Nulling interferometry is one of the direct detection methods assessed to find and characterize extrasolar planets and particularly telluric ones. Several projects such as Darwin [1;2], TPF-I [3;4], PEGASE [5;6] or FKSI , are currently considered. One of the main issues is the feasibility of a stable polychromatic null despite the presence of significant disturbances, induced by vibrations, atmospheric turbulence on the ground or satellite drift. Satisfying all these requirements is a great challenge and a key issue of these missions. In the context of the PEGASE mission, it was decided (in 2006), to build a laboratory demonstrator named PERSEE. It is the first laboratory setup which couples deep nulling interferometry with a free flying GNC simulator . It is developed by a consortium composed of CNES, IAS, LESIA, OCA, ONERA, and TAS. In this paper, we detail the main objectives, the set-up and the function of the bench. We describe all the subsystems and we focus particularly on two key points of PERSEE: the beam combiner and the Fringe tracker.
Although it has been recently postponed due to high cost and risks, nulling interferometry in space remains one of the very few direct detection methods able to characterize extrasolar planets and particularly telluric ones. Within this framework, several projects such as DARWIN , , TPF-I , , FKSI  or PEGASE , , have been proposed in the past years. Most of them are based on a free flying concept. It allows firstly to avoid atmosphere turbulence, and secondly to distribute instrumental function over many satellites flying in close formation. In this way, a very high angular resolution can be achieved with an acceptable launch mass. But the price to pay is to very precisely position and stabilize relatively the spacecrafts, in order to achieve a deep and stable extinction of the star. Understanding and mastering all these requirements are great challenges and key issues towards the feasibility of these missions. Thus, we decided to experimentally study this question and focus on some possible simplifications of the concept.
Since 2006, PERSEE (PEGASE Experiment for Research and Stabilization of Extreme Extinction) laboratory test bench is under development by a consortium composed of Centre National d’Etudes Spatiales (CNES), Institut d’Astrophysique Spatiale (IAS), Observatoire de Paris-Meudon (LESIA), Observatoire de la Côte d’Azur (OCA), Office National d’Etudes et de Recherches Aérospatiales (ONERA), and Thalès Alénia Space (TAS) . It is mainly funded by CNES R&D. PERSEE couples an infrared wide band nulling interferometer with local OPD and tip/tilt control loops and a free flying Guidance Navigation and Control (GNC) simulator able to introduce realistic disturbances. Although it was designed in the framework of the PEGASE free flying space mission, PERSEE can adapt very easily to other contexts like FKSI (in space, with a 10 m long beam structure) or ALADDIN  (on ground, in Antarctica) because the optical designs of all those missions are very similar. After a short description of the experimental setup, we will present first the results obtained in an intermediate configuration with monochromatic light. Then we will present some preliminary results with polychromatic light. Last, we discuss some very first more general lessons we can already learn from this experiment.
The PERSEE breadboard, developed by a consortium including CNES, IAS, LESIA, OCA, ONERA and TAS since 2006, is a nulling demonstrator that couples an infrared nulling interferometer with a formation flying simulator able to introduce realistic disturbances in the set-up. The general idea is to prove that an adequate optical design can considerably release the constraints applied at the spacecrafts level of a future interferometric space mission like Darwin/TPF or one of its precursors. The breadboard is now fully operational and the measurements sequences are managed from a remote control room using automatic procedures. A set of excellent results were obtained in 2011: the measured polychromatic nulling depth with non polarized light is 8.8x10-6 stabilized at 9x10-8 in the [1.65-2.45] μm spectral band (37% bandwidth) during 100s. This result was extended to a 7h duration thanks to an automatic calibration process. The various contributors are identified and the nulling budget is now well mastered. We also proved that harmonic disturbances in the 1-100Hz up to several tens of nm rms can be very efficiently corrected by a Linear Quadratic Control (LQG) if a sufficient flux is available. These results are important contributions to the feasibility of a future space based nulling interferometer.
