29 research outputs found
Direct imaging with highly diluted apertures. II. Properties of the point spread function of a hypertelescope
In the future, optical stellar interferometers will provide true images
thanks to larger number of telescopes and to advanced cophasing subsystems.
These conditions are required to have sufficient resolution elements (resel) in
the image and to provide direct images in the hypertelescope mode. It has
already been shown that hypertelescopes provide snapshot images with a
significant gain in sensitivity without inducing any loss of the useful field
of view for direct imaging applications. This paper aims at studying the
properties of the point spread functions of future large arrays using the
hypertelescope mode. Numerical simulations have been performed and criteria
have been defined to study the image properties. It is shown that the choice of
the configuration of the array is a trade-off between the resolution, the halo
level and the field of view. A regular pattern of the array of telescopes
optimizes the image quality (low halo level and maximum encircled energy in the
central peak), but decreases the useful field of view. Moreover, a
non-redundant array is less sensitive to the space aliasing effect than a
redundant array.Comment: 10 pages paper with referee in A&
A test for the search for life on extrasolar planets: Looking for the terrestrial vegetation signature in the Earthshine spectrum
We report spectroscopic observations (400 to 800nm, R = approx 100) of
Earthshine in June, July and October 2001 from which normalised Earth albedo
spectra have been derived. The resulting spectra clearly show the blue colour
of the Earth due to Rayleigh diffusion in its atmosphere. They also show the
signatures of oxygen, ozone and water vapour. We tried to extract from these
spectra the signature of Earth vegetation. A variable signal (4 to 10 +/-3%)
around 700nm has been measured in the Earth albedo. It is interpreted as being
due to the vegetation red edge, expected to be between 2 to 10% of the Earth
albedo at 700nm, depending on models. We discuss the primary goal of the
present observations: their application to the detection of vegetation-like
biosignatures on extrasolar planets.Comment: 7 pages, 7 figures. A&A, accepted 6 May 200
Tests with a Carlina-type diluted telescope; Primary coherencing
Studies are under way to propose a new generation of post-VLTI
interferometers. The Carlina concept studied at the Haute- Provence Observatory
is one of the proposed solutions. It consists in an optical interferometer
configured like a diluted version of the Arecibo radio telescope: above the
diluted primary mirror made of fixed cospherical segments, a helium balloon (or
cables suspended between two mountains), carries a gondola containing the focal
optics. Since 2003, we have been building a technical demonstrator of this
diluted telescope. First fringes were obtained in May 2004 with two
closely-spaced primary segments and a CCD on the focal gondola. We have been
testing the whole optical train with three primary mirrors. The main aim of
this article is to describe the metrology that we have conceived, and tested
under the helium balloon to align the primary mirrors separate by 5-10 m on the
ground with an accuracy of a few microns. The servo loop stabilizes the mirror
of metrology under the helium balloon with an accuracy better than 5 mm while
it moves horizontally by 30 cm in open loop by 10-20 km/h of wind. We have
obtained the white fringes of metrology; i.e., the three mirrors are aligned
(cospherized) with an accuracy of {\approx} 1 micron. We show data proving the
stability of fringes over 15 minutes, therefore providing evidence that the
mechanical parts are stabilized within a few microns. This is an important step
that demonstrates the feasibility of building a diluted telescope using cables
strained between cliffs or under a balloon. Carlina, like the MMT or LBT, could
be one of the first members of a new class of telescopes named diluted
telescopes.Comment: 18 pages, 17 figures, A&A, accepte
On-sky MOAO performance evaluation of RAVEN
This paper presents the AO performance we got on-sky with RAVEN, a Multi-Object Adaptive Optics (MOAO) technical and science demonstrator installed and tested at the Subaru telescope. We report Ensquared-Energy (EE) and Full Width at Half Maximum (FWHM) measured from science images on Subaru's IRCS taken during all of the on-sky observing runs. We show these metrics as function of different AO modes and atmospheric conditions for two asterisms of natural guide stars. The performances of the MOAO and Ground-Layer AO (GLAO) modes are between the classical Single-Conjugate AO (SCAO) and seeing-limited modes. We achieve the EE of 30% in H-band with the MOAO correction, which is a science requirement for RAVEN. The MOAO provides sightly better performance than the GLAO mode in both asterisms. One of the reasons which cause this small difference between the MOAO and GLAO modes may be the strong GL contribution. Also, the performance of the MOAO modes is affected by the accuracy of the on-sky turbulence profiling by the SLOpe Detection And Ranging (SLODAR) method
