KEYWORDS: Exoplanets, Distortion, Thermography, Space telescopes, Finite element methods, Thermal effects, Sun, Thermal modeling, Telescopes, Control systems
Starshades are an exoplanet direct-imaging architecture that uses a precisely-shaped screen to block the light from a star
in order to achieve high-contrast imaging of exoplanets. The shape of the deployable starshade structure must precisely
match the design shape in order to maintain the high level of starlight suppression. In this paper, we discuss analysis of
error sources from the starshade structure including manufacturing, dynamics, and thermal distortion to show that the
starshade can achieve the needed optical performance.
In the field of exoplanet detection and characterization, the use of a starshade, an external occulter in front of a telescope
at large separations, has been identified as one of the highly promising methods to achieve the necessary high contrast
imagery. Control of scattered sunlight from the edges of the starshade into the telescope has been identified as one of the
key technology development areas in order to make the starshade feasible. Modeling of the scattered light has resulted
in very different results so a campaign of experimentation with edge samples was undertaken to attempt to understand
the discrepancies.
Here, we present our results from the measurement of select samples of materials which would be suitable for manufacturing
the starshade edge, and related models. We have focused on coating metallic samples for ease of fabrication:
Titanium, Aluminum, and a Beryllium Copper alloy. Using standard machine shop methods, we fabricated samples
which had sharp edges with radius of curvature (RoC) between 15 and 20 μm. We then had these samples coated by two
suppliers to evaluate how well these coating types would conform to the edge and provide scatter suppression. The
results of scatter measurements of these coated edge samples are presented. These scatter results have been incorporated
into a new geometrical model in FRED which includes the details of the starshade mechanical model. This model
predicts both the magnitude and distribution of the scattered sunlight in the image plane of a nominal telescope. We
present these results, including a first effort at modeling the Solar System at 10 pc as seen by this mission architecture.
The external starshade is a prospective method for the direct detection and spectral characterization of terrestrial planets around other stars, a key goal identified in ASTRO2010. As part of an ongoing campaign to validate the starlight-suppression performance of the starshade, we present our first results from our most recent desert test campaign, completed June 2, 2014. Our preliminary contrast measurement is ~1x10-8, consistent with our previous result for the same 60 cm starshade. These data were collected with a 50% spectral bandpass, using a white-light LED as an incoherent light source, in a challenging outdoor test environment. Additional analysis may improve the calculated contrast and/or provide additional improvements to our test configuration, which is currently limited by a halo around the starshade, presumably caused by dust scatter in the atmosphere. The spectral coverage is limited only by the optics and detector in our test setup, not by the starshade itself. Our experimental setup is designed to provide starshade to telescope separation and telescope aperture size that are scaled as closely as possible to the canonical flight system. In this paper we describe key improvements to our test configuration and our latest results with the Hypergaussian starshade. Plans for the next phase of ground testing under a 2013 NASA TDEM award are discussed.
The external starshade is a prospective method for the direct detection and spectral characterization of terrestrial planets around other stars, a key goal identified in ASTRO2010. Validation of the numerical simulations that are critical to this approach has been challenging at very small scales (~4 cm) in the lab. We have successfully fabricated 60 cm starshades and begun a series of ground test experiments with them. We measured contrast better than 1×10-8 under challenging environmental conditions at outdoor test sites. Our experimental setup is designed to provide starshade to telescope separation and telescope aperture size that are scaled as closely as possible from the flight system. In this paper, we describe the test setup, the data acquisition, the reduction techniques, and a preliminary comparison of measured to modeled results.
The use of an external occulter, or starshade, has been proposed as one method for the direct detection and spectral
characterization of terrestrial planets around other stars, a key goal identified in ASTRO2010. Because of the
observational geometry, one of the concerns is stray light from the edge of the starshade that is scattered into the line of
sight of the telescope. We have developed a stray light model using physical properties of a realizable starshade edge
geometry and material to calculate the resulting stray light. The background signal due to stray light has been calculated
for the two telescope architectures adopted for study by the Exoplanet Exploration Program Analysis Group (ExoPAG),
a 4 m monolithic and an 8 m segmented mirror design. Using these results, we have developed a set of design
requirements and structure features that will result in a buildable system with stray light levels that meet the overall
system sensitivity requirements.
