MIRI, the mid-IR instrument of JWST (JamesWebb Space Telescope), will include a coronagraphic device for which the ultimate goal is the detection and caracterization of extrasolar planets around nearby stars. In this paper, we describe the optical concept of MIRI and the instrumental choices that were made in phase A and B regarding the coronagraphic system. Finally, we will present numerical simulations that we carried out to evaluate the capability for detecting extrasolar giant gazeous planets.
Erick Bondoux, Sandra Bosio, Rijuparna Chakraborty, Wassila Dali-Ali, Antoine Labeyrie, Bruno Lacamp, Jerome Maillot, Denis Mourard, Paul Nunez, Jordi Pijoan, Rémi Prudhomme, Pierre Riaud, Martine Roussel, Arun Surya, Bernard Tregon, Thomas Houllier, Thierry Lepine, Patrick Rabou, André Rondi, Yves Bresson, David Vernet
Hypertelescopes are large optical interferometric arrays, employing many small mirrors and a miniature pupildensifier before the focal camera, expected to produce direct images of celestial sources at high resolution. Their peculiar imaging properties, initially explored through analytical derivations, had been verified with simulations before testing a full-size testbed instrument. We describe several architectures and optical design solutions and present recent progress made on the Ubaye hypertelescope experiment. Arecibo-like versions with a fixed spherical primary meta-mirror, or an active aspheric one, have a suspended focal beam combiner equipped for pupil-drift accommodation, with a field-mosaic arrangement for observing multiple sources such as exoplanetary systems, globular clusters or active galactic nuclei. We have developed a cable suspension and drive system with tracking accuracy reaching a millimeter at 100m above ground.
The Vector Vortex Coronagraph (VVC) is one of the most attractive new-generation coronagraphs for ground- and
space-based exoplanet imaging/characterization instruments, as recently demonstrated on sky at Palomar and
in the laboratory at JPL, and Hokkaido University. Manufacturing technologies for devices covering wavelength
ranges from the optical to the mid-infrared, have been maturing quickly. We will review the current status of
technology developments supported by NASA in the USA (Jet Propulsion Laboratory-California Institute of
Technology, University of Arizona, JDSU and BEAMCo), Europe (University of Li`ege, Observatoire de Paris-
Meudon, University of Uppsala) and Japan (Hokkaido University, and Photonics Lattice Inc.), using liquid
crystal polymers, subwavelength gratings, and photonics crystals, respectively. We will then browse concrete
perspectives for the use of the VVC on upcoming ground-based facilities with or without (extreme) adaptive
optics, extremely large ground-based telescopes, and space-based internal coronagraphs.
The small angular distance (<100 mas) and the huge flux ratio (107) between an Earth-like exoplanet in the socalled
habitable zone and its host star makes it very difficult to direct image such systems. Nulling interferometry
consists of a very powerful technique that combines destructively the light from two or more collectors to dim
the starlight and to reveal faint companions in its vicinity. We have developed a new nulling experiment based
on the fiber nuller principle. This fully symmetric reflective nulling bench aims at testing broadband nulling
in both H and K bands as well as characterizing photonic fibers for modal filtering. We present in this paper the
design, the development as well as preliminary results of the experiment.
A unique statistical data analysis method has been developed for reducing nulling interferometry data. The idea
is to make use of the statistical distributions of the fluctuating null depths and beam intensities to retrieve the
astrophysical null depth in the presence of fluctuations. The approach yields an accuracy much better than is
possible with standard data reduction methods, because the accuracy of the null depth is not limited by the
sizes of the phase and intensity errors but by the uncertainties on their statistical distributions. The result is
an improvement in the instrumental null depth measurement limit of roughly an order of magnitude. We show
in this paper that broadband null depths of 10-4 can be measured in the lab with our infrared Fiber Nuller
without achromatic phase shifters. On sky results are also dramatically improved, with measured contrasts up to
a couple of 10-4 with our instrument mounted on the Hale telescope at the Palomar Observatory. This statistical
analysis is not specific to our instrument and may be applicable to other interferometers.
We recently presented a new concept for designing an achromatic phase shifter. An APS is required in nulling interferometry,
a technique that aims at directly detecting and characterizing planets around a star in the thermal infrared. Our solution
is based on two cellular mirrors (alternatively, transparent plates can be used) where cells have thickness which introduce
OPD that are respectively odd and even multiples of half the central wavelength, on the fraction of the wave it reflects. A
destructive interference is thus produced on axis for the central wavelength when recombining the two beams. We have
shown that if the thicknesses are distributed according to the Pascal triangle, a fair quasi-achromatism is also reached on
typically one octave in wavelength, provided there is a suffcient number of cells. The major interest of this solution is
that it allows a compact, simple and fully symmetric design, without complex sub-systems to adjust. In this paper, after
reminding the basic concept, we first present the theoretical estimations for the expected performances in the two possible
regimes of recombination: on axis and multi-axial (Fizeau). We then describe the laboratory setup of the demonstration
bench we are developing, as well as the first results obtained.
