The objective of the Evanescent Wave Coronagraph (EvWaCo) project is to develop a new kind of simple and cost effective coronagraph, first for ground-based telescopes and then for space-based telescopes. The principle involves the tunneling effect to separate the star light from the companion light. The star light is directed transmitted toward a WaveFront Sensor (WFS) that measures the wavefront distortions in the immediate proximity of the occulting mask with minimum non-common path errors. The beam reflected by the mask propagates toward the Lyot stop and forms the images of the companion and of the star residuals on the camera.<p> </p> The EvWaCo concept has been demonstrated and this instrument is achromatic over the I-band of the Johnson- Cousins photometric system in unpolarized light. We have measured over this photometric band an Inner Working Angle (IWA) equal to 6 λ/D and contrasts of a few 10<sup>-6</sup> at distances greater than 10 Airy radii from the star Point Spread Function (PSF) center.<p> </p> This paper describes the continuation of the project, from this setup of demonstration to the first prototype operating on the sky at horizon 2020. The objective is to show the capability of the full system to provide IWA and raw contrasts close to the state-of-art performance with the Thai National Telescope, by observing through an unobstructed elliptical pupil of major axis length equal to 1 m. The system will demonstrate over the full I-band an IWA close to 3 λ/D and raw contrasts equal to a few 10<sup>-4</sup> at a distance equal to the IWA from the PSF.
We present the recent developments preparing the construction of a new visible 6T beam combiner for the CHARA Array, called SPICA. This instrument is designed to achieve a large survey of stellar parameters and to image surface of stars. We first detail the science justification and the general idea governing the establishment of the sample of stars and the main guidance for the optimization of the observations. After a description of the concept of the instrument, we focus our attention on the first important aspect: optimizing and stabilizing the injection of light into single mode fibers in the visible under partial adaptive optics correction. Then we present the main requirements and the preliminary design of a 6T-ABCD integrated optics phase sensor in the H-band to achieve long exposures and reach fainter magnitudes in the visible.
We report on the VIII edition of the imaging reconstruction contest held in the optical/infrared interferometry community. This edition sported a VLTI+CHARA dataset of a simulated image of a star surrounded by a proto-planetary disk hosting a planet. 5 teams responded to the challenge and produced images very close the original image, showing the maturity of image reconstruction techniques for a well sampled u,v coverage. The contest was organised by the first author of this article, whereas the following authors provided entries to the contest.
We have recently shown that a posteriori co-phasing of multi-spectral interferograms was possible.<sup>1</sup> In this contribution, we extend our approach so that it can be applied to actual data as provided by Amber <sup>2</sup> or Matisse instruments. The main advantage of the proposed post-processing technique is that it requires no modifications of the instruments and yields interferometric observables with higher SNR and much fewer unknowns (in particular for the Fourier phase) than conventional measurements. In order to perform the co-phasing of a complete sequence of interferograms, we jointly estimate a global phase template and the frame dependent optical path errors due to the turbulence. We show that this strategy is effective for very low SNR data. We assess the effectiveness of our method on simulated and actual AMBER data. We also compare the lowest SNR that can be achieved to the theoretical bounds and estimate the gain in sensitivity compared to usual interferometric data.
In the coming year, the CHARA 1-meter telescopes will be equipped with Adaptive Optics (AO) systems. This improvement opens the possibility to apply, in the visible domain, the principle of spatial filtering with single mode fibers well demonstrated in the near-infrared. It will clearly open new astrophysical fields by taking benefit of an improved sensitivity and state-of-the-art precision and accuracy on interferometric observables. A demonstrator called FRIEND (Fibered and spectrally Resolved Interferometric Experiment - New Design) has been developed. FRIEND combines the beams coming from 3 telescopes after injection in single mode optical fibers and provides photometric channels as well as some spectral capabilities for characterization purposes. It operates around the R spectral band (from 600nm to 750nm) and uses the fast and sensitive analog detector OCAM2. On sky tests at the focus of the CHARA interferometer have been performed during the last year to get the optimal DIT or an estimation of the stability of the instrumental visibility. Complementary lab tests have permitted to characterize the birefringence of the fibers, and the characteristics of the detector. In this paper, we present the results of these tests.
