Christophe Buisset

Stable deep nulling in polychromatic unpolarized light with multiaxial beam combination

In the context of the space-based nulling mission ESA-Darwin, Thales Alenia Space has developed a nulling breadboard for the European Space Agency (ESA): the multiaperture imaging interferometer (MAII) to demonstrate deep and stable nulling and to investigate various subsystems of the ESA-Darwin interferometer. Recently, we have extended our investigations to the multiaxial beam combination. This combination scheme presents many advantages: simplicity, compactness, and a high coupling efficiency for a three-beam combination. The near-infrared (lambda approximately 1.55 microm) MAII breadboard has been upgraded to the multiaxial beam combination. Polarization and stability issues have been thoroughly investigated. We report on the recent results we have obtained with the multiaxial configuration of MAII in unpolarized light with a relative spectral bandwidth of 5%: nulling ratios of mean value N=1.7 x 10(-5), stable over 1 h with a standard deviation sigma( N )=5.7 x 10(-7). These results indicate that the multiaxial beam combination has the potential to meet Darwin requirements.

Multi-axial Nulling Interferometry: Demonstration of deep nulling and investigations of polarization effects.

The ESA-Darwin mission is devoted to direct detection and spectroscopic characterization of earth-like exoplanets in the thermal infrared domain by nulling interferometry in space. This technique yields the rejection of starlight so as to make detectable the faintly emitting planet in the neighborhood. In that context, Alcatel Alenia Space has developed a nulling breadboard for ESA in order to perform the rejection of an unresolved on-axis source. This device, the Multi Aperture Imaging Interferometer (MAII) demonstrated high rejection capability at a relevant level for exoplanets, in single-polarized and mono-chromatic conditions. In this paper we report on our late investigations using the MAII focussed on modal filtering. The dependence of the nulling ratio on the degeneracy of the guided modes in the modal filter is put into evidence.

Study of a coronagraphic mask using evanescent waves

The evanescent wave coronagraph (EvWaCo) is a specific kind of band-limited coronagraph using the frustrated total internal reflection phenomenon to produce the coronagraphic effect (removing starlight from the image plane in order to make the stellar environment detectable). In this paper, we present a theoretical and experimental study of the EvWaCo coronagraphic mask. First, we calculate the theoretical transmission and we show that this mask is partially achromatic. Then, we present the experimental results obtained in unpolarized light at the wavelength λ≈900 nm and relative spectral bandwidth Δλ/λ≈6%. In particular, we show that the coronagraph provides a contrast down to a few 10-6 at an angular distance of about ten Airy radii.

Multi-axial interferometry: demonstration of deep nulling

The ESA-Darwin mission is devoted to direct detection and spectroscopic characterization of earthlike exoplanets. Starlight rejection is achieved by nulling interferometry from space so as to make detectable the faintly emitting planet in the neighborhood. In that context, Alcatel Alenia Space has developed a nulling breadboard for ESA in order to demonstrate in laboratory conditions the rejection of an on-axis source. This device, the Multi Aperture Imaging Interferometer (MAII) demonstrated high rejection capability at a relevant level for exoplanets, in singlepolarized and mono-chromatic conditions. In this paper we report on the new multi-axial configuration of MAII and we summarize our late nulling results.

Preliminary design and performance estimate of a telescope dedicated to solar system planet imagery

The observations of the solar system Jovian planets performed by ground-based medium size telescopes can provide an efficient support to the space missions by performing observations of the planet atmospheres. In particular, ground-based medium size telescopes are able to provide high resolution images close to the diffraction limit of giant planets while observing through the Earth atmosphere by using some Lucky Imaging processing. These observations of the Jovian planet atmospheres ideally require i) an instrument with a high angular resolution close to the diffraction limit and ii) a high contrast, especially for the low and medium spatial frequencies that corresponds to the turbulence areas inside the atmosphere clouds bands. We thus decided to design one telescope that shall provide diffraction limited images (without the contribution of the atmosphere) over a circular Field of View (FOV) of diameter equal to 2 arcminutes. This, over the photometric spectral band V, R and I of the Johnson-Cousin photometric. In this paper, we present the design and the performance of a Ritchey-Chretien telescope dedicated to solar system planet imagery with a linear central obscuration lower than 0.15 and an active system to correct the low frequency distortions of the wavefront before each observation. First, we describe the optical design, then we establish the image quality budget. Finally we show that the stray light signal induced by the moon light scattering is negligible during the observations of Jupiter.

