The goal of a cophasing sensor (CS) is to measure the phase disturbances between the sub-apertures or inside each aperture of a telescope. Three CSs are currently studied at ONERA. A first CS for Earth imaging is based on phase diversity on extended sources (cf companion paper by L. Mugnier). A second CS for intercalibration uses phase retrieval on a point source. The third CS for nulling interferometry (“DWARF”, for the ESA/DARWIN mission) is based on similar algorithms. To test performance of these CSs, ONERA has defined and integrated a multipurpose bench, “BRISE”. Its main features are the simultaneous imaging of a point source and of an extended source, the minimisation of absolute and differential disturbances, the use of any aperture configuration and the generation of pure calibrated piston/tip/tilt aberrations on three sub-apertures by a dedicated PZT-based device. Preliminary experimental results are consistent with numerical simulations and confirm nanometric performance.
This paper, “Eléments-clés de la conception d'un instrument spatial à synthése d'ouverture optique," was presented as part of International Conference on Space Optics—ICSO 1997, held in Toulouse, France.
PEGASE, a spaceborne mission proposed to the CNES, is a 2-aperture interferometer for nulling and interferometric imaging. PEGASE is composed of 3 free-flying satellites (2 siderostats and 1 beam combiner) with baselines from 50 to 500 m. The goals of PEGASE are the spectroscopy of hot Jupiter (Pegasides) and brown dwarves, the exploration of the inner part of protoplanetary disks and the validation in real space conditions of nulling and visibility interferometry with formation flying. <p> </p>During a phase-0 study performed in 2005 at CNES, ONERA and in the laboratories, the critical subsystems of the optical payload have been investigated and a preliminary system integration has been performed. These subsystems are mostly the broadband (2.5-5 μm) nuller and the cophasing system (visible) dedicated to the real-time control of the OPD/tip/tilt inside the payload. A laboratory breadboard of the payload is under definition and should be built in 2007.
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.
This paper presents a summary of the phase-0 performed in 2005 for the Pegase mission. The main scientific goal is the spectroscopy of hot Jupiters (Pegasides) and brown dwarfs from 2.5 to 5 μm. The mission can extend to the exploration of the inner part of protoplanetary disks, the study of dust clouds around AGN,... The instrument is basically a two-aperture (D=40 cm) interferometer composed of two siderostats and one beam-combiner. The formation is linear and orbits around L2, pointing in the anti-solar direction within a +/-30° cone. The baseline is adjustable from 50 to 500 m in both nulling and visibility measurement modes. The angular resolution ranges from 1 to 20 mas and the spectral resolution is 60. in the nulling mode, a 2.5 nm rms stability of the optical path difference (OPD) and a pointing stability of 30 mas rms impose a two level control architecture. It combines control loops implemented at satellite level and control loops operating inside the payload using fine mechanisms. According to our preliminary study, this mission is feasible within an 8 to 9 years development plan using existing or slightly improved space components, but its cost requires international cooperation. Pegase could be a valuable Darwin/TPF-I pathfinder, with a less demanding, but still ambitious, technological challenge and a highly associated scientific return.
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<sup>-6</sup> stabilized at 9x10<sup>-8</sup> 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.
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.<p> </p>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.
We report pupillometry results corresponding to three studies. A first study aims at measuring 2D pupil geometry with high precision (below 2 microns) at high frequency (more than 450Hz). The two other studies aim at measuring 3D pupil movements, with and without a chin rest. Results of measurements over 42 subjects are presented.
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<sup>-6</sup> stabilized at 9 10<sup>-8</sup> 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.
Stabilizing a nulling interferometer at a nanometric level is the key issue to obtain deep null depths. The
PERSEE breadboard has been designed to study and optimize the operation of cophased nulling bench in the
most realistic disturbing environment of a space mission. This presentation focuses on the current results of the
PERSEE bench. In terms of metrology, we cophased at 0.33 nm rms for the piston and 60 mas rms for the tip/tilt.
