ESA’s <i>Gaia </i>space astrometry mission is performing an all-sky survey of stellar objects. At the beginning of the nominal mission in July 2014, an operation scheme was adopted that enabled <i>Gaia </i>to routinely acquire observations of all stars brighter than the original limit of G∼6, i.e. the naked-eye stars. Here, we describe the current status and extent of those observations and their on-ground processing. We present an overview of the data products generated for G<6 stars and the potential scientific applications. Finally, we discuss how the <i>Gaia </i>survey could be enhanced by further exploiting the techniques we developed.
The Gaia payload ensures maximum passive stability using a single material, SiC, for most of its elements. Dedicated metrology instruments are, however, required to carry out two functions: monitoring the basic angle and refocusing the telescope. Two interferometers fed by the same laser are used to measure the basic angle changes at the level of μas (prad, micropixel), which is the highest level ever achieved in space. Two Shack- Hartmann wavefront sensors, combined with an ad-hoc analysis of the scientific data are used to define and reach the overall best-focus. In this contribution, the systems, data analysis, procedures and performance achieved during commissioning are presented .
The ESA Gaia space astrometry mission will perform an all-sky survey of stellar objects complete in the nominal magnitude range G = [6.0 - 20.0]. The stars with G < 6.0, i.e. those visible to the unaided human eye, would thus not be observed by Gaia. We present an algorithm configuration for the Gaia on-board autonomous object observation system that makes it possible to observe very bright stars with G = [2.0-6.0). Its performance has been tested during the in-orbit commissioning phase achieving an observation completeness of ~ 94% at G = 3 – 5.7 and ~ 75% at G = 2 – 3. Furthermore, two targeted observation techniques for data acquisition of stars brighter than G=2.0 were tested. The capabilities of these two techniques and the results of the in-flight tests are presented. Although the astrometric performance for stars with G < 6.0 has yet to be established, it is clear that several science cases will benefit from the results of the work presented here.
In the summer of 2011, the first on-sky astrometric commissioning of PRIMA-Astrometry delivered a performance of 3 m″ for a 10 ″ separation on bright objects, orders of magnitude away from its exoplanet requirement of 50 μ″ ~ 20 μ″ on objects as faint as 11 mag ~ 13 mag in K band. This contribution focuses on upgrades and characterizations carried out since then. The astrometric metrology was extended from the Coudé focus of the Auxillary Telescopes to their secondary mirror, in order to reduce the baseline instabilities and improve the astrometric performance. While carrying out this extension, it was realized that the polarization retardance of the star separator derotator had a major impact on both the astrometric metrology and the fringe sensors. A local compensation of this retardance and the operation on a symmetric baseline allowed a new astrometric commissioning. In October 2013, an improved astrometric performance of 160 μ″ was demonstrated, still short of the requirements. Instabilities in the astrometric baseline still appear to be the dominating factor. In preparation to a review held in January 2014, a plan was developed to further improve the astrometric and faint target performance of PRIMA Astrometry. On the astrometric aspect, it involved the extension of the internal longitudinal metrology to primary space, the design and implementation of an external baseline metrology, and the development of an astrometric internal fringes mode. On the faint target aspect, investigations of the performance of the fringe sensor units and the development of an AO system (NAOMI) were in the plan. Following this review, ESO decided to take a proposal to the April 2014 STC that PRIMA be cancelled, and that ESO resources be concentrated on ensuring that Gravity and Matisse are a success. This proposal was recommended by the STC in May 2014, and endorsed by ESO.
Astrometry is a powerful technique to study the populations of extrasolar planets around nearby stars. It gives access to a unique parameter space and is therefore required for obtaining a comprehensive picture of the properties, abundances, and architectures of exoplanetary systems. In this review, we discuss the scientific potential, present the available techniques and instruments, and highlight a few results of astrometric planet searches, with an emphasis on observations from the ground. In particular, we discuss astrometric observations with the Very Large Telescope (VLT) Interferometer and a programme employing optical imaging with a VLT camera, both aimed at the astrometric detection of exoplanets. Finally, we set these e orts into the context of Gaia, ESA’s astrometry mission scheduled for launch in 2013, and present an outlook on the future of astrometric exoplanet detection from the ground.
The quality of the tracking performed by the fringe sensor units (FSUs) of PRIMA, the ESO’s dual feed facility
for the VLTI, is affected by the angular separation between the two objects being observed simultaneously, the
detector integration time (DIT) and the atmospheric observational conditions. We describe the algorithm we
developed to compute visibilities from the FSU data and discuss their validity for the study on the angular
anisoplanatism measured through the dependence of the visibility amplitudes on the angular separation.