Future X-ray astronomical missions will need to address a number of important goals such as studying the dynamics of
clusters of galaxies, determining how elements are created in the explosions of massive stars, and revealing most of the
"normal" matter in the universe which is currently thought to be hidden in hot filaments of gas stretching between
galaxies. In order to achieve these goals, spectrometers capable of high resolution and high throughput are necessary for
the lowest X-ray energies, 0.3-1.0 keV. We present recent progress in the development of off-plane reflection grating
technology for use on upcoming missions. Off-plane grating spectrometers consist of an array of gratings capable of
reaching resolutions above 3000 (λ/Δλ). Concept designs have been made for the International X-ray Observatory X-ray
Grating Spectrometer. More recently however, we have designed an Optics Module Assembly for WHIMex, an
Explorer mission concept that incorporates a Wolter telescope, steering flats, and an array of gratings. This paper will
discuss these designs and the application of off-plane arrays for the future.
The x-ray astronomy community has never flown a celestial source spectrograph that can resolve natural line widths in
absorption the way the ultraviolet community did with OAO-3 Copernicus back in 1972. Yet there is important science
to be mined there, and right now, the large flagship missions like the International X-ray Observatory are not
progressing toward launch. WHIMEx is an Explorer concept proposed earlier this year to open up that science regime in
the next few years. The concept features a modified off-plane grating spectrograph design that will support high
resolution (λ/δλ ~ 4000) in the soft x-ray band with a high packing density that will enable a modest cost space mission.
We discuss the design and capabilities for the WHIMEx mission. Its prime science goal is detecting high temperature
oxygen in the Intergalactic Medium, but it has a broad range of science potential cutting across all of x-ray astronomy
and should give us a new window on the Universe.
The International X-ray Observatory (IXO) is a collaborative effort between NASA, ESA, and JAXA. The IXO science
goals are heavily based on obtaining high quality X-ray spectra. In order to achieve this goal the science payload will
incorporate an array of gratings for high resolution, high throughput spectroscopy at the lowest X-ray energies, 0.3 - 1.0
keV. The spectrometer will address a number of important astrophysical goals such as studying the dynamics of clusters
of galaxies, determining how elements are created in the explosions of massive stars, and revealing most of the "normal"
matter in the universe which is currently thought to be hidden in hot filaments of gas stretching between galaxies. We
present here a mature design concept for an Off-Plane X-ray Grating Spectrometer (OP-XGS). This XGS concept has
seen recent significant advancements in optical and mechanical design. We present here an analysis of how the baseline
OP-XGS design fulfills the IXO science requirements for the XGS and the optical and mechanical details of this design.
An Off-Plane X-ray Grating Spectrometer (OP-XGS) concept is being developed to meet the needs of the International
X-ray Observatory (IXO). The OP-XGS will provide the required spectral resolution of R >3000 over the 0.3 - 1 keV
band with >1000 cm2 effective collecting area, using experience gained with the current generation of reflection gratings
already flown on rocket experiments. We have developed several potential configurations that meet or exceed these
requirements. This paper will focus on the mechanical design and requirements for one of these configurations, the
"tower" concept. This configuration mounts the grating modules to the instrument platform via a tower, allowing direct
alignment with the camera module. This reduces the complexity of the alignment problem while also minimizing the
overall mass of the XGS. We have developed an initial interface concept and resource requirements for this option to be
reviewed by the mission teams for design drivers. We contrast the resource requirements for this concept with those
required for other concepts which have been reviewed by the OP-XGS team. Further, we have identified those portions
of the tower design concept that will require potential technology demonstration to reach TRL 6 prior to the Preliminary
Design Review.
