METimage is a cross-purpose, medium resolution, multi-spectral optical imaging radiometer for meteorological applications on-board the MetOp-SG satellites. The fully representative instrument engineering model has been successfully tested and delivered to MetOp-SG. On METimage level this model served for instrument mechanical, thermal and EMC verification, at MetOp-SG level it supports the entire satellite test campaign. The instrument PFM integration is under finalisation by integrating the optical subsystems into the optical head. The preparation of the environmental test campaign and of the calibration and characterization campaign is ongoing.
METimage is a cross-purpose, medium resolution, multi-spectral optical imaging radiometer for meteorological applications onboard the MetOp-SG satellites. It is capable of measuring thermal radiance emitted by the Earth and solar backscattered radiation in 20 spectral bands from 443 to 13.345 nm. We provide an overview over the instrument design and present the instrument development status.
Modal wavefront filtering is mandatory in nulling interferometers dedicated to detect extrasolar planets. Several activities have been initiated by ESA for developing single-mode waveguides for the mid-infrared.
We present the development of fibres to be used for modal filtering within the European DARWIN mission and its scientific precursor GENIE: Chalcogenide fibres fit the wavelength range up to about 11 microns, while silver halide fibres can cover the full DARWIN wavelength range from 6.5 to 20 microns. A wide range of different manufacturing methods have been applied for producing step-index fibres. We also present the first results of manufacturing photonic crystal silver halide fibres.
We tested the modal wavefront filtering capability of the fibres in a Mach-Zehnder interferometer fed by a CO2-laser. In addition we recorded the transverse output beam profile for each fibre. The results of both measurements are strong indicators for single-mode operation.
We identified the critical issues experienced in the course of this manufacturing activity. The efficient removing of cladding modes and the required length of the fibres, commonly strongly underestimated, turned out as the keys for successful demonstration of singlemode behaviour. We found dedicated and compatible materials acting as mode stripper for both fibre materials used.
We highlight the required steps for further improvement of the manufactured fibres and for a reasonable continuation of the fibre development activities for DARWIN.
For the DARWIN mission the extremely low planet signal levels require an optical instrument design with utmost efficiency to guarantee the required science performance. By shaping the transverse amplitude and phase distributions of the receive beams, the singlemode fibre coupling efficiency can be increased to almost 100%, thus allowing for a gain of more than 20% compared to conventional designs. We show that the use of "tailored freeform surfaces" for purpose of beam shaping dramatically reduces the coupling degradations, which otherwise result from mode mismatch between the Airy pattern of the image and the fibre mode, and therefore allows for achieving a performance close to the physical limitations. We present an application of tailored surfaces for building a beam shaping optics that shall enhance fibre coupling performance as core part of a space based interferometer in the future DARWIN mission and present performance predictions by wave-optical simulations. We assess the feasibility of manufacturing the corresponding tailored surfaces and describe the proof of concept demonstrator we use for experimental performance verification.
The European DARWIN mission aims at the detection of Earth-like exo-planets and at the spectroscopic characterization of their atmospheres. By nulling interferometry in the mid-infrared wavelength regime the stellar flux may be rejected. By spatial and temporal modulation of the interferometer’s receive characteristic the planet signal may be extracted from the background signals. The DARWIN instrument consists of a flotilla of free-flying spacecraft, three to four spacecraft carrying the collector telescopes and one spacecraft carrying the control units and the beam recombination and detection unit. We present different system design concepts for the DARWIN instrument which have been elaborated within the DARWIN System Assessment Study. We discuss various aperture configurations and beam routing schemes as well as modulation methods and and beam recombination schemes.
EUCLID, a medium-class mission candidate of ESA's Cosmic Vision 2015–2025 Program, currently in Definition Phase (Phase A/B1), shall map the geometry of the Dark Universe by investigating dark matter distributions, the distance-redshift relationship, and the evolution of cosmic structures. EUCLID consists of a 1.2 m telescope and two scientific instruments for ellipticity and redshift measurements in the visible and nearinfrared wavelength regime. We present a design concept of the EUCLID mission which is fully compliant with the mission requirements. Preliminary concepts of the spacecraft and of the payload including the scientific instruments are discussed.
