The ESA Darwin mission is primarily devoted to the detection of earth-like exoplanets and the spectroscopic characterization of their atmospheres for key tracers of life. Darwin is implemented as a free-flying stellar interferometer operating in the 6.5-20 micron wavelength range, and passively cooled to 40 K. The stellar flux is suppressed by destructive interference (nulling) over the full optical bandwidth. The planetary signal is extracted from the zodiacal background signature by modulating the optical response of the interferometer. The Darwin mission concept has evolved considerably in the past years. The original concept, based on six 1.5 m telescopes, has been replaced by more efficient designs using three to four three-meter class apertures. A novel 3D architecture is being evaluated, together with the conventional planar one, bearing the potential for significant volume and mass savings and enhanced straylight rejection. A number of technology development activities have been successfully completed, including optical metrology, optical delay lines, and single-mode infrared optical fibers. A second iteration of the Darwin System Assessment Study has been kicked off end 2005, aiming to consolidate the overall mission architecture and the preliminary design of the Darwin mission concept. This paper illustrates the current status of the Darwin mission, with special emphasis on the optical configuration and the technology development programme in the area of optics.
A brief overview of the Darwin project in the context of the European Space Agency's Cosmic Vision program is given. The scientific goals in the context of the new approach with themes, is given. The goals are broken down into a stepwise approach first relating current ground based and immediate space based experiments (e.g. radial velocity measurements from the ground and the CNES/ESA COROT occultation mission). Then, the different approaches to how to achieving the full goal of a survey of the nearest stars is described. Then, a brief outline of steps following after the current objectives of Darwin have been reached will follow. Some focus is also given to the response of the European community on how to address these goals in a timely and technically correct fashion. This will lead up to scenarios likely to occur over the next 3 years. Darwin is developed through an active technology program, parts of which are described in other papers at this conference. A description of where the different elements fit will be given. Finally the international aspects as currently foreseen are presented.
Darwin is a mission under study by the European Space Agency, ESA. The mission objectives are detection
and characterization of exo-planets, with special emphasize on the planets likely to harbour earthlike life.
The mission cancels the light from the target star by nulling interferometry, while the light collected from
any orbiting planets will interfere constructively. In this way absorption features in the planetary light can
be detected and analysed. In the preceding years ESA has developed the required technology and elaborated
on and evaluated different mission concepts with the aim of reducing over-all mission cost. This has
resulted in a number of mission architectures, and various interferometric beam recombination techniques.
To consolidate the study results two parallel mission assessment studies were initiated September 2005,
taking benefit from the large number of technology developments as conducted since 2000. This article
reviews the Darwin mission and its architecture evolution from the feasibility study up to the currently
ongoing system assessment studies.
Future nulling space interferometers, such as Darwin and TPF, under study by the European Space Agency and NASA respectively, will rely on fast internal modulation techniques in order to extract the planet signal from the much larger background noise. In this modulation scheme, the outputs of a number of sub-arrays are combined with a variable, achromatic phase shift. In this paper, we discuss the use of well-known OPD modulation techniques in nulling interferometry. The main attractiveness of this approach is that a small OPD modulation at frequency f will modulate the stellar leakage at frequency 2f, since leakage does not depend from the sign of the OPD. In turn, a planet transiting a quasi-linear portion of the transmission map will induce a signal at frequency f at the nulled output, which can be extracted by coherent detection techniques. The properties of this modulation scheme are analyzed, using the Bracewell configuration as a test case. The significance of this technique for ESA's Darwin mission, and its ground-based technology precursor GENIE, are discussed.
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.
We investigate whether the design of the DARWIN nulling interferometer can be simplified, while maintaining or even improving its performance, by accepting somewhat higher levels of stellar leakage. We establish detailed requirements on stellar suppression for a nulling interferometer, considering a realistic DARWIN stellar target sample. The dominating noise source, represented by the local zodiacal cloud, is essentially constant for all target stars while the stellar leakage decreases as the inverse of the distance squared. This means that such stellar leakage has an effect on the integration times of near-by target stars, while for more distant targets its influence decreases significantly. We assess the impact of different types of nulling profiles and identify those stars for which the detection sensitivity can be maximised.
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.
