. These efforts, which would rely on both
human and robotic agents, presume routine operations at Earth-Sun L2, and servicing at an Earth-Moon L1 jobsite. The
latter is understood to be easily accessible to a lunar-capable Exploration program. This study bridges the interface
between Exploration technology and astronomical space observatory technology. Such an Exploration-enhanced version
of SAFIR can be seen as a strawman for more ambitious far future work, in which much larger science instruments that
cannot be packaged in a single launch vehicle are not only serviced and maintained in space, but also constructed there.
A decision-making framework to determine the value of serviceability in space telescopes
Show abstract
On-orbit servicing can provide significant benefits for scientific space programs through maintenance and upgrades
of scientific spacecraft. The Hubble Space Telescope (HST) captured these benefits throughout its life
because it was designed to be serviceable. However, serviceability has often been excluded from other telescope
programs since the cost of serviceability could not be quantitatively justified. This paper develops a framework
to determine the value of including serviceability in a space telescope. The framework incorporates three main
principles: separation of cost and benefits, calculation of value through comparison of servicing to replacement,
and the use of Monte-Carlo simulation and decision rule analysis to account for programmatic uncertainty and
management flexibility. To demonstrate how the framework can be used in practice, a case study was performed
with representative data from HST.
Instrumentation for the next generation cryogenic spaceborne far-IR observatories
Show abstract
We present scientific rationale, concepts and technologies for far-IR (λ=35-600 μm) instrumentation for the
cryogenic single-dish space telescopes envisioned for the next two decades. With the tremendous success of
Spitzer, the stage is set for larger (3-10 meter) actively-cooled telescopes and several are under consideration
including SPICA in Japan, and CALISTO/SAFIR in the US. The cold platforms offer the potential for far-IR
observations limited only by the zodiacal dust emission and other diffuse astrophysical foregrounds. Optimal
instrumentation for these missions includes large-format direct-detector arrays with sensitivity matched to the
low photon backgrounds. This will require major improvements relative to the current state of the art, especially
for wavelengths beyond the 38-micron silicon BIB cutoff, We review options and present progress with one
approach: superconducting bolometers.
We highlight in particular the scientific potential for moderate-resolution broadband spectroscopy. The large
cold telescopes can provide line sensitivities below 10-20 W m-2, enabling the first routine survey spectroscopy
of the redshift 0.5 to 5 galaxies that produced the cosmic far-IR background. These far-IR-bright dusty galaxies
account for half of the photon energy released since stars and galaxies began forming, and the new far-IR
spectroscopic capability will reveal their energy sources and chart their history. We describe concepts for the
background-limited IR-Submillimeter Spectrograph (BLISS) designed for this purpose. BLISS is a suite of
R~1000 spectrometer modules spanning the far-IR range, and is under study for SPICA; a similar but more
capable instrument can be scaled for CALISTO/SAFIR.
CALISTO: a cryogenic far-infrared/submillimeter observatory
Show abstract
We present a design for a cryogenically cooled large aperture telescope for far-infrared astronomy in the wavength
range 30 μm to 300 μm. The Cryogenic Aperture Large Infrared Space Telescope Observatory, or CALISTO, is
based on an off-axis Gregorian telesocope having a 4 m by 6 m primary reflector. This can be launched using an
Atlas V 511, with the only optical deployment required being a simple hinged rotation of the secondary reflector.
The off-axis design, which includes a cold stop, offers exceptionally good performance in terms of high efficiency
and minimum coupling of radiation incident from angles far off the direction of maximum response. This means
that strong astronomical sources, such as the Milky Way and zodiacal dust in the plane of the solar system,
add very little to the background. The entire optical system is cooled to 4 K to make its emission less than
even this low level of astronomical emission. Assuming that detector technology can be improved to the point
where detector noise is less than that of the astronomical background, we anticipate unprecedented low values
of system noise equivalent power, in the vicinity of 10-19 WHz-0.5, through CALISTO's operating range. This
will enable a variety of new astronomical investigations ranging from studies of objects in the outer solar system
to tracing the evolution of galaxies in the universe throughout cosmic time.