Current interferometers often collect data simultaneously in many spectral channels by using dispersed fringes. Such polychromatic data provide powerful insights in various physical properties, where the observed objects show particular spectral features. Furthermore, one can measure spectral differential visibilities that do not directly depend on any calibration by a reference star. But such observations may be sensitive to instrumental artifacts that must be taken into account in order to fully exploit the polychromatic information of interferometric data. As a specimen, we consider here an observation of P Cygni with the VEGA visible combiner on CHARA interferometer. Indeed, although P Cygni is particularly well modeled by the radiative transfer code CMFGEN, we observe questionable discrepancies between expected and actual interferometric data. The problem is to determine their origin and disentangle possible instrumental effects from the astrophysical information. By using an expanded model fitting, which includes several instrumental features, we show that the differential visibilities are well explained by instrumental effects that could be otherwise attributed to the object. Although this approach leads to more reliable results, it assumes a fit specific to a particular instrument, and makes it more difficult to develop a generic model fitting independent of any instrument.
In the next 2 or 3 years, the two major interferometric arrays, VLTI and CHARA, will equip their telescopes of 1.8m and 1m respectively with Adaptive Optics (AO hereafter) systems. This improvement will permit to apply with a reasonable e_ciency in the visible domain, the principle of spatial filtering with single mode fibers demonstrated in the near-infrared. It will clearly open new astrophysical fields by taking benefit of an improved sensitivity and state-of-the-art precision and accuracy on interferometric observables. To prepare this future possibility, we started the development of a demonstrator called FRIEND (Fibered and spectrally Resolved Interferometric Experiment - New Design). FRIEND combines the beams coming from 3 telescopes after injection in single mode optical fibers and provides some spectral capabilities for characterization purposes as well as photometric channels. It operates in the R spectral band (from 600<i>nm</i> to 750<i>nm</i>) and uses the world's fastest and more sensitive analogic detector OCAM2. Tests on sky at the focus of the CHARA interferometer are scheduled for December 2014. In this paper, we present the first interferometric tests of the OCAM2 detector performed on CHARA in November 2012 and the concept, the expected performance and the opto-mechanical design of FRIEND.
The so-called “<i>phase delay tracking</i>” attempts to estimate the effects of the turbulence on the phase of the interferograms in order to numerically cophase the measured complex visibilities and to coherently integrate them. This is implemented by the “<i>coherent fringe analysis</i>” of MIDI instrument<sup>1</sup> but has only been used for high SNR data. In this paper, we investigate whether the sensitivity of this technique can be pushed to its theoretical limits and thus applied to fainter sources. In the general framework of the maximum likelihood and exploiting the chromatic behavior of the turbulence effects, we propose a global optimization strategy to compute various estimators of the differential pistons between two data frames. The most efficient estimators appear to be the ones based on the phasors, even though they do not yet reach the theoretical limits.
This poster advertizes the Jean-Marie Mariotti Center software tools, databases and services aimed at facilitating the use of optical interferometry worldwide such as preparation of observations, data reduction and data analysis. Its mission and organization are presented before listing the current software suite. Finally some facts and perspectives are mentioned.
In this paper, we review the current performance of the VEGA/CHARA visible spectrograph and make a review of
the most recent astrophysical results. The science programs take benefit of the exceptional angular resolution, the
unique spectral resolution and one of the main features of CHARA: Infrared and Visible parallel operation. We
also discuss recent developments concerning the tools for the preparation of observations and important features
of the data reduction software. A short discussion of the future developments will complete the presentation,
directed towards new detectors and possible new beam combination scheme for improved sensitivity and imaging
In the framework of the understanding of extrasolar systems, the study of host stars is a fundamental point. We
need to understand the link between them and the presence of companions, i.e. what makes a star becoming
a host star. In this perspective, we used the instrument called VEGA, situated at Mount Wilson (California)
on the CHARA array to perform optical interferometric measurements. Interferometry at visible wavelengths
allows reaching very high spatial frequencies well adapted for very small (less than 1 millisecond of arc) angular
diameters. Therefore, we can access limb darkening measurements which is one of the very few directly measurable
constraints on the structure of the atmosphere of a star. From this we can derive stars fundamental
parameters. A precise measurement within spectral lines is also a very powerful tool to study the temperature
and density structure of the atmosphere of distant stars. Besides, the detection of exoplanets is also related to
this method. Combined with the radial velocity method and the transit method, one can study the atmosphere
of exoplanets and learn more about their internal structure. We started a large program of observations made of
40 stars hosting exoplanets and observable by VEGA/CHARA. We will measure their limb darkened diameters
and derive their parameters. We also aim at better understanding stellar noise sources like spots, and study
surface brightness relationships.
A challenge of adaptive optics (AO) on Extremely Large Telescopes (ELTs) is to overcome the difficulty of solving a huge
number of equations in real time, especially when atmospheric tomography is involved. This is particularly the case for
multi-conjugate or multi-objects AO systems. In addition, the quality of the wavefront estimation is crucial to optimize the
performances of the future systems in a situation where measurements are missing and noises are correlated.