A Linear Quadratic Gaussian (LQG) control coupled with an unsupervised vibration identification allows us to
maintain that level of correction, even with characteristic vibrations of nulling interferometry space missions.
These performances, with an accurate design and alignment of the bench, currently lead to a polychromatic
unpolarised null depth of 8.9 × 10<sup>-6</sup> stabilized at 2.7 × 10<sup>-7</sup> on the [1.65 - 2.45] μm spectral band (37%
bandwidth). With those significant results, 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.
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<sup>-5</sup> stabilized
at 3•10<sup>-6</sup>. 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.
Optical aberrations induced by turbulent flows are critical issues for the performance of an airborne optical system. In
that context experiments were performed on a test-body at Mach 3.7 with several high Reynolds number in the S3MA
wind tunnel of ONERA. A Shack-Hartman wave front sensing was performed (see companion paper of R. Deron).
The objective of this work was to develop a computational algorithm to model the wave front distortions in temporal
First the aerodynamical flow field is obtained by RANS computation around the test body and mass-density is
interpolated in an optical grid. Then wave front has been calculated using ray tracing from eikonal equations. For this
validation two Reynolds numbers were retained and the boundary layer is turbulent on downstream window. A planar
optical beam is emitted from the window and passes through the inhomogeneous media from the boundary layer to the
shock wave. Various pupil, angle positions and lines of sight are considered. Zernike decomposition and MTF
computation allow modal analysis in the near field condition and evaluation of the image quality respectively. Tilt effects
appear to be the dominant aberration while higher orders have a limited impact on the image quality, except for large
departures from the normal of the window. Discrepancies observed according to the line of sight variation are analysed.
Finally simulations results compare favorably with the measurements made with the Shack-Hartmann wave front
sensing. So this computational study is conforted and allows to complete the experiment.
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<sup>-4</sup> rejection rate and a 10<sup>-5</sup> 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.
The Cophasing Sensor (CS), which measures the disturbances between the sub-apertures, is a key component of multiple-aperture telescopes. As multiple-aperture telescopes become more ambitious, requirements for the CS become more demanding: low flux (for stellar interferometers), sub-nanometric accuracy (for interferometric nullers), image with very small contrast (for wide-field telescopes such as spaceborne Earth imagers), larger number of beams (for all applications). Focal-plane sensing is a solution to cope with all these requirements, with a very simple opto-mechanical setup. Two implementations have been investigated at ONERA: <i>phase retrieval</i>, using the sole focal-plane image, and <i>phase diversity</i>, based on the joint analysis of a focal and an extra-focal images. Phase diversity can measure any mode on any source, while phase retrieval is more suited to real-time piston/tip/tilt measurements on an unresolved (or partially resolved) source. To evaluate accurately the performance of CS or other high-resolution devices, ONERA has built a multipurpose bench called BRISE (Banc Reconfigurable d'Interferometrie sur Sources Etendues). BRISE mainly includes an extended scene and a reference point source, a deformable mirror, a focal-plane CS, afocal input/output ports to interface with other instruments, and a general purpose code MASTIC (Multiple-Aperture Software for Telescope Imaging and Cophasing). BRISE has already been (or will be) used for several applications, such as the validation of CSs for Earth imaging or nulling interferometry, or the exploration of advanced nulling techniques. This paper describes the bench and the investigated CSs, the experiments performed on BRISE, and reports main results such as nanometric accuracy or three-beam nulling.
The space based mission Pegase was proposed to CNES in the framework of its call for scientific proposals for formation
flying missions. This paper presents a summary of the phase-0 performed in 2005. The main scientific goal is the
spectroscopy of hot Jupiters (Pegasides) and brown dwarfs from 2.5 to 5 μm. The mission can extend to other objectives
such as the exploration of the inner part of protoplanetary disks, the study of dust clouds around AGN,... The instrument
is basically a two-aperture (D=40 cm) interferometer composed of three satellites, two siderostats and one beam-combiner.