The Annular Groove Phase Mask coronagraph (AGPM) is an intrinsically achromatic vectorial vortex. It consists
of integrated subwavelength optical elements whose space-variant polarization properties can be engineered and
optimized to synthesize one of the theoretically most efficient coronagraphs. This paper briefly recalls the
principles of the AGPM, presents the benefit of its implementation inside a polarimetric differential imager,
realistic numerical simulations assessing its performances, as well as the current status of the near-infrared and
visible prototype manufacturing operations.
Direct detection and characterization of a planet around a star by nulling interferometry, must be efficient in a large wavelength
domain in order to detect simultaneously the infrared bio-tracers CO2, O3 and H2O. This condition requires that an achromatic phase shift of π be implemented, with an accuracy sufficient for achieving a deep nulling at all considered
wavelengths. Several solutions have been presented. We present here a new concept for designing such an achromatic
phase shifter. It is based on two cellular mirrors (alternatively, transparent plates can be used) where cells have thickness
which are respectively odd and even multiples of a quarter of the central wavelength. Each cell introduces then a phase shift
of (2k + 1)π or of 2kπ, on the fraction of the wave it reflects. Each mirror is introduced in the collimated beam issued from
one or the other telescopes. Because of the odd/even distribution, a destructive interference is obviously produced on axis
for the central wavelength when recombining the two beams. The trick to obtain a quasi-achromatisation is to distribute
the thickness of the cells, so that the nulling is also efficient for a wavelength not too far from the central wavelength.
We show that if the thicknesses are distributed according to the Pascal triangle, a fair quasi-achromatism is reached. This
effect is the more efficient that the number of cells is large. For instance, with 256 × 256 cells, where phase shift range is
between -6π and +6π one shows that the nulling reaches 10-6 on the wavelength range [0.7λ0, 1.3λ0] which corresponds roughly to the DARWIN specification. In a second step, we study the optimum way to distribute the cells in the plane of the
pupil. The most important criterion is the isolation of the planet image from the residual image of the star. Several efficient
configurations are presented. Finally we consider some practical aspects on a device belonging to the real world and on the
bench we are developing. The major interest of this solution is that it allows a compact, simple and fully symmetric design,
with essentially no ajustable sub-systems ; extension to multi-telescopes interferometers with phase shift other than π can
also be envisioned.
We propose a new family of achromatic phase shifters that uses the modulated total internal reflection (TIR) phenomenon. These components can be seen as enhanced Fresnel rhombs for infrared applications like nulling interferometry and polarimetry. The TIR phenomenon comes with a differential phase shift between the polarization components of the incident light. Modulating the index transition at the TIR interface allows compensating for the intrinsic material dispersion in order to make the subsequent phase shift achromatic over broad bands. The modulation can be induced by a thin film of a well-chosen medium or a subwavelength grating whose parameters are specially optimized. We present results from theoretical simulations together with preliminary fabrication outcomes.
J. Surdej, O. Absil, P. Bartczak, E. Borra, J.-P. Chisogne, J.-F. Claeskens, B. Collin, M. De Becker, D. Defrère, S. Denis, C. Flebus, O. Garcet, P. Gloesener, C. Jean, P. Lampens, C. Libbrecht, A. Magette, J. Manfroid, D. Mawet, Th. Nakos, N. Ninane, J. Poels, A. Pospieszalska, P. Riaud, P.-G. Sprimont, J.-P. Swings
The entire funding has recently been obtained in Belgium for the construction of a 4m Liquid Mirror Telescope. Its prime focus will be equipped with a semi-conventional glass corrector allowing to correct for the TDI effect and a thinned, high quantum efficiency, 4K × 4K pixel equivalent CCD camera. It will be capable of subarcsecond imaging in the i'(760 nm) and possibly r', g' band(s) over a field of ~ 30' in diameter. This facility will be entirely dedicated to a deep photometric and astrometric variability survey over a period of ~ 5 years. In this paper, the working principle of liquid mirror telescopes is first recalled, along with the advantages and disadvantages of the latter over classical telescopes. Several science cases are described. For a good access to one of the galactic poles, the best image quality sites for the ILMT are located either in Northern Chile (latitude near -29°30') or in North-East India (Nainital Hills, latitude near +29°30'). At those geographic latitudes, a deep (i' = 22.5 mag.) survey will approximately cover 90 square degrees at high galactic latitude, which is very useful for gravitational lensing studies as well as for the identification of various classes of interesting galactic and extragalactic objects (cf. microlensed stars, supernovae, clusters, etc.). A description of the telescope, its instrumentation and the handling of the data is also presented.
Phase mask coronagraphy is a technique aiming at accommodating both high dynamic and high angular resolution imaging of faint sources around bright objects such as exo-planets orbiting their parent stars or host galaxies around Active Galactic Nuclei. We present two new phase mask coronagraphs implemented with subwavelength diffractive optical elements consisting of optimized surface-relief gratings. The first one is an evolution of the Four Quadrant Phase Mask coronagraph, which resolves the π phase shift chromaticity problem: the Four Quadrant Zeroth Order Grating (4QZOG). The second
one is a totally new design consisting of an optical vortex induced by a space-variant grating: the Annular Groove Phase Mask (AGPM) coronagraph is fully symmetric and free from any "shaded zones". Some manufacturing hints are given.