The Fractal Iterative Method has been introduced as a fast iterative algorithm for minimum variance wavefront reconstruction
and control on ELTs. This method has been successfully tested on Classical Single Conjugate AO systems on
Octopus numerical simulator at ESO. But the minimum variance approach is expected to be mostly useful with atmospheric
We present the first results obtained with FrIM in the context of atmospheric tomography. We recall the principle of
the algorithm and we summarize the formalism used for modeling the measurements obtained from laser guide stars that
entail spot elongation and tip/tilt indetermination, mixed with low order measurements from natural guide stars. We show
the respective effects of tip/tilt indetermination, spot elongation, unseen modes on various configurations, as well as the
usefulness of priors and correct noise models in the reconstruction.
This analysis is essential for balancing the various errors that combine in a quite complex way and to optimize the
configuration of the future AO systems for specific science cases and instrument requirements.
This paper presents the current status of the VEGA (Visible spEctroGraph and polArimeter) instrument installed
at the coherent focus of the CHARA Array, Mount Wilson CA. Installed in september 2007, the first science
programs have started during summer 2008 and first science results are now published. Dedicated to high angular (0.3mas) and high spectral (R=30000) astrophysical studies, VEGA main objectives are the study of circumstellar environments of hot active stars or interactive binary systems and a large palette of new programs dedicated to fundamental stellar parameters. We will present successively the main characteristics of the instrument and its current performances in the CHARA environment, a short summary of two science programs and finally we will develop some studies showing the potential and difficulties of the 3 telescopes mode of VEGA/CHARA.
The VEGA spectrograph and polarimeter has been recently integrated on the visible beams of the CHARA
Array. With a spectral resolution up to 35000 and thanks to operation at visible wavelengths, VEGA brings
unique capabilities in terms of spatial and spectral resolution to the CHARA Array. We will present the main
characteristics of VEGA on CHARA, some results concerning the performance and a preliminary analysis of the
first science run.
LITpro is a software for fitting models on data obtained from various stellar optical interferometers, like the VLTI. As a
baseline, for modeling the object, it provides a set of elementary geometrical and center-to-limb darkening functions, all
combinable together. But it is also designed to make very easy the implementation of more specific models with their
own parameters, to be able to use models closer to astrophysical considerations. So LITpro only requires the modeling
functions to compute the Fourier transform of the object at given spatial frequencies, and wavelengths and time if needed.
From this, LITpro computes all the necessary quantities as needed (e.g. visibilities, spectral energy distribution, partial
derivatives of the model, map of the object model). The fitting engine, especially designed for this kind of optimization, is
based on a modified Levenberg-Marquardt algorithm and has been successfully tested on real data in a prototype version.
It includes a Trust Region Method, minimizing a heterogeneous non-linear and non-convex criterion and allows the user
to set boundaries on free parameters. From a robust local minimization algorithm and a starting points strategy, a global
optimization solution is effectively achieved. Tools have been developped to help users to find the global minimum. LITpro
is also designed for performing fitting on heterogeneous data. It will be shown, on an example, how it fits simultaneously
interferometric data and spectral energy distribution, with some benefits on the reliability of the solution and a better
estimation of errors and correlations on the parameters. That is indeed necessary since present interferometric data are
The current projects of Extremely Large Telescopes rely on adaptive optics systems using several sodium laser guide
stars (LGSs). Because of the thickness of the sodium layer in the mesosphere, the subapertures of a Shack-Hartmann
wavefront sensor will see the LGS all the more elongated as its position is distant from the launching point of the laser.
This effect is significant and prompts the lasers to be launched from behind the secondary instead of from around the
telescope. The elongations increase the centroiding errors and new smarter algorithms have been designed to mitigate
this effect, but the loss of accuracy is still significant. Further, the measurement uncertainties are no more uniform across
the pupil and correlations are introduced between the two coordinates of the gradients. From numerical simulations, we
analyze the benefit of taking into account this structured correlations in wavefront reconstruction algorithms and compare
the reconstruction accuracy when using least squares, weighted least squares, or minimum variance using von Karman
turbulence priors. For a single LGS launched behind the secondary, numerical simulations show effective improvements
when using both noise correlations and priors in wavefront reconstruction. When combining the measurements from
several LGSs in a Ground Layer adaptive optics system, we show that taking into account the noise covariances yields
better reconstructions when LGSs are launched from around the telescope than from behind the secondary. Further, results
indicate that we could discard the measurements along the elongated direction where this elongation is greater than a given
We describe a project for the installation of a visible focal instrument at the CHARA Array, named VEGA for Visible spEctroGraph and polArimeter. This new instrument will further open the visible domain and offer both spectral and polarimetric capabilities at the CHARA Array. It will create a new and unique scientific niche for the CHARA Array, especially in the context of international competition. The combination of the visible domain and high spectral resolution mode combined with a good sensitivity will allow VEGA/CHARA to carve out a new piece of observational phase space and compliment many existing or planned near-infrared interferometers. VEGA will help make CHARA the interferometer with the largest spectral and spatial resolution worldwide.