The formation is linear and orbits around L2, pointing in the anti-solar direction within a +/-30° cone. The
baseline is adjustable from 50 to 500 m in both nulling and visibility measurement modes. The angular resolution ranges
from 1 to 20 mas and the spectral resolution is 60. In the nulling mode, a 2.5 nm rms stability of the optical path
difference (OPD) and a pointing stability of 30 mas rms impose a two level control architecture. It combines control
loops implemented at satellite level and control loops operating inside the payload using fine mechanisms. According to our preliminary study, this mission is feasible within an 8 to 9 years development plan using existing or slightly
improved space components, but its cost requires international cooperation. Pegase could be a valuable Darwin/TPF-I
pathfinder, with a less demanding, but still ambitious, technological challenge and a high associated scientific return.
Multiple-Aperture Optical Telescopes (MAOTs) are a promising solution for very high resolution imaging. In the Michelson configuration, the instrument is made of sub-telescopes distributed in the pupil and combined by a common telescope via folding periscopes. The phasing conditions of the sub-pupils lead to specific optical constraints in these subsystems. The amplitude of main contributors to the wavefront error (WFE) is given as a function of high level requirements (such as field or resolution) and free parameters, mainly the sub-telescope type, magnification and diameter. It is shown that for the periscopes, the field-to-resolution ratio is the main design driver and can lead to severe specifications. The effect of sub-telescopes aberrations on the global WFE can be minimized by reducing their diameter. An analytical tool for the MAOT design has been derived from this analysis, illustrated and validated in three different cases: LEO or GEO Earth observation and astronomy with extremely large telescopes. The last two cases show that a field larger than 10 000 resolution elements can be covered with a very simple MAOT based on Mersenne paraboloid-paraboloid sub-telescopes. Michelson MAOTs are thus a solution to be considered for high resolution wide-field imaging, from space or ground.
The Real Time Computer RTC is a key component of the Nasmyth Adaptive Optics System, controlling the 185 actuators of the deformable mirror from a 144 Shack-Hartmann subapertures wavefront sensor at a maximum frequency of 500 Hz. It also provides additional capabilities such as real time optimization of the control loop which is the warranty for NAOS to achieve a very good Strehl Ratio in a broad magnitude range (Mv equals 8 up to 18), on-line turbulence and performance estimations and finally capability to store and process the data necessary to the off-line PSF reconstruction algorithm. This RTC is also designed to be easily upgraded as for Laser Guide Star. Moreover all softwares can be easily adapted to control a curvature sensor as well as the hardware which can be used with the two types of wave front sensors.
We present the Active Stabilization in Stellar Interferometry (ASSI) beam combining optical table which was installed on the 2- telescope interferometer (I2T) of the Observatoire de la Cote d'Azur in 1993. To achieve very high angular resolution, the 26- centimeter telescopes can be positioned along a 140-meter North- South baseline. The limiting magnitude of the instrument depends dramatically on its ability to stabilize the fringe pattern despite the atmospheric disturbances. The function of the ASSI table is to perform this task. Three adaptive mirrors are used. The first two are fine pointing mirrors which correct the fluctuations of the angle of arrival of the two wavefronts. The other corrects the optical path difference fluctuations between the two telescopes. These corrections, e.g. tip-tilt and piston phase, are required to obtain high precision visibility measurements. We present our first observing results obtained on bright stars that have allowed the evaluation of the ASSI table performance in image tracking.
We report preliminary results on fringe pattern acquisition and stabilization as performed on a Mach-
Zehnder set up representative of Stellar Interferometry needs. The system algorithm is based on "white
light" fringe tracking controlled from a reference interferometer synchronous detection (central fringe
locked interferometry). This servo-system drives a simple two-stages delay line for real time
compensation of the optical path delays either due to atmospheric or instrumental errors. Contrasts (nonoptimized)
of 70 % on the stabilized fringe pattern were measured, strongly encouraging to pursue the
development of the technique.