133 research outputs found
Direct imaging with highly diluted apertures. I. Field of view limitations
Future optical interferometric instrumentation mainly relies on the
availability of an efficient cophasing system: once available, what has so far
postponed the relevance of direct imaging with an interferometer will vanish.
This paper focuses on the actual limits of snapshot imaging, inherent to the
use of a sparse aperture: the number of telescopes and the geometry of the
array impose the maximum extent of the field of view and the complexity of the
sources. A second limitation may arise from the beam combination scheme.
Comparing already available solutions, we show that the so called
hypertelescope mode (or densified pupil) is ideal. By adjusting the direct
imaging field of view to the useful field of view offered by the array, the
hypertelescope makes an optimal use of the collected photons. It optimizes
signal to noise ratio, drastically improves the luminosity of images and makes
the interferometer compatible with coronagraphy, without inducing any loss of
useful field of view.Comment: 13 pages, 6 figures, accepted for publication in MNRAS.
Full-resolution version available at
http://www.obs-hp.fr/~lardiere/publi/2006-Lardiere-MNRAS.pd
Multi-spectral piston sensor for co-phasing giant segmented mirrors and multi-aperture interferometric arrays
This paper presents the optical design of a multi-spectral piston sensor
suitable to co-phasing giant segmented mirrors equipping the Future Extremely
Large Telescopes (ELTs). The general theory of the sensor is described in
detail and numerical simulations have been carried out, demonstrating that
direct piston and tip-tilt measurements are feasible within accuracies
respectively close to 20 nm and 10 nano-radians. Those values are compatible
with the co-phasing requirements, although the method seems to be perturbed by
uncorrected atmospheric seein
Two Wide Planetary-mass Companions to Solar-type Stars in Upper Scorpius
At wide separations, planetary-mass and brown dwarf companions to solar-type stars occupy a curious region of
parameter space not obviously linked to binary star formation or solar system scale planet formation. These
companions provide insight into the extreme case of companion formation (either binary or planetary), and
due to their relative ease of observation when compared to close companions, they offer a useful template
for our expectations of more typical planets. We present the results from an adaptive optics imaging survey
for wide (~50–500 AU) companions to solar-type stars in Upper Scorpius. We report one new discovery of a
~14 M_J companion around GSC 06214−00210and confirm that the candidate planetary-mass companion 1RXS
J160929.1−210524 detected by Lafrenière et al. is in fact comoving with its primary star. In our survey, these
two detections correspond to ~4% of solar-type stars having companions in the 6–20 M_J mass and ~200–500 AU
separation range. This figure is higher than would be expected if brown dwarfs and planetary-mass companions
were drawn from an extrapolation of the binary mass function. Finally, we discuss implications for the formation
of these objects
Speckle Control with a remapped-pupil PIAA-coronagraph
The PIAA is a now well demonstrated high contrast technique that uses an
intermediate remapping of the pupil for high contrast coronagraphy
(apodization), before restoring it to recover classical imaging capabilities.
This paper presents the first demonstration of complete speckle control loop
with one such PIAA coronagraph. We show the presence of a complete set of
remapping optics (the so-called PIAA and matching inverse PIAA) is transparent
to the wavefront control algorithm. Simple focal plane based wavefront control
algorithms can thus be employed, without the need to model remapping effects.
Using the Subaru Coronagraphic Extreme AO (SCExAO) instrument built for the
Subaru Telescope, we show that a complete PIAA-coronagraph is compatible with a
simple implementation of a speckle nulling technique, and demonstrate the
benefit of the PIAA for high contrast imaging at small angular separation.Comment: 6 figures, submitted to PAS
Spiral FFAG lattice design tools - Application to 6-D tracking
Ray-tracing based methods for 3-D modeling of magnetic field and particle motion in spiral scaling FFAG accelerators have been developed. They allow efficient simulation of particle motion in presence of the strong field non-linearities proper to FFAG magnets, and of possible field overlapping in configurations of neighboring magnets, thus yielding a performing tool for spiral FFAG lattice design and optimizations, and for 6-D tracking studies. It is applied for illustration to a principle design of a 200 MeV medical class proton FFAG now under study in the frame of the RACCAM hadrontherapy project
Artificial Incoherent Speckles Enable Precision Astrometry and Photometry in High-Contrast Imaging
State-of-the-art coronagraphs employed on extreme adaptive optics enabled instruments are constantly improving the contrast detection limit for companions at ever-closer separations from the host star. In order to constrain their properties and, ultimately, compositions, it is important to precisely determine orbital parameters and contrasts with respect to the stars they orbit. This can be difficult in the post-coronagraphic image plane, as by definition the central star has been occulted by the coronagraph. We demonstrate the flexibility of utilizing the deformable mirror in the adaptive optics system of the Subaru Coronagraphic Extreme Adaptive Optics system to generate a field of speckles for the purposes of calibration. Speckles can be placed up to 22.5 λ/D from the star, with any position angle, brightness, and abundance required. Most importantly, we show that a fast modulation of the added speckle phase, between 0 and π, during a long science integration renders these speckles effectively incoherent with the underlying halo. We quantitatively show for the first time that this incoherence, in turn, increases the robustness and stability of the adaptive speckles, which will improve the precision of astrometric and photometric calibration procedures. This technique will be valuable for high-contrast imaging observations with imagers and integral field spectrographs alike