DESTINY: the dark energy space telescope
Show abstract
We have proposed the development of a low-cost space telescope, Destiny, as a concept for the NASA/DOE
Joint Dark Energy Mission. Destiny is a 1.65m space telescope, featuring a near-infrared (0.85-1.7m) survey
camera/spectrometer with a large flat-field Field Of View (FOV). Destiny will probe the properties of dark
energy by obtaining a Hubble diagram based on Type Ia supernovae (SN) and a large-scale mass power
spectrum derived from weak lensing distortions of field galaxies as a function of redshift.
Telescopes and Mirrors I
Comparison of on-axis three-mirror-anastigmat telescopes
Show abstract
We compare and contrast the Korsch (1972) full-field three-mirror anastigmat telescope (TMA) to the Korsch (1977)
annular-field TMA. Both TMAs offer flat fields with comparably good aberration correction and comparably good
telephoto advantage. Both offer good accessibility of the focal plane. The advantages of the FFTMA are its extremely
uniform focal length over its field, its nearly telecentric final focus, and the fact that there is no hole in the center of its
field. The advantages of the AFTMA are its complete accessible cold stop (essential if a warm telescope is to be used to
image the sky at near-IR wavelengths) and its low sensitivity to mirror location error. Either alternative can deliver
diffraction-limited visible-wavelength images over a one degree diameter field with a two meter aperture.
Manufacturing and control of the aspherical mirrors for the telescope of the French satellite Pleiades
Show abstract
Pleiades is the last generation of French satellite for earth observation. For this space program, SESO has been awarded the
contract (fully completed end 2006), for the manufacturing of the whole set of telescope mirrors (EM, QM and FM, primary
mirror with 700mm CA). These works did also include the mechanical design, manufacturing and mounting of the
attachment flexures (MFDs) between the mirrors and the telescope main structure. This presentation will be focused on the
different steps of manufacturing and control of these mirrors, as well as a presentation of the existing SESO facilities and
capabilities to produce such kind of aspherical components/sub-assemblies.
Unique space telescope concepts using CFRP composite thin-shelled mirrors and structures
Show abstract
Presented are unique concepts for space telescopes and optics, based on carbon fiber reinforced polymer (CFRP) thin-shelled
mirror technology. Thin-shell CFRP mirrors have been proven for IR and longer wavelengths and to a large
extent, visible wavelength optics. The unique structural/mechanical and lightweight characteristics of thin shells open
the design possibilities for advanced space telescopes with active/adaptive mirrors. Low weight and general ease of
manufacturing of CFRP structures can result in reduced part-count and inexpensive lightweight telescopes for space
applications. Three advanced mirror concepts will be presented in this paper, 1) Advanced stowage of thin-shell mirrors
for segmented telescopes, 2) advanced deformable mirror concepts, and 3) simple and inexpensive fabrication concepts
using simplified molding tools for space telescope mirrors. Also presented will be empirical data of CFRP thin-shell
mirrors and composite structures produced supporting their use for space telescope applications.
Telescopes and Mirrors II
Primary mirror shape control for athermalization using embedded sensors
Show abstract
The next generation of space telescopes will be required to meet very challenging science goals. In order to
achieve these goals, the size of the primary mirror will need to be increased. However, since current
telescopes are reaching their limits in terms of size and mass, new designs will require advanced
technologies such as lightweight mirrors and active optical control. Traditional shape control of the primary
mirror relies on feedback from a wavefront sensor located in the optical path. However, a wavefront sensor
reduces the amount of light available for image formation. Therefore, to view very dim objects, it will be
necessary to use a different type of sensor. In this work, a quasi-static shape control algorithm is developed
to correct errors in the mirror due to thermal disturbances using only sensors embedded in the mirror.
Control algorithms are presented for both embedded strain gages and temperature sensors. Finite element
models of both a simple flat plate mirror and a rib-stiffened mirror are generated and analyzed using
Nastran. The flat plate model, with surface-parallel actuation is used to compare the two algorithms.
Following this, the parametric model for a rib-stiffened mirror is used to analyze the effects of the shape
control algorithm as the mirror geometry is changed. It is shown that correction of a mirror can be
achieved using these embedded sensors.
Integrated modeling of point-spread function stability of the SNAP telescope
Show abstract
SNAP is a proposed space-based experiment designed to study dark energy and alternate explanations of the acceleration
of the universe's expansion by performing a series of complementary systematic-controlled astrophysical measurements.
