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 The original concept, based on six 1.5 m telescopes, has been replaced by more efficient designs using three to four three-meter class apertures.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.
The Laser Interferometer Space Antenna, as well as its reformulated European-only evolution, the New Gravitational-Wave Observatory, both employ heterodyne laser interferometry on million kilometer scale arm lengths in a triangular spacecraft formation, to observe gravitational waves at frequencies between 3 × 10−5 Hz and 1 Hz. The Optical Bench as central payload element realizes both the inter-spacecraft as well as local laser metrology with respect to inertial proof masses, and provides further functions, such as point-ahead accommodation, acquisition sensing, transmit beam conditioning, optical power monitoring, and laser redundancy switching.
These functions have been combined in a detailed design of an Optical Bench Elegant Breadboard, which is currently under assembly and integration. We present an overview of the realization and current performances of the Optical Bench subsystems, which employ ultraprecise piezo mechanism, ultrastable assembly techniques, and shot noise limited RF detection to achieve translation and tilt metrology at Picometer and Nanoradian noise levels.
For observation of gravitational waves at frequencies between 30 μHz and 1 Hz, the LISA mission will be implemented in a triangular constellation of three identical spacecraft, which are mutually linked by laser interferometry in an active transponder scheme over a 5 million kilometer arm length. On the end point of each laser link, remote and local beam metrology with respect to inertial proof masses inside the spacecraft is realized by the LISA Optical Bench. It implements further- more various ancillary functions such as point-ahead correction, acquisition sensing, transmit beam conditioning, and laser redundancy switching.
A comprehensive design of the Optical Bench has been developed, which includes all of the above mentioned functions and at the same time ensures manufacturability on the basis of hydroxide catalysis bonding, an ultrastable integration technology already perfected in the context of LISA's technology demonstrator mission LISA Pathfinder. Essential elements of this design have been validated by dedicated pre-investigations. These include the demonstration of polarizing heterodyne interferometry at the required Picometer and Nanoradian performance levels, the investigation of potential non-reciprocal noise sources in the so-called backlink fiber, as well as the development of a laser redundancy switch breadboard.
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.
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.
Two competitive design studies for the Ground-based European Nulling Interferometer Experiment (GENIE) have been initiated by the European Space Agency and the European Southern Observatory in November 2003. The GENIE instrument will most probably consist of a two-telescope Bracewell interferometer, using the 8-m Unit Telescopes and/or the 1.8-m Auxiliary Telescopes of the VLTI, and working in the infrared L' band (3.5 - 4.1 microns). A critical issue affecting the overall performance of the instrument is its capability to compensate for the phase and intensity fluctuations produced by the atmospheric turbulence. In this paper, we present the basic principles of phase and intensity control by means of real-time servo loops in the context of GENIE. We then propose a preliminary design for these servo loops and estimate their performance using GENIEsim, the science simulation software for the GENIE instrument.
Homothetic mapping is an aperture synthesis technique that allows interferometric imaging over a wide field-of-view. A laboratory experiment was set up to demonstrate the feasibility of this technique. Here, we present the first static experiments on homothetic mapping that have been done on the Delft Testbed for Interferometry (DTI). Before a changeable telescope configuration is provided, we first took a fixed telescope configuration and tested the algorithms for their ability to provide an exit pupil configuration before beam combination, that was an exact copy of this telescope configuration. By doing so, we created a homothetic imaging system. This is an imaging system that acts as a masked aperture monolithic telescope, but consists of (in our case) three telescopes of which the light follow their own optical trains.
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.
The prime objective of GENIE (Ground-based European Nulling Interferometry Experiment) is to obtain experience with the design, construction and operation of an IR nulling interferometer, as a preparation for the DARWIN / TPF mission. In this context, the detection of a planet orbiting another star would provide an excellent demonstration of nulling interferometry. Doing this through the atmosphere, however, is a formidable task. In this paper we assess the prospects of detecting with nulling interferometry on ESO's VLTI, low-mass companions in orbit around their parent stars. With the GENIE science simulator (GENIEsim) we can model realistic detection scenarios for the GENIE instrument operating in the VLTI environment, and derive detailed requirements on control-loop performance, IR background subtraction and the accuracy of the photometry calibration. We analyse the technical feasibility of several scenarios for the detection of low-mass companions in the L'-band.
The far-infrared (FIR) wavelength regime has become of prime importance for astrophysics. Observations of ionic, atomic and molecular lines, many of them present in the FIR, provide important and unique information on the star and planet formation process occurring in interstellar clouds, and on the lifecycle of gas and dust in general.
