MATISSE is the second-generation mid-infrared spectrograph and imager for the Very Large Telescope Interferometer (VLTI) at Paranal. This new interferometric instrument will allow significant advances by opening new avenues in various fundamental research fields: studying the planet-forming region of disks around young stellar objects, understanding the surface structures and mass loss phenomena affecting evolved stars, and probing the environments of black holes in active galactic nuclei. As a first breakthrough, MATISSE will enlarge the spectral domain of current optical interferometers by offering the L and M bands in addition to the N band. This will open a wide wavelength domain, ranging from 2.8 to 13 μm, exploring angular scales as small as 3 mas (L band) / 10 mas (N band). As a second breakthrough, MATISSE will allow mid-infrared imaging - closure-phase aperture-synthesis imaging - with up to four Unit Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. Moreover, MATISSE will offer a spectral resolution range from R ∼ 30 to R ∼ 5000. Here, we present one of the main science objectives, the study of protoplanetary disks, that has driven the instrument design and motivated several VLTI upgrades (GRA4MAT and NAOMI). We introduce the physical concept of MATISSE including a description of the signal on the detectors and an evaluation of the expected performances. We also discuss the current status of the MATISSE instrument, which is entering its testing phase, and the foreseen schedule for the next two years that will lead to the first light at Paranal.
MATISSE is the mid-infrared spectrograph and imager for the Very Large Telescope Interferometer (VLTI) at Paranal. This second generation interferometry instrument will open new avenues in the exploration of our Universe. Mid-infrared interferometry with MATISSE will allow significant advances in various fundamental research fields: studies of disks around young stellar objects where planets form and evolve, surface structures and mass loss of stars in late evolutionary stages, and the environments of black holes in active galactic nuclei. MATISSE is a unique instrument. As a first breakthrough it will enlarge the spectral domain used by optical interferometry by offering the L & M bands in addition to the N band, opening a wide wavelength domain, ranging from 2.8 to 13 μm on angular scales of 3 mas (L/M band) / 10 mas (N band). As a second breakthrough, it will allow mid-infrared imaging – closure-phase aperture-synthesis imaging – with up to four Unit Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. MATISSE will offer various ranges of spectral resolution between R~30 to ~5000. In this article, we present some of the main science objectives that have driven the instrument design. We introduce the physical concept of MATISSE including a description of the signal on the detectors and an evaluation of the expected performance and discuss the project status. The operations concept will be detailed in a more specific future article, illustrating the observing templates operating the instrument, the data reduction and analysis, and the image reconstruction software.
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.
KEYWORDS: Telescopes, Interferometers, Astatine, Interferometry, Large telescopes, Observatories, Systems engineering, Control systems, Mirrors, Sensors
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 and increase sensitivity. New configurations of the
ATs have been offered and are frequently discussed with the science users of the VLTI and implemented to
optimize the scientific return. The PRIMA instrument, bringing astrometry capability to the VLTI and phase
referencing to the instruments is being commissioned. The visitor instrument PIONIER is now fully operational
and bringing imaging capability to the VLTI.
The current status of the VLTI is described with successes and scientific results, and prospects on future
evolution are presented.
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.
MAMMUT (Mirror vibrAtion Metrolology systeM for the Unit Telescope) is an ESO funded feasibility project
for the development of a fiber interferometer prototype designed for optical path laser-metrology along the optical train
of the Unit Telescopes (UT) of the Very Large Telescope Interferometer (VLTI). Fast mechanical vibrations originating
in the VLTI cause fast variations of the optical path difference between two arms of the stellar interferometer, thus
reducing the contrast of measured interference fringes. MAMMUT aims at monitoring in real time the optical path
variations inside the Coudé train of the UT, for active control purposes.
MAMMUT features a 250-meter-long optical fiber which can be used to deliver and inject a laser beam at 1353 nm into
the UT. The injected beam can be dropped from the telescope in the Coudé room and interfered with a phase reference,
provided by the second 250-meter-long arm of the fiber interferometer. The optical path variations are measured
by means of an active homodyne scheme. Coherence between the beam at the injection point and the phase reference
is provided by active fiber stabilization, made possible by the implementation of an internal metrology channel
in MAMMUT.
Here we present the initial laboratory performance results of the MAMMUT prototype, which will be able to sense
optical path variations of +/- 5 μm with sub-10 nm precision within a bandwidth of at least 100 Hz.
KEYWORDS: Calibration, Phase shifts, Sensors, Phase measurement, Control systems, Observatories, Polarization, Telescopes, Interferometers, Chemical elements
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 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 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.
FINITO (the VLTI three beam fringe-tracker) has been offered in September 2007 to the astronomical community
for observations with the scientific instruments AMBER and MIDI. In this paper, we describe the last
improvements of the fringe-tracking loop and its actual performance when operating with the 1.8m Auxiliary
Telescopes. We demonstrate the gain provided to the scientific observations. Finally, we discuss how FINITO
real-time data could be used in post-processing to enhance the scientific return of the facility.
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 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.
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.
The ESO Very Large Telescope Interferometer (VLTI) is the first general-user interferometer that offers near- and mid-infrared long-baseline interferometric observations in service and visitor mode to the whole astronomical community. Over the last two years, the VLTI has moved into its regular science operation mode with the two science instruments, MIDI and AMBER, both on all four 8m Unit Telescopes and the first three 1.8m Auxiliary Telescopes. We are currently devoting up to half of the available time for science, the rest is used for characterization and improvement of the existing system, plus additional installations. Since the first fringes with the VLTI on a star were obtained on March 17, 2001, there have been five years of scientific observations, with the different instruments, different telescopes and baselines. These observations have led so far to more than 40 refereed publications. We describe the current status of the VLTI and give an outlook for its near future.