The Phase Referenced Imaging and Micro Arcsecond Astrometry (PRIMA) facility for the Very Large Telescope
Interferometer (VLTI), is being installed and tested in the observatory of Paranal. Since January 2011 the
integration and individual testing of the different subsystem has come to a necessary minimum. At the same
time the astrometric commissioning phase has begun.
In this contribution we give an update on the status of the facility and present some highlights and difficulties
on our way from first dual-feed fringe detection to first astrometric measurements. We focus on technical and
operational aspects. In particular, within the context of the latter we are going to present a modified mode of
operation that scans across the fringes. We will show that this mode, originally only intended for calibration
purposes, facilitates the detection of dual-fringes.
The Extrasolar Planet Search with PRIMA project (ESPRI) aims at characterising and detecting extrasolar planets by measuring the host star's reflex motion using the narrow-angle astrometry capability of the PRIMA facility at the Very Large Telescope Interferometer. A first functional demonstration of the astrometric mode was achieved in early 2011. This marked the start of the astrometric commissioning phase with the purpose of characterising the instrument's performance, which ultimately has to be sufficient for exoplanet detection. We show results obtained from the observation of bright visual binary stars, which serve as test objects to determine the instrument's astrometric precision, its accuracy, and the plate scale. Finally, we report on the current status of the ESPRI project, in view of starting its scientific programme.
The ESO Very Large Telescope Interferometer (VLTI) offers access to the four 8-m Unit Telescopes (UT) and the four
1.8-m Auxiliary Telescopes (AT) of the Paranal Observatory located in the Atacama Desert in northern Chile. The two
VLTI instruments, MIDI and AMBER deliver regular scientific results. In parallel to the operation, the instruments
developments are pursued, and new modes are studied and commissioned to offer a wider range of scientific possibilities
to the community. New configurations of the ATs array are discussed with the science users of the VLTI and
implemented to optimize the scientific return. The monitoring and improvement of the different systems of the VLTI is a
continuous work. The PRIMA instrument, bringing astrometry capability to the VLTI and phase referencing to the
instruments has been successfully installed and the commissioning is ongoing. The possibility for visiting instruments
has been opened to the VLTI facility.
The Phase Referenced Imaging and Micro Arcsecond Astrometry (PRIMA) facility for the Very Large Telescope
Interferometer (VLTI), is being installed and tested in the observatory of Paranal. Most of the tests have been
concentrated on the characterization of the Fringe Sensor Unit (FSU) and on the automation of the fringe
tracking in preparation of dual-field observations. The status of the facility, an analysis of the FSU performance
and the first attempts towards dual-field observations will be presented in this paper. In the FSU, the phase
information is spatially encoded into four independent combined beams (ABCD) and the group delay comes from
their spectral dispersion over 5 spectral channels covering the K-band. During fringe tracking the state machine
of the optical path difference controller is driven by the Signal to Noise Ratio (SNR) derived from the 4 ABCD
measurements. We will describe the strategy used to define SNR thresholds depending on the star magnitude
for automatically detecting and locking the fringes. Further, the SNR as well as the phase delay measurements
are affected by differential effects occurring between the four beams. We will shortly discuss the contributions
of these effects on the measured phase and SNR noises. We will also assess the sensitivity of the group delay
linearity to various instrumental parameters and discuss the corresponding calibration procedures. Finally we
will describe how these calibrations and detection thresholds are being automated to make PRIMA as much as
possible a user-friendly and efficient facility.
The fringe sensor unit (FSU) is the central element of the phase referenced imaging and micro-arcsecond astrometry
(PRIMA) dual-feed facility for the Very Large Telescope interferometer (VLTI). It has been installed
at the Paranal observatory in August 2008 and is undergoing commissioning and preparation for science operation.
Commissioning observations began shortly after installation and first results include the demonstration
of spatially encoded fringe sensing and the increase in VLTI limiting magnitude for fringe tracking. However,
difficulties have been encountered because the FSU does not incorporate real-time photometric correction and its
fringe encoding depends on polarisation. These factors affect the control signals, especially their linearity, and
can disturb the tracking control loop. To account for this, additional calibration and characterisation efforts are
required. We outline the instrument concept and give an overview of the commissioning results obtained so far. We describe the effects of photometric variations and beam-train polarisation on the instrument operation and propose possible solutions. Finally, we update on the current status in view of the start of astrometric science operation with PRIMA.