Darwin is one of the most challenging space projects ever considered by the European Space Agency (ESA). Its principal objectives are to detect Earth-like planets around nearby stars and to characterise their atmospheres. Darwin is conceived as a space "nulling interferometer" which makes use of on-axis destructive interferences to extinguish the stellar light while keeping the off-axis signal of the orbiting planet. Within the frame of the Darwin program, the European Space Agency (ESA) and the European Southern
Observatory (ESO) intend to build a ground-based technology demonstrator called GENIE (Ground based European Nulling Interferometry Experiment). Such a ground-based demonstrator built
around the Very Large Telescope Interferometer (VLTI) in Paranal will
test some of the key technologies required for the Darwin Infrared Space Interferometer. It will demonstrate that nulling interferometry can be achieved in a broad mid-IR band as a precursor to the next phase of the Darwin program. The instrument will operate in the L' band around 3.8 μm, where the thermal emission from the telescopes and the atmosphere is reduced. GENIE will be able to operate in two different configurations, i.e. either as a single Bracewell nulling interferometer or as a double-Bracewell nulling interferometer with an internal modulation scheme.
The reduction of the thermal background emission from the local and exozodiacal dust clouds is a critical element for the success of ESA's space mission, DARWIN. Internal modulation, a technique using fast signal chopping, isolating the planetary signal from these noise sources, was proposed by Mennesson and Léger. In this paper, a short review of internal modulation is given, and new configurations with internal modulation are proposed to reduce the complexity of the beam-combining optics. A modification to the implementation of internal modulation is then investigated. It provides similar performance with a single detector and a greatly simplified optical layout: the number of beam-combiners is reduced by a factor of about two. The principle of inherent modulation is different from internal modulation in that no sub-interferometers are used: different phase shifts are applied to the input beams before recombination such that an asymmetric transmission map is obtained directly, without plus or minusπ/2 modulation as used in internal modulation. By combining the phase shifts and the input beams differently a transposed transmission map is obtained, allowing the signal to be chopped. During operations, multiplexing between the two interferometers is performed, such that at any time only one interferometer is being used.
Darwin is one of the most challenging space projects ever
considered by the European Space Agency (ESA). Its principal
objectives are to detect Earth-like planets around nearby stars and to characterize their atmospheres. Darwin is conceived as a space
"nulling interferometer" which makes use of on-axis destructive
interferences to extinguish the stellar light while keeping the
off-axis signal of the orbiting planet. Within the frame of the Darwin program, the European Space Agency (ESA) and the European Southern Observatory (ESO) intend to build a ground-based technology
demonstrator called GENIE (Ground based European Nulling
Interferometry Experiment). Such a ground-based demonstrator built
around the Very Large Telescope Interferometer (VLTI) in Paranal will
test some of the key technologies required for the Darwin Infrared Space Interferometer. It will demonstrate that nulling interferometry can be achieved in a broad mid-IR band as a precursor to the next phase of the Darwin program. The present paper will describe the objectives and the status of the project.
In the DARWIN study, the European Space Agency has for several years investigated the possibilities for a mission, dedicated to direct detection of earth-like life on extra- solar planets. The detection technique is based on nulling interferometry, i.e. suppression of the on-axis star, such that only light from orbiting planet(s) remains. The concept of Generalized Angel's Cross (GAC) is introduced. A GAC is a nulling interferometer made up of four telescopes, all at equal distance from the interferometer's center and flown in one plane perpendicular to the common optical axis.
An Image Management And Communication (IMAC) system adapted to the X-ray department at Sahlgrenska University Hospital has been developed using standard components. Two user demands have been considered primary: Rapid access to (display of) images and an efficient worklist management. To fulfil these demands a connection between the IMAC system and the existing Radiological Information System (RIS) has been implemented. The functional modules are: check of information consistency in data exported from image sources, a (logically) central storage of image data, viewing facility for high speed-, large volume-, clinical work, and an efficient interface to the RIS. Also, an image related database extension has been made to the RIS. The IMAC system has a strictly modular design with a simple structure. The image archive and short term storage are logically the same and acts as a huge disk. Through NFS all image data is available to all the connected workstations. All patient selection for viewing is through worklists, which are created by selection criteria in the RIS, by the use of barcodes, or, in singular cases, by entering the patient ID by hand.
A method for modeling reflectionless conductive dispersive media is presented. The media are either half-spaces or finite dispersive slabs with a spatially varying impedance. The problem of finding reflectionless media for plane waves at normal incidence is formulated as an inverse problem where the constitutive relation is to be determined as a function of depth given a reflection kernel which is zero. The inverse problem is solved by a time domain Green functions technique. It is seen that non-reflecting half-spaces can be constructed in a number of different ways, whereas non-reflecting slabs only can be found if the backwall is non- conducting.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.