An Achromatic Focal Plane Mask for High-Performance Broadband Coronagraphy
Developments in coronagraph technology are close to achieving the technical requirements necessary to observe the faint signal of an Earth-like exoplanet in monochromatic light. An important remaining technological challenge is to achieve high contrast in broadband light. Coronagraph bandwidth is largely limited by chromaticity of the focal plane mask, which is responsible for blocking the stellar PSF. The size of a stellar PSF scales linearly with wavelength; ideally, the size of the focal plane mask would also scale with wavelength. A conventional hard-edge focal plane mask has a fixed size, normally sized for the longest wavelength in the observational band to avoid starlight leakage. The conventional mask is oversized for shorter wavelengths and blocks useful discovery space. We present a new focal plane mask which operates conceptually as an opaque disk occulter, but uses a phase mask technique to improve performance and solve the "size chromaticity" problem. This achromatic focal plane mask would maximize the potential planet detection space without allowing starlight leakage to degrade the system contrast. Compared with a conventional opaque disk focal plane mask, the achromatic mask allows coronagraph operation over a broader range of wavelengths and allows the detection of exoplanets closer to their host star. We present the generalized design for the achromatic focal plane mask, implementation within the Subaru Coronagraph Extreme Adaptive Optics instrument, and laboratory results which demonstrate the size-scaling property of the mask
Efficient injection from large telescopes into single-mode fibres: Enabling the era of ultra-precision astronomy
Photonic technologies offer numerous advantages for astronomical instruments
such as spectrographs and interferometers owing to their small footprints and
diverse range of functionalities. Operating at the diffraction-limit, it is
notoriously difficult to efficiently couple such devices directly with large
telescopes. We demonstrate that with careful control of both the non-ideal
pupil geometry of a telescope and residual wavefront errors, efficient coupling
with single-mode devices can indeed be realised. A fibre injection was built
within the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument.
Light was coupled into a single-mode fibre operating in the near-IR (J-H bands)
which was downstream of the extreme adaptive optics system and the pupil
apodising optics. A coupling efficiency of 86% of the theoretical maximum limit
was achieved at 1550 nm for a diffraction-limited beam in the laboratory, and
was linearly correlated with Strehl ratio. The coupling efficiency was constant
to within <30% in the range 1250-1600 nm. Preliminary on-sky data with a Strehl
ratio of 60% in the H-band produced a coupling efficiency into a single-mode
fibre of ~50%, consistent with expectations. The coupling was >40% for 84% of
the time and >50% for 41% of the time. The laboratory results allow us to
forecast that extreme adaptive optics levels of correction (Strehl ratio >90%
in H-band) would allow coupling of >67% (of the order of coupling to multimode
fibres currently). For Strehl ratios <20%, few-port photonic lanterns become a
superior choice but the signal-to-noise must be considered. These results
illustrate a clear path to efficient on-sky coupling into a single-mode fibre,
which could be used to realise modal-noise-free radial velocity machines,
very-long-baseline optical/near-IR interferometers and/or simply exploit
photonic technologies in future instrument design.Comment: 15 pages, 16 figures, 1 table, published in A&
The FFAG R&D and medical application project RACCAM
JACoW web site http://accelconf.web.cern.ch/AccelConf/e06/Pre-Press/WEPCH161.pdf WEPCH161International audienceThe RACCAM project (Recherche en ACCelerateurs et Applications Medicales) has recently obtained fundings, extending over three years (2006-2008), from the French National Research Agency (ANR). RACCAM is a tripartite collaboration, involving (i) the CNRS Laboratory IN2P3/LPSC, (ii) the French magnet industrial SIGMAPHI, and (iii) the nuclear medecine Departement of Grenoble Hospital. The project concerns fixed field alternating gradient accelerator (FFAG) research on the one hand, and on the other hand their application as hadrontherapy and biology research machines. RACCAM's goal is three-fold, (i) participate to the on-going international collaborations in the field of FFAGs and recent concepts of "non-scaling" FFAGs, with frames for instance, the Neutrino Factory (NuFact) and the EMMA project of an electron model of a muon FFAG accelerator, (ii) design, build and experiment a prototype of an FFAG magnet proper to fulfil the requirements of rapid cycling acceleration, (iii) develop the concepts, and show the feasibility, of the application of such FFAG beams to hadrontherapy and to biology research
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