The ESO Very Large Telescope Interferometer (VLTI) offers the unique access to the combination of the four 8-meter
Unit Telescopes (UT) of Cerro Paranal. The quality of the scientific observations in interferometric mode is strongly
related to the stability of the optical path difference (OPD) between the telescopes. Vibrations at the level of the
telescopes and affecting the mirrors were shown to be an important source of perturbation for the OPD.
ESO has thus started an important effort on the UTs and VLTI to tackle this effect. Active controls based on
accelerometers and phase measurements have been developed to provide real-time correction of the variation of OPD
introduced by vibrations. Systematic studies and measurement of the sources of vibration (instruments, wind, telescope
altitude, ...) have been performed. Solutions to reduce the vibrations via design modification and/or new operation
configurations are studied and implemented. To ensure good operational conditions, the levels of vibrations are regularly
monitored to control any environmental change. This document will describe the modifications implemented and
foreseen and give a status of the VLTI-UT vibrations evolution.
The dual feed astrometric instrument software of PRIMA (PACMAN) that is currently being integrated at the VLTI will
use two spatially modulated fringe sensor units and a laser metrology system to carry out differential astrometry. Its
software and hardware compromises a distributed system involving many real time computers and workstations
operating in a synchronized manner. Its architecture has been designed to allow the construction of efficient and flexible
calibration and observation procedures. In parallel, a novel scheme of integrating M-code (MATLAB/OCTAVE) with
standard VLT (Very Large Telescope) control software applications had to be devised in order to support numerically
intensive operations and to have the capacity of adapting to fast varying strategies and algorithms. This paper presents
the instrument software, including the current operational sequences for the laboratory calibration and sky calibration.
Finally, a detailed description of the algorithms with their implementation, both under M and C code, are shown together
with a comparative analysis of their performance and maintainability.
The Phase-Referenced Imaging and Micro-arcsecond Astrometry (PRIMA) facility is scheduled for installation
in the Very Large Telescope Interferometer observatory in Paranal, Chile, in the second half of 2008. Its goal
is to provide an astrometric accuracy in the micro-arcsecond range. High precision astrometry can be applied
to explore the dynamics of the dense stellar cluster. Especially models for the formation of stars near super
massive black holes or the fast transfer of short-lived massive stars into the innermost parsec of our galaxy can
be tested. By measuring the orbits of stars close the the massive black hole one can probe deviations from a
Keplerian motion. Such deviations could be due to a swarm of dark, stellar mass objects that perturb the point
mass solution. At the same time the orbits are affected by relativistic corrections which thus can be tested. The
ultimate goal is to test the effects of general relativity in the strong gravitational field. The latter can be probed
with the near infrared flares of SgrA* which are most likely due to accretion phenomena onto the black hole.
We study the expected performance of PRIMA for astrometric measurements in the Galactic Center based on
laboratory measurements and discuss possible observing strategies.
In stellar interferometry fringe-tracking is a method of stabilizing the Optical Pathlength Difference (OPD) from the
observed astronomical source to the instrument detector via different telescopes in an interferometric array.
At the ESO Very Large Telescope Interferometer, which includes four 8.2 m class Unit Telescopes (UTs), stabilization
to better than a tenth of the observing wavelength is required in order to improve the quality and sensitivity of fringe
measurements on the interferometer's scientific instruments.
Unfortunately, fast mechanical vibrations due to myriad sources in the observatory infrastructure propagate to various
mirrors in the optical path and must be compensated for in real time. Due to its limited bandwidth the fringe tracking
loop cannot be used for this purpose. Alternative approaches must therefore be adopted.
Vibrations imparted to the primary, secondary and tertiary mirrors of the UTs are currently measured by a grid of
suitably placed accelerometers, converted to optical pathlengths and cancelled by a wideband feedforward compensation
algorithm to a downstream optical delay line.
Although very effective, it is obvious that this system can not compensate for vibrations originating elsewhere on the
optical path. We present here an adaptive narrow-band cancellation algorithm that can compensate remaining vibrations
measured on the stellar signal on condition that they are sufficiently stable in amplitude and frequency.
The Fringe Sensor Unit (FSU) is the central element of the dual-feed facility PRIMA at the VLT Interferometer
(VLTI). Two identical FSU fringe detectors deliver real-time estimates of phase delay, group delay and signal-to-noise ratio for the two observed targets. They serve both as the scientific instrument for astrometry with
PRIMA and as sensor for the fringe tracking system of the interferometer. Prior to its installation at the VLTI
scheduled for mid-2008, the FSU is going through an extensive laboratory test phase. It is therefore embedded in
a semi-realistic environment, involving a VLTI-like control system and a laser metrology. This allows us to probe
the system response to atmospheric piston jitter, tip-tilt disturbances and higher order aberrations, as they are
expected at the observatory. We report on the system test results, outline the optimisation of the calibration
procedure and we evaluate the FSU fringe tracking performance under realistic conditions. Finally, we compare
the obtained performances to the scientific and technical requirements.
