44 research outputs found
Chromaticity in all-reflective telescopes for astrometry
Chromatic effects are usually associated with refractive optics, so
reflective telescopes are assumed to be free from them. We show that
all-reflective optics still bears significant levels of such perturbations,
which is especially critical to modern micro-arcsecond astrometric experiments.
We analyze the image formation and measurement process to derive a precise
definition of the chromatic variation of the image position, and we evaluate
the key aspects of optical design with respect to chromaticity. The fundamental
requirement related to chromaticity is the symmetry of the optical design and
of the wavefront errors. Finally, we address some optical engineering issues,
such as manufacturing and alignment, providing recommendations to minimize the
degradation that chromaticity introduces into astrometry.Comment: 10 pages, 8 figure
A simple beam combination for stellar interferometry
In stellar interferometry, image quality improves significantly with the inclusion of more telescopes and the use of phase closure. We demonstrate, using first coherent and then partially coherent white light, a compact and efficient pair-wise combination of twelve or more beams. The input beams are lined up and spread through a cylindrical lens into a comb of parallel ellipses, which interferes with a perpendicular copy of itself to form a matrix of interferograms between all pairs. The diagonal elements show interference of each beam with itself, for in-tensity calibration. The measured white-light visibilities were high and stable
Fringe Tracker for the VLTI Spectro-Imager
The implementation of the simultaneous combination of several telescopes
(from four to eight) available at Very Large Telescope Interferometer (VLTI)
will allow the new generation interferometric instrumentation to achieve
interferometric image synthesis with unprecedented resolution and efficiency.
The VLTI Spectro Imager (VSI) is the proposed second-generation near-infrared
multi-beam instrument for the Very Large Telescope Interferometer, featuring
three band operations (J, H and K), high angular resolutions (down to 1.1
milliarcsecond) and high spectral resolutions. VSI will be equipped with its
own internal Fringe Tracker (FT), which will measure and compensate the
atmospheric perturbations to the relative beam phase, and in turn will provide
stable and prolonged observing conditions down to the magnitude K=13 for the
scientific combiner. In its baseline configuration, VSI FT is designed to
implement, from the very start, the minimum redundancy combination in a nearest
neighbor scheme of six telescopes over six baselines, thus offering better
options for rejection of large intensity or phase fluctuations over each beam,
due to the symmetric set-up. The planar geometry solution of the FT beam
combiner is devised to be easily scalable either to four or eight telescopes,
in accordance to the three phase development considered for VSI. The proposed
design, based on minimum redundancy combination and bulk optics solution, is
described in terms of opto-mechanical concept, performance and key operational
aspects.Comment: 11 pages, to be published in Proc. SPIE conference 7013 "Optical and
Infrared Interferometry", Schoeller, Danchi, and Delplancke, F. (eds.
A Dynamical Mass Constraint for Pre-Main-Sequence Evolutionary Tracks: The Binary NTT 045251+3016
We present an astrometric/spectroscopic orbital solution for the
pre-main-sequence binary NTT 045251+3016. Our measurements for the primary and
secondary masses are 1.45 +/- 0.19 M_sun and 0.81 +/- 0.09 M_sun, respectively,
and 145 +/- 8 pc for the distance of the system, consistent with prior
estimates for the Taurus-Auriga star-forming region. The evolutionary tracks of
D'Antona & Mazzitelli (1997), Baraffe et al. (1998), and Palla & Stahler (1999)
are tested against these dynamical mass measurements. Due to the intrinsic
color/T_eff variation within the K5 spectral class, each pre-main-sequence
model provides a mass range for the primary. The theoretical mass range derived
from the Baraffe et al. (1998) tracks that use a mixing length parameter
alpha=1.0 is closest to our measured primary mass, deviating between 1.3 and
1.6 sigma. The set of Baraffe et al. (1998) tracks that use alpha=1.9 deviate
between 1.6 and 2.1 sigma from our measured primary mass. The mass range given
by the Palla & Stahler (1999) tracks for the primary star deviate between 1.6
and 2.9 sigma. The D'Antona & Mazzitelli (1997) tracks give a mass range that
deviates by at least 3.0 sigma from our derived primary mass, strongly
suggesting that these tracks are inconsistent with our observation.