The principal mission activities are the construction of an accurate Type Ia supernova Hubble diagram (the supernova
program) and conducting a wide-area weak gravitational lensing (WL) survey. WL measurements require highly
constant point spread function (PSF) second moments (ellipticity), and the aim of this study is to expand on the 2005
Sholl, et al. preliminary work, specifically via use of the Ball Aerospace integrated modeling tool, EOSyM (End-to-end
Optical System Model). This modeling environment combines thermal, structural and optical effects, including
alignment errors, manufacturing residuals and diffraction, in an integrated model of the telescope. Thermo-mechanically
induced motions and deformations of the mirrors are modeled as well as other disturbances, and corresponding ellipticity
variations of the PSF are quantified for typical operational scenarios. In this study, the effects of seasonal variations in
solar flux, transients introduced when pointing the body-fixed Ka-band antenna toward Earth, 90° roll maneuvers
(planned every three months of operations) and structure dimensional changes associated with composites desorption are
quantified and introduced into the optical system. Uncertainty in the telescope ellipticity distribution may be reduced by
examination of foreground stars within the field of view. Reference is made to ongoing work on the use of foreground
stars in quantifying the PSF.
Thermal calculation and structure analysis of space Main Optical Telescope
Show abstract
Primary mirror with Φ 1m and f 3.5m is the most important optical part in the space Main Optical Telescope (MOT).
Since its required surface error is less than λ/40(rms.), where λ is about 0.6μm, the mirror deformation induced by space
heat and gravity must be within 0.015μm, it's necessary to make thermal calculation and structural analysis to improve
its structure. In this paper, the MOT structure and its finite element model is described. The mechanical properties are
then analyzed in order to verify whether this structure can meet the optical requirements of sufficient strength, stiffness,
and thermal stability. Mechanical analysis is carried out with MSC.Nastran software under 3 different load cases: gravity
influence on-ground, dynamic impact during launching, weightlessness and heat environment in-orbit. Space thermal
analyses are also done to simulate the space environment. The coupled deformation of heat and structure is finally
analyzed. Calculation results show that different support ways and support forces will be the keys to determine the
surface precision of primary mirror. The structure can meet the optical demands, but the thermal deformation can not,
especially in an asymmetric temperature distribution, which should be tested and controlled by some strict methods.
Wavefront Sensing and Control (WFSC)
Dynamic wavefront control for lightweight mirrors in space telescopes
Show abstract
Future space telescopes require larger apertures to continue to improve performance. However, balancing
the large, high performance optics with the desire for lightweight systems proves quite challenging. One
way to achieve both goals is to utilize active, on-orbit wavefront control. A promising method of wavefront
control implementation is surface-parallel piezo-electric actuation. The primary mirror backplane is ribbed
to provide increased stiffness even at very low areal densities, with piezo-electric actuators embedded at the
top of each rib. When the piezo-electrics expand or contract, they bend the surface of the mirror and can be
used to directly correct for dynamic distortions of the wavefront. In addition, rigid-body petal control can be
used to allow for the possibility of systems with segmented primary mirrors.
This paper examines the implementation of both the piezoelectric deformable mirror and petal wavefront
controllers, along with their implications on both optical performance and stability robustness. The systems
analyzed in this paper are integrated models of the entire space telescope system, considering the
transmission of disturbances and vibrations from the reaction wheels in the bus through the structure,
isolators, and bipods to the aperture. The deformable mirror control is performed using a Linear Quadratic
Gaussian (LQG) controller, while the mirror segment control is performed using a positive position feedback
(PPF) controller. For all cases, the wavefront error is the primary optical performance metric and is
calculated using the Zernikes of the primary mirror. The major deterrents to the use of control are
complexity and the loss of stability robustness. The integrated model allows for the calculation of all
metrics together to enable the examination of the potential benefits of implementing dynamic wavefront
control.
Adaptive cross-correlation algorithm and experiment of extended scene Shack-Hartmann wavefront sensing
Show abstract
We have developed a new, adaptive cross-correlation (ACC) algorithm to estimate with high accuracy the shift as large
as several pixels in two extended-scene images captured by a Shack-Hartmann wavefront sensor (SH-WFS). It
determines the positions of all extended-scene image cells relative to a reference cell using an FFT-based iterative
image-shifting algorithm. It works with both point-source spot images as well as extended scene images. We have also
set up a testbed for extended-scene SH-WFS, and tested the ACC algorithm with the measured data of both point-source
and extended-scene images. In this paper we describe our algorithm and present our experimental results.