As these regions are heavily obscured by dust, FIR observations are the only means of getting insight in the physical and chemical conditions and their evolution. These investigations require, besides high spectral, also high angular resolution in order to match the small angular sizes of star forming cores and circum-stellar disks. We present here a mission concept, ESPRIT, which will provide both, in a wavelength regime not accessible from ground by ALMA (Atacama Large Millimeter Array), nor with JWST (James Webb Space Telescope).
ESA's DARWIN will be an interferometric mission carrying out high-resolution astrophysical observations as well as the detection/characterization of earthlike exoplanets. In this paper, the current status and development perspectives of the Darwin imaging mode are discussed. First, overall system aspects are addressed including expected sensitivity, and baseline reconfiguration needs. Subsequently, the current instrumental concept is reviewed. This is based on a phase-referencing architecture supporting simultaneous observation of the science object, and an off-axis reference target for OPD stabilization purposes. The reference and science beams are wavelength-multiplexed and propagate along a common path through the interferometer. The viability of the cophasing approach is discussed, with emphasis on crosstalk control for multiplexed beam transfer, real-time compensation of the astrometric OPD, and associated metrology requirements. Studies have shown that imaging capabilities can be implemented within the current nulling beam combiner concept, which avoids the complexity and cost of developing a dedicated imaging beam combiner spacecraft. However, this approach has important drawbacks for the imaging mission
The Delft Testbed Interferometer (DTI) will be presented. The main
purpose for the DTI is to demonstrate the feasibility of
homothetic mapping, both fixed and under scanning conditions. The
driving design issues behind the DTI will be presented together
with a list of experiments to be conducted with the DTI system in
the field of wide field imaging.
The start of NEVEC was initiated by the opportunity in the Netherlands to reinstate instrumental efforts in astronomy through a funding program for 'Top Research Schools,’ which brought about the creation of NOVA. The fact that considerable experience exists in Radio Astronomical imaging through interferometry (the Westerbork Synthesis Radio Telescope started in 1970), and the relatively small size at the time of ESO's VLTI Team made it opportune to aim for a win-win situation through collaboration. So presently an MOU between ESO and NOVA is in force, which stipulates that 10 out of the 18 man-years funded by NOVA for NEVEC until 2005 [new personnel, in university setting (Leiden) but on project money] shall be used on tasks that are mutually agreed between NOVA and ESO.
The tasks presently are found in the domain of observing modes, calibration and modeling, as well as contributing to the commissioning of new instruments and thinking about future instruments. Another task, outside these 10 FTE, has been the data handling and analysis software for MIDI, and again contributing to its commissioning. Delivery of the first operational version in Heidelberg has just taken place (summer 2002) contributing to the successful Preliminary Acceptance in Europe for MIDI on September 10, 2002. The actual state of 'products and deliveries' and the future outlook are reviewed.
An update of the current status and schedule of PRIMA (Phase-Referenced Imaging and Micro-arcsecond Astrometry) developed for the Very Large Telescope Interferometer (VLTI) is given, with emphasis on the astrometric objectives, performances and technological challenges. PRIMA will allow to observe simultaneously two fields separated by 2 to 60 arcsec, to detect and track the fringes on the brightest object, to detect the fringes on the faintest, and to measure the phase of the secondary set of fringes relative to the primary one, with an accuracy of (lambda) /1000 at 2 micrometers .
The interferometric mode of the ESO very large telescope (VLT) permits coherent combination of stellar light beams collected by four telescopes with 8m diameter and by several auxiliary telescopes of the 2m class. While the position of the 8m telescopes is fixed, auxiliary telescopes can be moved on rails, and can operate from 30 stations distributed on the top of the observatory site for efficient UV coverage. Coherent beam combination can be achieved with the 8m telescopes alone, with the auxiliary telescopes alone, or with any combination, up to eight telescopes in total. A distinct feature of the interferrometric mode is the high sensitivity due to the 8m pupil of the main telescopes which will be compensated by adaptive optics in the near-infrared spectral regime. The VLT interferometer (VLTI) part of the VLT program is conceived as an evolutionary program where a significant fraction of the interferometer's functionality is initially funded, and more capability may be added later while experience is gained and further funding becomes available. Major subsystems of the present baseline VLTI include: three auxiliary telescopes, three delay lines which permit combining the light from up to four telescopes, and a laboratory which contains an imaging beam combiner telescope, and enough space to accomodate a number of experimental setups. This paper presents a general overview of the recent evolution of the project and its future development.