Preliminary design and performance estimate of a prime focus camera for the 2.3m Thai National telescope

The National Astronomical Research Institute of Thailand (NARIT) is currently developing a five lenses prime focus camera in order to enlarge the field of view of the 2.3 m Thai National Telescope to a one degree diameter circle. The instrument shall operate in the spectral bands B, V, R and I of the Johnson-Cousins photometric system with an angular resolution better than 2 arcsecond. In this paper, we describe the camera design, we estimate the theoretical performance in the V-band and we show that the theoretical angular resolution after tolerancing is better that 1.3 arcsecond. Then, we present the results of the stray light analysis and we show that the system is free of critical ghost images.

The evanescent wave coronagraph project: setup results and demonstrator preliminary design

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. 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 10-6 at distances greater than 10 Airy radii from the star Point Spread Function (PSF) center. 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 10-4 at a distance equal to the IWA from the PSF.

Development status and performance of the evanescent wave coronagraph testbed

The Evanescent Wave Coronagraph (EvWaCo) is a new kind of “band-limited coronagraph” that involves the tunneling effect to suppress the starlight, thus producing the coronagraphic effect. The first advantage is that this mask adapts itself to the wavelength due to the evanescent wave properties, yielding nearly an achromatization of the star extinction. The second advantage is that the starlight can be collected for astrometry and/or wavefront analysis and correction. NARIT has developed a specific optical setup operating over the spectral band [780 nm, 880nm] to demonstrate highlevel contrasts and inner working angles in line with the requirements for exoplanet detection. Our aims are: to test and characterize the EvWaCo performance in diffraction-limited regime, to install a simulator of turbulence and an adaptive optics setup to simulate ground-based observations, and to define the best scheme for the wavefront correction. The preliminary results obtained in diffraction-limited regime demonstrated contrasts equal to a few 10-6 at a distance between 10 and 20 λ/D from the Point Spread Function (PSF) center with an unpolarized source emitting at λ1 = 880 nm with a relative spectral bandwidth Δλ/λ1 ≈ 6%. In this paper, we first describe the upgraded setup and present the results of the performance characterization that investigates the variation of the contrast with the wavelength and with the polarization. Then, we show the results obtained on the star channel and demonstrate the capability to measure in real time the star PSF profile and position. Finally, we discuss the future improvements to optimize the performance.

Programmable spectrometer using MOEMs devices for space applications

A new class of spectrometer can be designed using programmable components such as MOEMS which enable to tune the beam in spectral width and central wavelength. It becomes possible to propose for space applications a spectrometer with programmable resolution and adjustable spectral bandwidth. The proposed way to tune the output beam is to use the diffraction effect with the so-called PMDG (Programmable Micro Diffraction Gratings ) diffractive MEMS. In that case, small moving structures can form programmable gratings, diffracting or not the incoming light. In the proposed concept, the MOEMS is placed in the focal plane of a first diffracting stage (using a grating for instance). With such implementation, the MOEMS component can be used to select some wavelengths (for instance by reflecting them) and to switch-off the others (for instance by diffracting them). A second diffracting stage is used to recombine the beam composed by all the selected wavelengths. It becomes then possible to change and adjust the filter in λ and Δλ. This type of implementation is very interesting for space applications (Astronomy, Earth observation, planetary observation). Firstly because it becomes possible to tune the filtering function quasi instantaneously. And secondly because the focal plane dimension can be reduced to a single detector (for application without field of view) or to a linear detector instead of a 2D matrix detector (for application with field of view) thanks to a sequential acquisition of the signal.