Co-adding techniques for image-based wavefront sensing for segmented-mirror telescopes
Show abstract
Image-based wavefront sensing algorithms are being used to characterize the optical performance for a variety of current
and planned astronomical telescopes. Phase retrieval recovers the optical wavefront that correlates to a series of
diversity-defocused point-spread functions (PSFs), where multiple frames can be acquired at each defocus setting.
Multiple frames of data can be co-added in different ways; two extremes are in "image-plane space," to average the
frames for each defocused PSF and use phase retrieval once on the averaged images, or in "pupil-plane space," to use
phase retrieval on each PSF frame individually and average the resulting wavefronts. The choice of co-add methodology
is particularly noteworthy for segmented-mirror telescopes that are subject to noise that causes uncorrelated motions
between groups of segments. Using models and data from the James Webb Space Telescope (JWST) Testbed Telescope
(TBT), we show how different sources of noise (uncorrelated segment jitter, turbulence, and common-mode noise) and
different parts of the optical wavefront, segment and global aberrations, contribute to choosing the co-add method. Of
particular interest, segment piston is more accurately recovered in "image-plane space" co-adding, while segment tip/tilt
is recovered in "pupil-plane space" co-adding.
TPF External Occulter I
External occulters for direct observation of exoplanets: an overview
Show abstract
Perhaps the most compelling piece of science and exploration now under discussion for future space missions is the direct
study of planets circling other stars. Indirect means have established planets as common in the universe but have given us
a limited view of their actual characteristics. Direct observation holds the potential to map entire planetary systems, view
newly forming planets, find Earth-like planets and perform photometry to search for major surface features. Direct
observations will also enable spectroscopy of exoplanets and the search for evidence of simple life in the universe. Recent
advances in the design of external occulters - starshades that block the light from the star while passing exoplanet light -
have lowered their cost and improved their performance to the point where we can now envision a New Worlds Observer
that is both buildable and affordable with today's technology. We will summarize recent studies of such missions and
show they provide a very attractive alternative near term mission.
The terrestrial planet finder-occulter (TPF-O) science program
Show abstract
We describe the TPF-O science program composed of interleaved imagery and spectroscopy of planets in
the habitable zones of nearby stars. We give the rationale for the science program and argue that TPF-O
offers the best approach to achieving the original goals set for the Terrestrial Planet Finder.
TPF-O design reference mission
Show abstract
The Terrestrial Planet Finder-Occulter (TPF-O) is a proposed mission to find and characterize planets around nearby
stars. It uses a telescope and an external occulter to suppress the starlight so that the planets close to the star can be
observed. We have constructed Design Reference Missions (DRMs) that show that the TPF-O architecture can achieve
the science requirements. A 4.0 meter telescope and occulter system should be able to find Earth-like planets in the
equivalent search space of 42.7 continuous habitable zones (CHZ) and characterize the planets including detection of
water (at 1000 ppm) and oxygen (at 21%) in the planet's atmosphere. With a smaller telescope (2.4 meter) and occulter,
we can still probe 21.9 CHZs and detect water and oxygen in many of the planets detected.
TPF External Occulter II
New worlds observer optical performance
Show abstract
This article was originally published online on 20 September 2007.
The following errors were discovered by the authors after publication: missing author (Park J. McGraw) and missing references.
A UV/optical telescope for the New Worlds Observer mission
Show abstract
The New Worlds Observer (NWO) mission uses a large external occulter, or "starshade," to block the light from
nearby stars and cast a deep shadow over the entrance aperture of a space telescope, enabling it to detect and characterize
Exo-Solar Planets. Since these planets are intrinsically faint (30th to 32nd magnitude), the telescope must have a large
aperture (2.4 to 4 meters) and the starshade must be large enough (25 to 50 meters) to create a shadow that is deep
enough (108 to 1010 starlight suppression) and large enough (5 to 10 meters in diameter) to envelop the telescope. The
telescope must also be far enough from the starshade (30,000 to 80,000 kilometers) that planets close to the star (50 to 65
milli-arc-seconds) are not occulted. Since the starshade's performance is inversely proportional to the wavelength of the
starlight, the telescope must operate in the visible and near infrared. The telescope should also have a significant capability
for general astrophysics observations, since it will have more than half its time available for other observations while
the starshade is moving from one target to the next.