EUCLID is a mission to accurately measure the accelerated expansion of the universe. It has been selected for implementation with a launch planned for 2020. EUCLID will map the large-scale structure of the Universe over 15.000 deg2 of the extragalactic sky and it will measure galaxies out to redshifts of z=2 EUCLID consists of a 1.2 m telescope and two scientific instruments for ellipticity and redshift measurements in the visible and near infrared wavelength regime.
We present a design for the EUCLID space segment, targeting optimum performance in terms of image quality and stability and maximum robustness with respect to performance, resources and instrument interfaces.
METimage is a cross-purpose medium resolution, multi-spectral optical imaging instrument dedicated for operational meteorology, oceanography, and climate applications. It is implemented as passive imaging spectro-radiometer, capable of measuring thermal radiance emitted by the Earth and solar backscattered radiation in a broad spectral range.
The detection of terrestrial exo-planets in the habitable zone of
Sun-like stars as well as the proof of biomarkers
is one of the most exciting goals in Astrophysics today. A nulling interferometer operated in the mid-infrared wavelength regime allows for overcoming the obstacles of huge contrast ratio and small angular separation between star and planet. Dedicated missions, as ESA's DARWIN or NASA's TPF-I, are implemented as a closely controlled formation of free-flying spacecraft which carry the distributed payload. We discuss various implementation alternatives and present an optimized design of the DARWIN instrument
including the science payload and the formation-flying subsystem. We analyze the achievable scientific performance
of the DARWIN instrument by taking into account the target properties and the instrument performance. We show that the DARWIN mission is feasible and that the mission goals can be fulfilled.
Spectroscopy of exoplanets around near-by stars is one of the most fascinating but also most challenging science goals of
our days. The ESA DARWIN mission as well as NASA TPF-I rely on nulling interferometry. The measurement
principle underlying their nulling science mode is essentially nonlinear. On the one hand in terms of null depth as a
function of amplitude and phase noise, and on the other hand in terms of fiber coupling as function of science beam
pointing and lateral offset. We present a performance breakdown and an end-to-end performance simulation for
DARWIN with focus on principal limitations, and with a clear distinction between static null depth contributors,
dynamic error contributors, and so-called instability noise within the overall system. We additionally discuss the derived
next-step development efforts for critical subsystems.
The missions DARWIN and TPF-I (Terrestrial Planet Finder-Interferometer) aim at the search and analysis of
terrestrial exo-planets orbiting nearby stars. The major technical challenge is the huge contrast ratio and the
small angular separation between star and planet. The observational method to be applied is nulling interferometry.
It allows for extinguishing the star light by several orders of magnitude and, at the same time, for resolving
the faint planet.
The fundamental performance of the nulling interferometer is determined by the aperture configuration, the
effective performance is driven by the actual instrument implementation. The x-Array, an aperture configuration
with 4 telescopes allowing for phase chopping and decoupling of the nulling and imaging properties, provides
highest instrument performance. The scientific goals necessitate an instrument setup of high efficiency and utmost
symmetry between the beams concerning optical path length, beam profile and state of polarization. Non-planar
spacecraft formations allow for a simpler spacecraft design which comes at the cost of inherent constellation and
beam asymmetry, of increased complexity of the beam relay optics and of instrumental errors synchronous to
the planet signal demodulation frequency. Planar formations allow for perfect efficiency and symmetry but need
deployable structures for the secondary mirror and the sunshield due to launcher accommodation constraints.
We present a discussion of planar and non-planar implementations of the x-Array aperture configuration and
identify for both the critical items and design drivers. We compare the achievable instrument performance and
point out the constraints for each spacecraft formation.
The European DARWIN mission aims at detection and characterization of Earth-like exo-planets as well as at aperture synthesis imaging. The method to be applied is nulling interferometry in the mid-infrared wavelength regime. The DARWIN instrument consists of a flotilla of free-flying spacecraft, one spacecraft carrying the optics for beam recombination and three or more spacecraft carrying the large collector telescopes. We provide a trade-off of different configuration, payload, and mission concepts. We discuss various two and three-dimensional aperture configurations with three or four telescopes, beam routing schemes, phase modulation methods, and beam recombination and detection schemes as well as different launch vehicle configurations, launch scenarios, and orbits. We trade the different DARWIN concepts by assessing the performance in terms of science return, development risk, and planning.