The International X-ray Observatory (IXO) is a merger of the former ESA XEUS and NASA Constellation-X missions,
with additional collaboration from JAXA, proposed for launch ~2020. IXO will address the leading astrophysical
questions in the 'hot universe' through its breakthrough capabilities in X-ray spectroscopy. The mission covers the 0.1
to 40 keV energy range, complementing the capabilities of the next generation observatories, such as ALMA, LSST,
JWST and 30 meter ground-based telescopes. An X-ray Grating Spectrometer is baselined to provide science in the
energy range 0.3-1.0 keV at a spectral resolution of E/ΔE > 3,000 with an effective area greater than 1,000 cm2. This
will require an array of soft X-ray enhanced CCDs operating at a modest frame rate to measure the diffracted light in
both position and energy. Here we describe the baseline camera for the Off-plane XGS instrument using mature CCD
technology.
A dispersive spectrometer onboard the International X-ray Observatory (IXO) provides a method for high throughput and
high spectral resolution at X-ray energies below 1 keV. An off-plane reflection grating array maximizes these
capabilities. We present here a mature mechanical design that places the grating array on the spacecraft avionics bus
13.5 m away from the focal plane. In addition, we present the technology development plan for advancing the
Technology Readiness Level to 6 for the Off-Plane X-ray Grating Spectrometer.
A dispersive spectrometer onboard the International X-ray Observatory (IXO) provides a method for high throughput and
high spectral resolution at X-ray energies below 1 keV. An off-plane reflection grating array maximizes these
capabilities. We present here a mature mechanical design that places the grating array on the spacecraft avionics bus
13.5 m away from the focal plane.
KEYWORDS: Space telescopes, James Webb Space Telescope, Mirrors, Telescopes, Space operations, Optical design, Observatories, Optical instrument design, Packaging, Stray light
A Single Aperture, Far InfraRed Observatory, called SAFIR, is a proposed NASA mission to observe the universe at wavelengths from ~30 to 800 microns. To achieve the mission objectives, the telescope must be of order 10-m in diameter and cooled to ~4K to obtain background limited performance. Northrop Grumman Space Technology (NGST) has developed a conceptual design based on our James Webb Space Telescope (JWST) and Terrestrial Planet Finder (TPF) mission architectures that utilizes a deployable telescope and a large sunshade to achieve the desired mirror temperature. Our design concept includes a 12-m diameter on-axis Gregorian telescope, which provides the wide fields of view desired by the SAFIR science team. We describe the optical design, a packaging concept that allows this telescope to fit in a standard 5- launch vehicle fairing, and initial concepts for the telescope thermal control system.
NASA wants to launch a Terrestrial Planet Finder (TPF) mission in 2014 to detect and characterize Earth-like planets around nearby stars, perform comparative planetology studies, and obtain general astrophysics observations. The detection of a 30th magnitude planet located within 80 milli-arcseconds of a 5th (Visual) magnitude star is an exceptionally challenging objective. Observations in the thermal infrared (7-17 mm) are somewhat easier since the planet is 'only' 15m fainter than the star at these wavelengths, but many severe challenges must still be overcome, including: Designing a spacecraft, a telescope and an IR coronagraph for star-planet separations equal to λ/D;(i) Providing a stable (~30K) thermal environment for the optics and isolating them from vibration sources; (ii)Developing a deployment scheme for a 28-m space telescope that can fit in an existing launch vehicle; (iii) Minimizing telescope mass to enable launch to L2 or a driftway orbit with a single launch vehicle; (iv) Generating a manufacturing plan that will permit TPF to be developed at a reasonable cost and schedule; (v) Identifying the key enabling technologies for TPF. This paper describes the IR Coronagraph we designed during our recent TPF Mission Architecture study in an effort to meet these challenges.