This paper describes our conceptual design for the NWO telescope, including its instrument suite and operations concept.
We note that in addition to comparative planetology studies and the detection and characterization of terrestrial
planets, the telescope could provide a UV/Optical observing capability for the general astronomical community in the
post-HST era.
Conceptual design of the TPF-O SC buses
Show abstract
The Terrestrial Planet Finder - Occulter (TPF-O) mission has two Spacecraft (SC) buses, one for a formation-flying
occulter and the other for a space telescope. These buses supply the utilities (support structures, propulsion, attitude
control, power, communications, etc) required by the payloads: a deployable shade for the occulter and a telescope with
instruments for the space telescope. Significant requirements for the occulter SC bus are to provide the large delta V
required for the slewing maneuvers of the occulter and communications for formation flying. The TPF-O telescope SC
bus shares some key features of the one for the Hubble Space Telescope (HST) in that both support space telescopes
designed to observe in the visible to near infrared range of wavelengths with comparable primary mirror apertures (2.4 m
for HST, 2.4 - 4.0 m for TPF-O). Significant differences from HST are that 1) the TPF-O telescope is expected to have a
Wide Field Camera (WFC) that will have a Field of View (FOV) large enough to provide fine guidance, 2) TPF-O is
designed to operate in an orbit around the Sun-Earth Lagrange 2 (SEL2) point which requires TPF-O (unlike HST) to
have a propulsion system, and 3) the velocity required for reaching SEL2 and the limited capabilities of affordable
launch vehicles require both TPF-O elements to have compact, low-mass designs. Additionally, it is possible that TPF-O
may utilize a modular design derived from that of HST to allow robotic servicing in the SEL2 orbit.
Externally occulted terrestrial planet finder coronagraph: simulations and sensitivities
Show abstract
A multitude of coronagraphic techniques for the space-based direct detection and characterization of exo-solar terrestrial
planets are actively being pursued by the astronomical community. Typical coronagraphs have internal shaped focal
plane and/or pupil plane occulting masks which block and/or diffract starlight thereby increasing the planet's contrast
with respect to its parent star. Past studies have shown that any internal technique is limited by the ability to sense and
control amplitude, phase (wavefront) and polarization to exquisite levels - necessitating stressing optical requirements.
An alternative and promising technique is to place a starshade, i.e. external occulter, at some distance in front of the
telescope. This starshade suppresses most of the starlight before entering the telescope - relaxing optical requirements to
that of a more conventional telescope. While an old technique it has been recently been advanced by the recognition that
circularly symmetric graded apodizers can be well approximated by shaped binary occulting masks. Indeed optimal
shapes have been designed that can achieve smaller inner working angles than conventional coronagraphs and yet have
high effective throughput allowing smaller aperture telescopes to achieve the same coronagraphic resolution and similar
sensitivity as larger ones.
Herein we report on our ongoing modeling, simulation and optimization of external occulters and show sensitivity
results with respect to number and shape errors of petals, spectral passband, accuracy of Fresnel propagation, and show
results for both filled and segmented aperture telescopes and discuss acquisition and sensing of the occulter's location
relative to the telescope.
New Worlds Observer tolerance overview
Show abstract
New Worlds Observer (NWO) is a formation flying mission that combines a starshade with a telescope to study Earthlike
exoplanets around neighboring stars. The general architecture consists of a telescope and detector that share one
spacecraft platform pointed toward a nearby solar system. Planets in the solar system are revealed by blocking the bright
star with a starshade, on its own spacecraft, positioned between the telescope and its target. Questions arise regarding the
type of precision, tolerances, and diffraction control required when considering the practicality of such an endeavor. We
address the generalities here by presenting an overview of requirements necessary for this type of system. Basic
tolerances are described at both the mission and starshade level.