Observations of the secondary are less constraining than those of the primary,
but the deviations between the dynamical mass of the secondary and the mass
inferred for the secondary from the various pre-main-sequence tracks mirror the
deviations of the primary star. All of the pre-main-sequence tracks are
consistent with coevality of the components of NTT 045251+3016.Comment: 27 pages, 6 figures, 5 tables -- accepted by A
Metrology on-board PROBA-3: The Shadow Position Sensor (SPS) subsystem
PROBA-3 is an ESA Mission whose aim is to demonstrate the in-orbit Formation Flying and attitude control capabilities of its two satellites by means of closed-loop, on-board metrology. The two small spacecraft will form a giant externally occulted coronagraph that will observe in visible polarized light the inner part of the solar corona. The SPS subsystem is composed of eight sensors that will measure, with the required sensitivity and dynamic range, the penumbra light intensity around the coronagraph instrument entrance pupil
SOXS: a wide band spectrograph to follow up transients
SOXS (Son Of X-Shooter) will be a spectrograph for the ESO NTT telescope
capable to cover the optical and NIR bands, based on the heritage of the
X-Shooter at the ESO-VLT. SOXS will be built and run by an international
consortium, carrying out rapid and longer term Target of Opportunity requests
on a variety of astronomical objects. SOXS will observe all kind of transient
and variable sources from different surveys. These will be a mixture of fast
alerts (e.g. gamma-ray bursts, gravitational waves, neutrino events), mid-term
alerts (e.g. supernovae, X-ray transients), fixed time events (e.g. close-by
passage of minor bodies). While the focus is on transients and variables, still
there is a wide range of other astrophysical targets and science topics that
will benefit from SOXS. The design foresees a spectrograph with a
Resolution-Slit product ~ 4500, capable of simultaneously observing over the
entire band the complete spectral range from the U- to the H-band. The limiting
magnitude of R~20 (1 hr at S/N~10) is suited to study transients identified
from on-going imaging surveys. Light imaging capabilities in the optical band
(grizy) are also envisaged to allow for multi-band photometry of the faintest
transients. This paper outlines the status of the project, now in Final Design
Phase.Comment: 12 pages, 14 figures, to be published in SPIE Proceedings 1070
VSI: the VLTI spectro-imager
The VLTI Spectro Imager (VSI) was proposed as a second-generation instrument
of the Very Large Telescope Interferometer providing the ESO community with
spectrally-resolved, near-infrared images at angular resolutions down to 1.1
milliarcsecond and spectral resolutions up to R=12000. Targets as faint as K=13
will be imaged without requiring a brighter nearby reference object. The unique
combination of high-dynamic-range imaging at high angular resolution and high
spectral resolution enables a scientific program which serves a broad user
community and at the same time provides the opportunity for breakthroughs in
many areas of astrophysic including: probing the initial conditions for planet
formation in the AU-scale environments of young stars; imaging convective cells
and other phenomena on the surfaces of stars; mapping the chemical and physical
environments of evolved stars, stellar remnants, and stellar winds; and
disentangling the central regions of active galactic nuclei and supermassive
black holes. VSI will provide these new capabilities using technologies which
have been extensively tested in the past and VSI requires little in terms of
new infrastructure on the VLTI. At the same time, VSI will be able to make
maximum use of new infrastructure as it becomes available; for example, by
combining 4, 6 and eventually 8 telescopes, enabling rapid imaging through the
measurement of up to 28 visibilities in every wavelength channel within a few
minutes. The current studies are focused on a 4-telescope version with an
upgrade to a 6-telescope one. The instrument contains its own fringe tracker
and tip-tilt control in order to reduce the constraints on the VLTI
infrastructure and maximize the scientific return.Comment: 12 pages, to be published in Proc. SPIE conference 7013 "Optical and
Infrared Interferometry", Schoeller, Danchi, and Delplancke, F. (eds.). See
also http://vsi.obs.ujf-grenoble.f
Milli-arcsecond astrophysics with VSI, the VLTI spectro-imager in the ELT era
Nowadays, compact sources like surfaces of nearby stars, circumstellar
environments of stars from early stages to the most evolved ones and
surroundings of active galactic nuclei can be investigated at milli-arcsecond
scales only with the VLT in its interferometric mode. We propose a
spectro-imager, named VSI (VLTI spectro-imager), which is capable to probe
these sources both over spatial and spectral scales in the near-infrared
domain. This instrument will provide information complementary to what is
obtained at the same time with ALMA at different wavelengths and the extreme
large telescopes.Comment: 8 pages. To be published in the proceedings of the ESO workshop
"Science with the VLT in the ELT Era", held in Garching (Germany) on 8-12
October 2007, A. Moorwood edito
System overview of the VLTI Spectro-Imager
The VLTI Spectro Imager project aims to perform imaging with a temporal
resolution of 1 night and with a maximum angular resolution of 1
milliarcsecond, making best use of the Very Large Telescope Interferometer
capabilities. To fulfill the scientific goals (see Garcia et. al.), the system
requirements are: a) combining 4 to 6 beams; b) working in spectral bands J, H
and K; c) spectral resolution from R= 100 to 12000; and d) internal fringe
tracking on-axis, or off-axis when associated to the PRIMA dual-beam facility.
The concept of VSI consists on 6 sub-systems: a common path distributing the
light between the fringe tracker and the scientific instrument, the fringe
tracker ensuring the co-phasing of the array, the scientific instrument
delivering the interferometric observables and a calibration tool providing
sources for internal alignment and interferometric calibrations. The two
remaining sub-systems are the control system and the observation support
software dedicated to the reduction of the interferometric data. This paper
presents the global concept of VSI science path including the common path, the
scientific instrument and the calibration tool. The scientific combination
using a set of integrated optics multi-way beam combiners to provide
high-stability visibility and closure phase measurements are also described.
Finally we will address the performance budget of the global VSI instrument.
The fringe tracker and scientific spectrograph will be shortly described
<i>Gaia</i> Data Release 1. Summary of the astrometric, photometric, and survey properties
Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7.
Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release.
Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue.
Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the HIPPARCOS and Tycho-2 catalogues – a realisation of the Tycho-Gaia Astrometric Solution (TGAS) – and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of ∼3000 Cepheid and RR-Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yr−1 for the proper motions. A systematic component of ∼0.3 mas should be added to the parallax uncertainties. For the subset of ∼94 000 HIPPARCOS stars in the primary data set, the proper motions are much more precise at about 0.06 mas yr−1. For the secondary astrometric data set, the typical uncertainty of the positions is ∼10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to ∼0.03 mag over the magnitude range 5 to 20.7.
Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data