The PERSEE breadboard, developed by a consortium including CNES, IAS, LESIA, OCA, ONERA and
TAS since 2005, is a nulling demonstrator that couples an infrared nulling interferometer with a formation flying
simulator able to introduce realistic disturbances in the set-up. The general idea is to prove that an adequate optical
design can considerably relax the constraints applying at the spacecrafts level of a future interferometric space mission
like Darwin/TPF or one of its precursors. The breadboard is now fully operational and the measurements sequences are
managed from a remote control room using automatic procedures. A set of excellent results were obtained in 2011. The measured polychromatic nulling depth with non polarized light is 8.8 10-6 stabilized at 9 10-8 in the 1.65-2.45 μm spectral band (37 % bandwidth) during 100 s. This result was extended to a 7h duration thanks to an automatic calibration process. The various contributors are identified and the nulling budget is now well mastered. We also proved that harmonic disturbances in the 1-100 Hz up to several ten’s of nm rms can be very efficiently corrected by a Linear Quadratic Control (LQG) if a sufficient flux is available. These results are important contributions to the feasibility of a future space based nulling interferometer.
On-going developments on the PERSEE nulling testbench include the realization of a focal plane simulator featuring one central star, an extra-solar planet orbiting around it, and an Exo-Zodiacal Cloud (EZC) surrounding the observed stellar system. PERSEE (Pégase Experiment for Research and Stabilization of Extreme Extinction) is a laboratory testbench jointly developed by a Consortium of six French institutes and companies, incorporating Observatoire de la Côte d'Azur (OCA) who is in charge of the manufacturing and procurement of the future Star and Planet Simulator (SPS). In this
communication is presented a complete description of the SPS, including general requirements, techniques employed for
simulating the observed planet and EZC, opto-mechanical design and expected performance. The current status of the SPS activities is summarized in the conclusion, pending final integration on the PERSEE test bench in September 2011.
Nulling interferometry is still a promising method to characterize spectra of exoplanets. One of the main issues
is to cophase at a nanometric level each arm despite satellite disturbances. The bench PERSEE aims to prove
the feasibility of that technique for spaceborne missions. After a short description of PERSEE, we will first
present the results obtained in a simplified configuration: we have cophased down to 0.22 nm rms in optical
path difference (OPD) and 60 mas rms in tip/tilt, and have obtained a monochromatic null of 3 · 10-5 stabilized
at 3•10-6. The goal of 1 nm with additional typical satellite disturbances requires the use of an optimal
control law; that is why we elaborated a dedicated Kalman filter. Simulations and experiments show a good
rejection of disturbances. Performance of the bench should be enhanced by using a Kalman control law, and we
should be able to reach the desired nanometric stability. Following, we will present the first results of the final
polychromatic configuration, which includes an achromatic phase shifter, perturbators and optical delay lines.
As a conclusion, we give the first more general lessons we have already learned from this experiment, both at
system and component levels for a future space mission.
Less than 20 years after the discovery of the first extrasolar planet, exoplanetology is rapidly growing with more than
one discovery every week on average since 2007. An important step in exoplanetology is the chemical characterization
of exoplanet atmospheres. It has recently been shown that molecular signatures of transiting exoplanets can be studied
from the ground. To advance this idea and prepare more ambitious missions such as THESIS, a dedicated spectrometer
named the New Mexico Tech Extrasolar Spectroscopic Survey Instrument (NESSI) is being built at New Mexico Tech
in collaboration with the NASA Jet Propulsion Laboratory. NESSI is a purpose-built multi-object spectrograph that
operates in the J, H, and K-bands with a resolution of R = 1000 in each, as well as a lower resolution of R = 250 across
the entire J/H/K region.
We present the second-generation VLTI instrument GRAVITY, which currently is in the preliminary design phase.