White-light demonstration of one hundred parts per billion irradiance suppression in air by new starshade occulters
Show abstract
A new mission concept for direct imaging of exo-solar planets called New Worlds Observer (NWO) has been proposed. It involves flying a meter-class space telescope in formation with a newly-conceived, specially-shaped, deployable star-occulting shade several meters across at a separation of some tens of thousands of kilometers. The telescope would make its observations from behind the starshade in a volume of high suppression of incident irradiance from the star around which planets orbit. For an efficacious mission, the required level of irradiance suppression by the starshade is of order 0.1 to 10 parts per billion in broadband light. We discuss an experiment to accurately measure the irradiance suppression ratio at the null position behind candidate starshade forms to these levels. We also present results of broadband measurements which demonstrated suppression levels of less than 100 parts per billion in air using the Sun as a light source. A simulated spatial irradiance distribution surrounding the null from an analytical model developed for starshades is compared with a photograph of actual irradiance captured in situ behind a candidate starshade.
Formation Flying
Formation flying system design for a planet-finding telescope-occulter system
Show abstract
The concept of flying an occulting shade in formation with an orbiting space telescope to enable astronomical
imaging of faint targets while blocking out background noise primarily from starlight near distant Earth-like
planets has been studied in various forms over the past decade. Recent analysis has shown that this approach
may offer comparable performance to that provided by a space-based coronagraph with reduced engineering and
technological challenges as well as overall mission and development costs. This paper will present a design of
the formation flying architecture (FFA) for such a collection system that has potential to meet the scientific
requirements of the National Aeronautics and Space Administration's (NASA's) Terrestrial Planet Finder mission.
The elements of the FFA include the relative navigation, intersatellite communication, formation control,
and the spacecraft guidance, navigation, and control (GN&C) systems. The relative navigation system consists
of the sensors and algorithms to provide necessary range, bearing or line-of-sight, and relative attitude between
the telescope and occulter. Various sensor and filtering (estimation) approaches will be introduced. A formation
control and GN&C approach will be defined that provides the proper alignment and range between the spacecraft,
occulter, and target to meet scientific objectives. The state of technology will be defined and related to
several formation flying and rendezvous spacecraft demonstration missions that have flown.
Formation control and reconfiguration through synthetic imaging formation flying testbed (SIFFT)
Show abstract
The objective of the Synthetic Imaging Formation Flying Testbed (SIFFT) is to develop and demonstrate algorithms for
autonomous centimeter-level precision formation flying. Preliminary tests have been conducted on SIFFT at the Flat
Floor facility at NASA's Marshall Space Flight Center (MSFC). The goal of the testing at MSFC was to demonstrate
formation reconfiguration of three "apertures" by rotation and expansion. Results were very successful and demonstrate
the ability to position and reconfigure separate apertures. The final configuration was with three satellites floating in an
equilateral triangle. The two Follower satellites expand the formation with respect to the Master satellite, which
executes a 10° rotation. Testing was performed successfully under various initial conditions: initial Follower rotation,
initial Follower drift, and initial significant position error of each Follower. Results show roughly 10cm steady state
error and ±5cm precision. Formation capturing technique, where satellites search for each other without prior
knowledge of the position of the other satellites, were also developed and demonstrated both on the 2D flat table and in
the 3D International Space Station environment. Future work includes using a minimum set of beacons for estimation
and implementing a search algorithm so satellites can acquire each other from any initial orientation.
Satellite formation flight and realignment maneuver demonstration aboard the International Space Station
Show abstract
The Synchronized Position Hold Engage and Reorient Experimental Satellites (SPHERES), developed by the MIT Space
Systems Laboratory, enable the maturation of control, estimation, and autonomy algorithms for distributed satellite
systems, including the relative control of spacecraft required for satellite formation flight. Three free-flyer
microsatellites are currently on board the International Space Station (ISS). By operating under crew supervision and by
using replenishable consumables, SPHERES creates a risk-tolerant environment where new high-risk yet high-payoff
algorithms can be demonstrated in a microgravity environment. Through multiple test sessions aboard the ISS, the
SPHERES team has incrementally demonstrated the ability to perform formation flight maneuvers with two and three
satellite formations.
The test sessions aboard the Space Station include evaluation of coordinated maneuvers which will be applicable to
interferometric spacecraft formation missions. The satellites are deployed as a formation and required to rotate around a
common center about a given axis, mimicking an interferometer. Various trajectories are then implemented to point the
synthetic aperture in a different orientation by changing the common axis of revolution. Observation-time optimizing
synchronization strategies and fuel balancing/fuel optimizing trajectories are discussed, compared and evaluated
according to resulting mission duration and potential scientific output.