The VLTI's PRIMA (Phase-Referenced Imaging and Microarcsecond Astrometry) instrument is designed to
provide tens of microarcseond astrometry and faint-object imaging for the interferometer facility. Astrometry is
to be enabled by PRIMA between object pairs that are separated on the sky by one isoplanatic patch (roughly 60
seconds of arc at Cerro Paranal in the <i>K</i> band), with at least one of the two objects being bright (<i>K</i> <10), and a
second fainter object (Δ<i>K</i> < 7) that is nominally a 'stable' astrometric reference. The original expectation was
for star-star pairs to be observed by PRIMA in its astrometric mode; however, we are exploring the possibility
of also utilizing background galaxies in this role. Advantages of such source selection is eliminating the need to
solve for dim object parallax and proper motion before obtaining similar values for the bright foreground star.
Additionally, data from the galaxy may be of scientific interest as well, potentially leading to characterizations
of object morphology at milliarcsecond scales. Towards that end, we have begun observations with ESO's
NTT to explore the suitability of qualifying star-galaxy asterisms as potential PRIMA targets. Commissioning
observations for PRIMA are slated to begin in Fall of 2008.
The ESO Very Large Telescope Interferometer (VLTI) offers access to the four 8 m Unit Telescopes (UT) and the four
1.8 m Auxiliary Telescopes (AT) of the Paranal Observatory located in the Atacama Desert in northern Chile. The fourth
AT has been delivered to operation in December 2006, increasing the flexibility and simultaneous baselines access of the
VLTI. Regular science operations are now carried on with the two VLTI instruments, AMBER and MIDI. The FINITO
fringe tracker is now used for both visitor and service observations with ATs and will be offered on UTs in October
2008, bringing thus the fringe tracking facility to VLTI instruments. In parallel to science observations, technical periods
are also dedicated to the characterization of the VLTI environment, upgrades of the existing systems, and development
of new facilities. We will describe the current status of the VLTI and prospects on future evolution.
The VLTI control architecture is based on a real time distributed system involving dozens of specialized computers.
Several control loops are required to run the VLTI, e.g. for fringe tracking, angle tracking, injection optimization and
vibration cancellation. These control systems rely on a low latency, deterministic shared memory mechanism. It
communicates in the form of a close ring, which includes all devices involved in those loops. Through this ring, sensor
data, intermediate filtered signals, final actuator set-points and feedbacks flow at rates up to 8 kHz. Data in this ring can
be consumed by any node asynchronously. In many cases, those signals are also the astronomical observable (e.g. the
beam combiner fluxes for astrometry) or are used offline, in order to improve the quality of the scientific data reduction
and to debug the system. With the purpose of relieving the control applications of the simultaneous need to record their
signals, a centralized generic recording device has been designed and implemented at the VLTI. In this paper, we
describe its architecture and show that by over-sampling, streaming and posterior filtering on a separate computer it is
possible to overcome the asynchronous nature of the system. We demonstrate that it is feasible to capture data in real
time, verify time reference consistency and store on disk at rates up to ~50 Mbit/s, fulfilling the current VLTI
One of the key components of the planned VLTI dual feed facility PRIMA is the Fringe Sensor Unit (FSU). Its basic
function is the instantaneous measurement of the Optical Path Difference (OPD) between two beams. The FSU acts as
the sensor for a complex control system involving optical delay lines and laser metrology with the aim of removing any
OPD introduced by the atmosphere and the beam relay. We have initiated a cooperation between ESO and MPE with the
purpose of systematically testing this Fringe Tracking Control System in a laboratory environment. This testbed facility
is being built at MPE laboratories with the aim to simulate the VLTI and includes FSUs, OPD controller, metrology and
in-house built delay lines. In this article we describe this testbed in detail, including the environmental conditions in the
laboratory, and present the results of the testbed subsystem characterisation.
PRIMA, the Phase-Referenced Imaging and Micro-arcsecond Astrometry facility for the Very Large Telescope Interferometer, is now nearing the end of its manufacturing phase. An intensive test period of the various sub-systems (star separators, fringe sensor units and incremental metrology) and of their interactions in the global system will start in Garching as soon as they are delivered. The status and performances of the individual sub-systems are presented in this paper as well as the proposed observation and calibration strategy to reach the challenging goal of high-accuracy differential astrometry at 10 μas level.