NASA plans to launch a Terrestrial Planet Finder (TPF) mission in 2014 to detect and characterize Earth-like planets around nearby stars, to perform comparative planetology studies, and to obtain general astrophysics observations. As part of our recently completed TPF Mission Architecture study for NASA/JPL we developed the conceptual design for a Large Aperture IR Coronagraph that meets these mission objectives. This paper describes the optical design of the telescope and the coronagraph to detect and characterize exo-solar planets. The telescope design was optimized to provide a well-corrected image plane that is large enough to feed several instruments and control scattered light while accommodating packaging for launch and manufacturing limitations. The coronagraph was designed to provide a well corrected field of view with a radius > 5 arcsec around the star it occults in the 7-17 microns wavelength region. A design for this instrument as well as results of a system simulation model are presented. The methodology for wavefront error correction and control of scattered and diffracted light are discussed in some detail as they are critical parameters to enable detecting planets at separations of down to ~λ/D.
The tremendous potential for hyperspectral imagery as a remote sensing tool has driven the development of TRW's TRWIS III hyperspectral imager. This instrument provides 384 contiguous spectral channels at 5 nm to 6.25 nm spectral resolution covering the 400 nm to 2450 nm wavelength range. The spectra of each pixel in the scene are gathered simultaneously at signal to noise ratios of several hundred to one for typical Earth scenes. Designed to fly on a wide range of aircraft and with variable frame rate, the ground resolution can be varied from approximately 50 cm to 11 m depending on the aircraft altitude and speed. Meeting critical performance requirements for image quality, co- registration of spectral samples, spectral calibration, noise, and radiometric accuracy are important to the success of the instrument. TRWIS III performance has been validated and the instrument has been radiometrically calibrated using TRW's Multispectral Test Bed. This paper discusses the characterization and calibration process and results of the measurements. An example of results from a flight at the end of 1996 is included.
This paper describes the performance of a unique new IR array camera system which provides simultaneous imaging at two wavelengths in the near IR. Two-color imaging is achieved with a dichroic beam splitter which yields two independent beams, one short-wave (SW) from 1 to 2.5 micrometers and one long-wave (LW) from 2 to 5 micrometers . A Rockwell NICMOS 3 256 by 256 HgCdTe array is used in the SW channel and the LW channel has an InSb 256 by 256 array from SBRC. The instrument, which is designed for the University of California's Lick Observatory 3-m telescope and for the f/15 focus of the 10-m W.M. Keck telescope, employs a closed cycle refrigerator and a compact array control/data acquisition system based on transputers with a host 486 PC. On the Lick 3-m telescope the pixel size is 0.7' which gives a field of view of about 180' by 180'. Facilities are also provided for spectroscopy and polarimetry. Recent observational results are reported to illustrate the performance of this system.
This paper describes a new infrared imaging system being developed at UCLA for use on both the Lick Observatory 3-m telescope and the W.M. Keck 10-m telescope. The instrument has a relatively wide field of view on each telescope and is intended for infrared surveys and deep imaging. To enhance efficiency, the new instrument incorporates a dichroic beam splitter to provide two simultaneous imaging systems, one short-wave (SW) from 1 - 2.5 micrometers and one long-wave (LW) from 2 - 5 micrometers . Each wavelength channel is independently optimized. The SW channel contains a Rockwell NICMOS3 256 X 256 HgCdTe array and the LW channel has an SBRC 256 X 256 InSb array. The thermal design employs a closed cycle cooler. A control and data acquisition system based on transputers and high speed analog electronics is being developed to handle the high data rates.
This paper describes an infrared imaging system developed to match the physical pixel sizes of the near infrared arrays used in the UCLA IR camera to the typical seeing disk at the f/15 focus of the W. M. Keck telescope. A field of view of 64 arcseconds (') at 0.25' per pixel is required, no internal focussing mechanism is possible and the system must perform under vacuum at 77 K. In our design, an achromatic triplet lens of calcium and barium fluoride provides an image of the entrance pupil and collimates the beam before it passes through a dichroic beam-splitter which divides the system into two independent channels; the wavelength split occurs at approximately 2.5 micrometers . Each beam is re-imaged onto IR arrays with 30 and 40 micrometers pixels to yield 0.25'/pixel and each imager is independently optimized to be achromatic and aberration free. Spot diagrams and aberration plots will be given. We also describe the steps required to compensate for environmental changes, since these lenses are used at LN2 temperature, and we discuss AR coatings and throughput.
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