GRAVITY is specifically designed to observe highly relativistic motions of matter close to the event horizon of Sgr A*,
the massive black hole at center of the Milky Way. We have identified the key design features needed to achieve this
goal and present the resulting instrument concept. It includes an integrated optics, 4-telescope, dual feed beam combiner
operated in a cryogenic vessel; near infrared wavefront sensing adaptive optics; fringe tracking on secondary sources
within the field of view of the VLTI and a novel metrology concept. Simulations show that the planned design matches
the scientific needs; in particular that 10µas astrometry is feasible for a source with a magnitude of K=15 like Sgr A*,
given the availability of suitable phase reference sources.
Spectral characterization of exo-planets can be made by nulling interferometers. In this context, several projects
have been proposed such as DARWIN, FKSI, PEGASE and TPF, space-based, and ALADDIN, ground-based. To
stabilize the beams with the required nanometric accuracy, a cophasing system is required, made of piston/tip/tilt
actuators on each arm and piston/tip-tilt sensors. The demonstration of the feasibility of such a cophasing system
is a central issue.
In this goal, a laboratory breadboard named PERSEE is under integration. Main goals of PERSEE are the
demonstration of a polychromatic null from 1.65 μm to 3.3 μm with a 10-4 rejection rate and a 10-5 stability
despite the introduction of realistic perturbations, the study of the interfaces with formation-flying spacecrafts
and the joint operation of the cophasing system with the nuller.
We describe the principle of the cophasing system made by Onera, operating in the [0.8 - 1] μm (tip/tilt)
and [0.8 - 1.5] μm (piston) spectral bands. Emphasis is put on the piston sensor and its close integration with
Nulling interferometry is one of the most promising methods to study habitable extrasolar systems. Several
projects, such as Darwin, TPF, Pegase, FKSI or Aladdin, are currently considered and supported by R&D
One of the main issues of nulling interferometry is the feasibility of a stable polychromatic null despite the
presence of significant disturbances, induced by vibrations, atmospheric turbulence on the ground or satellite
drift for spaceborne missions. To reduce cost and complexity of the whole system, it is necessary to optimize not
only the control loop performance at platform and payload levels, but also their interaction.
In this goal, it was decided in 2006 to build a laboratory demonstrator named Persee. Persee is mostly
funded by CNES and built by a consortium including CNES, IAS, LESIA, OCA, ONERA and TAS. After a
definition phase in 2006, the implementation of the sub-systems has now begun and the integration near Paris
by GIS-PHASE (LESIA, ONERA and GEPI) is planned in 2009.
This paper details the main objectives of PERSEE, describes the definition of the bench, presents the current
status and reports results obtained with the first sub-systems.
Nulling interferometry requires, among other things, a symmetric recombination module and an optical path difference
control system. The symmetric recombination stage has been particularly studied over the last ten years and several
concepts are now well known. One of them is the "Modified Mach Zehnder" (MMZ) interferometer, proposed by
Serabyn and Colavita (2001) . In this paper, we describe a new version of the MMZ beam combiner which provides a
deep null signal in the science channel and, at the same time, phase-sensitive signals in the so-called co-phasing channel.
From the latter, accurate optical path difference measurements can be derived. This beam combiner works in the 0.8 to
3.3 μm spectral range (0.8 to 1.5 μm for the co-phasing channel and 1.65 to 3.3 μm for the science channel). Both optical
functions can be implemented in the same device thanks to an original optical design involving dedicated phase shifts. In
this paper, we describe its principle and detail the optical and mechanical design.
GRAVITY, a VLTI second generation instrument, requires a fringe tracker combining four beams. Its design is
driven by the observation of the Galactic Center and implies stringent fringe sensor specifications. We present
the simulations of the fringe tracking closed-loop performance with an optical path difference (OPD) turbulence
spectrum using a Kolmogorov model of the atmosphere for typical seeing conditions at VLTI (r0 = 0.95 m,
t0 = 47 ms at 2.2 μm). We show that the total residual OPD standard deviation can be as low as λ/12 at
a sampling frequency of 350 Hz on a guide star with a magnitude of mK = 10. To obtain this performance,
we compared several 4-beam pairwise co-axial combination conceptual architectures and show that the optimal
4-beam combination is the one measuring the OPD on the six baselines.