Using the multi-object adaptive optics demonstrator RAVEN to observe metal-poor stars in and towards the Galactic Centre
The chemical abundances for five metal-poor stars in and towards the Galactic bulge have been determined from the H-band infrared spectroscopy taken with the RAVEN multi-object adaptive optics science demonstrator and the Infrared Camera and Spectrograph at the Subaru 8.2-m telescope. Three of these stars are in the Galactic bulge and have metallicities between −2.1 < [Fe/H] < −1.5, and high [α/Fe] ∼ +0.3, typical of Galactic disc and bulge stars in this metallicity range; [Al/Fe] and [N/Fe] are also high, whereas [C/Fe] < +0.3. An examination of their orbits suggests that two of these stars may be confined to the Galactic bulge and one is a halo trespasser, though proper motion values used to calculate orbits are quite uncertain. An additional two stars in the globular cluster M22 show [Fe/H] values consistent to within 1σ, although one of these two stars has [Fe/H] = −2.01 ± 0.09, which is on the low end for this cluster. The [α/Fe] and [Ni/Fe] values differ by 2σ, with the most metal-poor star showing significantly higher values for these elements. M22 is known to show element abundance variations, consistent with a multipopulation scenario though our results cannot discriminate this clearly given our abundance uncertainties. This is the first science demonstration of multi-object adaptive optics with high-resolution infrared spectroscopy, and we also discuss the feasibility of this technique for use in the upcoming era of 30-m class telescope facilities
Asteroseismology and Interferometry
Asteroseismology provides us with a unique opportunity to improve our
understanding of stellar structure and evolution. Recent developments,
including the first systematic studies of solar-like pulsators, have boosted
the impact of this field of research within Astrophysics and have led to a
significant increase in the size of the research community. In the present
paper we start by reviewing the basic observational and theoretical properties
of classical and solar-like pulsators and present results from some of the most
recent and outstanding studies of these stars. We centre our review on those
classes of pulsators for which interferometric studies are expected to provide
a significant input. We discuss current limitations to asteroseismic studies,
including difficulties in mode identification and in the accurate determination
of global parameters of pulsating stars, and, after a brief review of those
aspects of interferometry that are most relevant in this context, anticipate
how interferometric observations may contribute to overcome these limitations.
Moreover, we present results of recent pilot studies of pulsating stars
involving both asteroseismic and interferometric constraints and look into the
future, summarizing ongoing efforts concerning the development of future
instruments and satellite missions which are expected to have an impact in this
field of research.Comment: Version as published in The Astronomy and Astrophysics Review, Volume
14, Issue 3-4, pp. 217-36
Coronagraphic imaging on the VLTI with VIDA
In a few years, the VLTI will be able to combine light from 4 to 8
telescopes equipped with adaptive optics. In order to exploit the
full VLTI infrastructure, a second generation instrument, based on
the densified pupil concept (VIDA), has been proposed for very
high resolution direct imaging and coronagraphy. This paper
presents some possible coronagraphic devices providing a total
star extinction on the VLTI when there are no phase errors.
Lastly, the expected performances considering cophasing and
adaptive optics residual errors are also presented
Pupil densification: a panorama
The technique of pupil densification bridges the gap existing between
conventionnal optical astronomy observing techniques and optical
interferometry: it indeed leads to the concept of hypertelescope: an
instrument that can provide direct images at the focus of an interferometer.
The hypertelescope is the open sesame for high dynamic imaging with an
interferometer: indeed, the elementary remapping of the pupil operated by a
densifier not only maximizes the dynamic range and the signal to noise ratio
of images but also makes the interferometer compatible with most existing
coronagraphic devices.
Moreover, a careful discussion about field of view show that for a diluted
array, the pupil densification preserves all the relevant high angular
resolution information collected by the interferometer and therefore induces
no field loss
Optimization of the direct imaging properties of an optical fibered long baseline interferometer
Labeyrie (1996) has described the possibility of making direct snapshot images with interferometric arrays, leading to high dynamic imaging properties well suitable for stellar surface imaging and also for coronagraphy for exoplanets findin