To achieve high rejection ratios with a nulling interferometer, the beams to be combined have to be as equal as possible, especially concerning the transverse field distribution. This requires highly symmetric beam recombination stages as well as modal wavefront filtering. Usually co-axial beam recombination employing beam splitters is used, ideally in highly symmetrizing double-pass configurations. Considering that a single-mode waveguide is required anyway for spatial wavefront filtering, it seems reasonable to incorporate the functionalities of beam recombination and modal filtering in a single device. If the beams to be combined are injected multi-axially into the single-mode waveguide, co-axial beam recombination is obtained within the waveguide. We show that such beam recombination stages may outperform conventional symmetric co-axial realizations with respect to optical throughput and system complexity.
Modal filtering is mandatory in nulling interferometers dedicated to direct detection of extrasolar terrestrial planets. However, up to date no appropriate waveguides to act as wavefront filter were available for the mid-infrared wavelengths in question. We present the development of silver-halide fibers and chalcogenide fibers to be used for modal filtering within the European DARWIN mission. We give a trade-off of suitable waveguides geometries, possible materials, and fabrication technologies and present measurements of the beam profiles, the insertion loss, and of the modal filtering capability of the developed fiber samples.
Nulling interferometry of exo-solar planets requires as a minimum two telescopes, of which one is phase shifted by 180 degrees, such that the on-axis stellar object is cancelled, while the light from the off-axis planet interferes constructively. Improvement of the nulling performance and the introduction of chopping leads to space interferometers of four or more telescopes and a separate spacecraft dedicated to beam recombination, as currently baselined for Darwin and TPF.
It has recently been demonstrated that the stellar leaks mainly affects the integration times for near-by target stars [o,c]. Considering that there are only a few near-by targets and that the integrations times for each of these is short compared to that of distant stars, it appears advantageous to simplify the interferometer, by accepting higher levels of stellar leaks for near-by targets.
A simple, chopping nulling interferometer can be obtained by adding one equal size telescope to the basic two telescope nulling interferometer. Modulation is obtained by applying time-varying phase-shifts to the beams before recombination, i.e. inherent modulation [d].
The recombination of 3 multi-axial beams is achieved by coupling into a single mode waveguide, leading to high modulation and coupling efficiencies, and a single focal plane [i]. Linear and circular telescope configurations are proposed and investigated, including a discussion on the need of a separate spacecraft for beam recombination. The associated transmission and modulation maps and efficiencies are calculated and discussed.
Spatial or modal filters are essential parts of highly rejecting nulling interferometers. We review the principle of operation of both types of filters and explain the fundamental physical difference. We point out the filter's individual properties and potentials, and analyze practical limitations. For modal filters we discuss implementation alternatives, also with regard to their suitability for mid-infrared operation. For a single-mode fibre filter we analyze the broadband performance and the minimum length ensuring a prescribed filter action. We further present simulation results of a DARWIN-representative nulling interferometer breadboard which confirm the distinct improvement in rejection ratio due to spatial or modal filtering.
In ESA's Infrared Space Interferometry mission, a multi-aperture interferometer fed by telescopes will serve to analyse exoplanets orbiting bright stars. Spectroscopy of the planet's radiation could give hints on the possibility of the existence of life. However, for a Sun/Earth-like constellation, a star light rejection ratio of some 80 dB is required. This is the factor by which the star light is suppressed, when comparing the interferometer with a standard, wide-field-of-view telescope. We investigate the nulling capability of space-based interferometers, realized either in fiber or bulk optics, in the presence of imperfections of the structure and of optical components. Mismatch of amplitude, optical path length, and polarization among the interferometer arms is taken into account, as well as multiple reflections and telescope imperfections. The parameters describing the interferometer's receive characteristic, which are actively controlled or influenced by environmental disturbances, are modeled stochastically. We analyse Sun/Earth-like constellations by numerical simulation for a wavelength range of 6 to 18 microns. The expected value of the star light rejection ratio is calculated for several interferometer configurations. The exemplary numerical results confirm the extreme requirements for interferometer uniformity and give a quantitative insight into the dependence of the attainable rejection ratio on individual and/or combined interferometer imperfections.
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