We give an overview of recent results obtained with the GI2T interferometer. On the technical side, great improvements have been obtained on photon counting detectors, especially in terms of quantum efficiency and of photon centroiding algorithms. Piston measurements with the GI2T dispersed fringes have been made during
coordinate observations with the Generalized Seeing Monitor GSM. These observations have lead to wavefront outer scale determinations. The last topic we will present concerns the polarimetric measurements done with the SPIN device on the GI2T spectrograph. We conclude this paper by a summary of the results obtained with the GI2T during its scientific life.
We present a summary of the global system analysis that led to the current definition of the AMBER instrument. AMBER is a near infrared multi-beam combiner for the Very Large Telescope Interferometer. This analysis goes through the following issues: atmospheric systematics including atmospheric turbulence and dispersion, analysis of single mode optical fibers, photometry calibration, spectral dispersion, background noise, data reduction and calibration steps.
SPID aims at offering a high spectral resolution in both short-exposure (speckle imaging) and long-exposure (adaptive optics with partial compensation) modes. It offers an adjustable spectral resolution (from 60 up to 3000) in the range 400 - 750 nm. For differential observation of astronomical objects, SPID gives images in two spectral bandwidths at the same time. The width and the central wavelength of each bandwidth can be chosen independently. A high image quality is achieved thanks to a new design derived from a Courtes' monochromator. SPID also includes a wavefront sensor for post-compensation processing. A short-exposure mode allows us to achieve diffraction limited images but with a low signal-to-noise ratio. Depending on the object brightness and on the seeing quality, adaptive optics will allow us to improve significantly the signal-to-noise ratio and sometimes to observe a diffraction limited core in long-exposure mode. Depending on the scientific goal, the availability of the two modes will drive the best choice. The current status of SPID is presented together with first results obtained at CFHT in the short-exposure mode.
AMBER is a focal instrument for the Very Large Telescope Interferometer working in the near infrared from 1.1 to 2.4 micrometers . It has been designed having in mind the General User of interferometric observations and the full range of his possible astrophysical programs. However the three programs used to define the key specifications have been the study of Young Stellar Objects, the study of Active Galactic Nuclei dust tori and broad line regions and the measure of masses and spectra of hot Extra Solar Planets. AMBER combines up to three beams produced by the VLTI 8 m Unit Telescopes equipped with Adaptive Optics and/or by the 1.8 m Auxiliary Telescopes. The fringes are dispersed with resolutions ranging from 35 to 10000. It is optimized for high accuracy single mode measurements of the absolute visibility, of the variation of the visibility and phase with wavelength (differential interferometry) and of phase closure relations with three telescopes. The instrument and its software are designed to allow a highly automated user friendly operation and an easy maintenance.
This paper presents the optical layout of the REGAIN beam combiner including the optical delay line LAROCA with its variable curvature mirror, the field rotator devices, the image and pupil tracking systems and the dedicated visible spectrography. Preliminary studies of foreseen improvements, such as adaptive optics, IR spectrograph and addition of a third telescope, will be discussed.
A new focal instrumentation for the Grand Interferometre a 2 Telescopes (GI2T) called REGAIN (REcombinaison pour GrAnd INterferometre) is under study at the Observatoire de la Cote d'Azur (OCA) and the Laboratoire d'Astronomie Spatiale (LAS) in Marseille, France. The objectives of the REGAIN project are multiple. Priority number 1 is a more efficient astrophysical exploitation of the GI2T. Next is the possibility for observing simultaneously at visible and near-infrared wavelengths. Finally REGAIN should ensure the test of the OVLA prototype telescope added to the present GI2T. Therefore, the resulting GI3T could be used for phase-closure imaging with 1.5-m apertures. At the same time reservations will be made for implementing adaptive optics units for each telescope whilst the VLT interferometer fringe- sensor currently studied at the OCA, should be tested on the GI3T.
This paper describes the present status of the Grand Interferometre a 2 Telescopes (GI2T). We review the general features of this instrument and present the scientific programs pursued by our group. Attention is given here to procedures of instrumental and visibility calibration, including the response of both the detector and spectrometer. We discuss the method of data analysis and the attainable accuracy of astrophysical parameters. The current limitations of the GI2T and development of our new optical table are presented.