Direct detection and characterization of Earth-like planets orbiting nearby stars are possible with nulling interferometry and coronagraphy in the thermal infrared. With these techniques, the star is extinguished by interference, with a ratio greater than 105 to allow planet detection. Generally, these techniques require a π phase shifter. In this paper, we investigate how thin film coatings can be used to design π achromatic phase shifters. The design, which provides a π phase shift upon reflection, is basically formed with a high reflectance mirror and an antireflection structure that can be designed without any constraint on their own phase properties. The principle of the design is detailed and numerical examples illustrates this concept.
The Four Quadrant Phase Mask is a key component for the design of advanced coronagraphs that may be used to search exo-planets. The validity of this concept has been demonstrated in the visible and need now to be demonstrated in the mid infrared. For this purpose, two components are manufactured for wavelengths 4.75 and 16.25 μm. This manufacturing requires the deposition of ZnSe layers using Ion Assisted Deposition, followed by a lift off process.
MIRI is the mid infrared instrument planned for the NGST. Working in the 5-28 μm band, it includes 3 units: a spectrograph, an imager and a calibration facility. We describe here the optical design of the MIRI imager channel as it is at teh end of the phase A study. The MIRI imager provides 3 observing modes: an imaging mode with a field of view of 1.3 arcmin x 1.7 arcmin and a Pixel Field of View of 0.1 arcsec/pixel, a coronagraphic mode and a low resolution spectroscopic mode for point sources, between 5 μm and 10 μm, with a spectral resolution R = λ/Δλ around 100.
Numerical simulations have been carried out to assess the opportunity to detect extrasolar planets with MIRI: the mid-IR instrument of NGST. Several coronagraphs and telescope designs have been investigated. As a result, we found that very young planets (50Myr) as well as old planets (5Gyr) can be imaged in the thermal-IR (5μm to 20μm) down to a few Jupiter masses if an appropriate high-contrast coronagraph is in used. Promising results of numerical simulations are presented.
The concept we recently introduced of a coronagraph using a four-quadrant phase mask has been the subject of detailed model calculations and of laboratory validation proving its great potential in planet finding. A nulling factor of 12500 is already demonstrated in the laboratory (Riaud et al., this conference). We first remind the principle of the 4QC: a destructive interference between the two equal fractions of the amplitude with opposite signs produces a very efficient nulling of the_star light. We propose to install such a device on several ground-based and space instruments, including present (NAOS/CONICA) or future (Planet Finder) VLT instruments and MIRI, the mid-IR camera of the NGST. The present paper focus on the question of direct imaging of exoplanets using this type of device. Indeed, one advantage of the four-quadrant coronagraph is to permit probing the vicinity of a star down to smaller angular distances than a classical Lyot mask. We examine the sources of uncertainties in different cases of optimized ground-based and space experiments and different situations of planet/star couples, using as far as possible realistic models of planetary evolution. On the VLT, even with an extremely powerful adaptive optics system, the speckle noise will be the main limitation: contrast in magnitude as large as Dm = 15 are however possible in the K band. The combination of a 4QC and differential imaging at two wavelength is likely the most promising concept for direct planet Detection from the ground. On the other hand, we show that with a 4QC on MIRI, a classical Jupiter is indeed detectable from space and at 20 μm for a star closer than 10 pc, while the more favorable cases of a young (hot) giant planet allows detection at 6 μm for a star belonging to the closest star forming region at 50 pc.
Only in the recent years did it become realized that multi-aperture interferometric arrays could provide direct snapshot images and coronagraphic images in a non-Fizeau mode. Whereas homothetic mapping of entrance pupil to exit pupil is useless when the aperture is higly diluted, a "densified-pupil" or "hypertelescope" imaging mode can concentrate most light into a high-resolution Airy peak. In addition to the luminosity gain, there is a contrast gain particularly valuable for stellar coronagraphy and exoplanets finding. The current VLTI is able to combine light from two telescopes coherently. In subsequent phases, a combiner is planned for applying closure phase with up to eight telescopes (UT and AT). The small number of apertures currently considered at the VLTI, does not take full advantage of hypertelescope imaging, but still performs significantly better than other observing modes (+3.8mag gain in comparison with Fizeau mode). We propose some possible optical scheme for a densified-pupil combiner for the VLTI. Beyond its science value, the proposed instrument can serve as a precursor for many-element post-VLTI hypertelescopes.
The 8-m class telescopes are now in full operation, while 100-m
baseline interferometers (VLTI, KeckI) are starting routine
operation too. A working group from the French high angular
resolution community tried to identify what could be our
post-VLT/VLTI instruments after 2010. Possible future instruments,
ground or space-based, can be split into three main categories:
Extremely large filled aperture telescopes, diluted
interferometric arrays for direct imaging, and diluted
interferometric arrays for aperture synthesis imaging. These
concepts are compared in terms of observing capabilities and
performances (spatial resolution, field of view, imaging
capability, sensitivity, photometric dynamical range, etc.),
technological issues (adaptive optics, phasing, instrument mount,
etc.) and R&D priorities.
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