FINITO is the first generation VLTI fringe sensor, optimised for three beam observations, recently installed at Paranal and currently used for VLTI optimisation. The PRIMA FSU is the second generation, optimised for astrometry in dual-feed mode, currently in construction. We discuss the constraints of fringe tracking at VLTI, the basic functions required for stabilised interferometric observations, and their different implementation in the two instruments, with remarks on the most critical technical aspects. We provide an estimate of the expected performance and describe some of their possible observing and calibration modes, with reference to the current scientific combiners.
The ARAL system of the VLTI is a multipurpose facility that helps to
have the interferometric instruments ready for night observations. It
consists of an artificial source (allowing a Mach-Zehnder mode of the
interferometric instruments for autotest), an alignment unit (verifying the position of the celestial target in the VLTI field-of-view), and an optical path router (controlling the optical switchyard and the instrument feeding-optics in the VLTI laboratory). With the multiplication of VLTI instruments and their specific features (wavelength coverage, number of beams), an upgrade of ARAL (from its November 2002 version) had to be carried out: the alignment unit has been redesigned, as well as the artificial source. This source will provide a point in the visible and in J, H, K and N infrared bands, split into four beams (with a zero optical path difference at the reference position). After a description of the optomechanics and of the computer architecture of ARAL, we detail the difficulties of building an interferometric artificial source with a wide spectral range.
The Very Large Telescope Interferometer (VLTI) on Cerro Paranal (2635 m) in Northern Chile reached a major milestone in September 2003 when the mid infrared instrument MIDI was offered for scientific observations to the community. This was only nine months after MIDI had recorded first fringes. In the meantime, the near infrared instrument AMBER saw first fringes in March 2004, and it is planned to offer AMBER in September 2004.
The large number of subsystems that have been installed in the last two years - amongst them adaptive optics for the 8-m Unit Telescopes (UT), the first 1.8-m Auxiliary Telescope (AT), the fringe tracker FINITO and three more Delay Lines for a total of six, only to name the major ones - will be described in this article. We will also discuss the next steps of the VLTI mainly concerned with the dual feed system PRIMA and we will give an outlook to possible future extensions.
The detector mounted in the VLTI fringe sensor FINITO is a 256x256 HgCdTe array with a cut-off wavelength of 1.9 micron. The same arrays having cut-off wavelengths of 2.5 micron will be used in the tip tilt sensor IRIS and the PRIMA instrument of the VLT interferometer. The arrays are part of an active control loop with integration times as short as a few hundred microseconds. The fringe tracker FINITO uses only 7 pixels of the array. To take advantage of the four parallel channels of the PICNIC multiplexer, the pixels illuminated in each quadrant are positioned at the same location within the quadrants. A noise analysis of the PICNIC array shows that the main sensitivity limitation of the array is contained in the low frequency part of the noise power spectrum. Similar behaviour has been observed with other infrared arrays. In an effort to optimize the unit cell pixel buffer to achieve high speed and low noise, a prototype multiplexer is being developed at Rockwell for adaptive optics. However, low frequency noise may still be the limiting factor dominating the noise performance of infrared arrays. To overcome this noise barrier, detector architectures have to be envisaged which should allow double correlated sampling on shorter time scales than a full exposure. This might be accomplished by some kind of gate in the IR material which allows charge to be shifted from an integrating well in the infrared pixel to a small sensing node capacitance of the multiplexer unit cell buffer.
On March 17, 2001, the VLT interferometer saw for the first time interferometric fringes on sky with its two test siderostats on a 16m baseline. Seven months later, on October 29, 2001, fringes were found with two of the four 8.2m Unit Telescopes (UTs), named Antu and Melipal, spanning a baseline of 102m. First shared risk science operations with VLTI will start in October 2002. The time between these milestones is used for further integration as well as for commissioning of the interferometer with the goal to understand all its characteristics and to optimize performance and observing procedures. In this article we will describe the various commissioning tasks carried out and present some results of our work.
The Very Large Telescope (VLT) Observatory on Cerro Paranal (2635 m) in Northern Chile is approaching completion in this year when the fourth of the 8-m Unit Telescopes will see first light. At the same time, the preparation for first fringes of the VLT Interferometer (VLTI) is advancing rapidly with the goal of having the first fringes with two siderostats within this year. In this article we describe the status of the VLTI and its subsystems, we discuss the planning for first fringes with the different telescopes and instruments. Eventually, we present an outlook for the future of interferometry with Very Large Telescopes.
The pupil transfer, from the individual telescopes to the interferometric laboratory, is an unique feature of the VLT Interferometer allowing to have a 2 arcsec interferometric field available at the instruments entrance. This capability is the result of a careful analysis pursued from the very beginning of the VLTI until today in the interferometric laboratory layout. For this goal it has been necessary to develop a new optical device, the Variable Curvature Mirror (VCM), and also to design all the optical systems located after the delay-lines, as the beam compressors for instance, according to these interferometric field-of-view and pupil transfer requirements. This pupil transfer and the role/design of the various optical systems are presented for the major configurations of the VLTI. A special section is dedicated to the VCM system as this component is the most critical one and required special studies, using large deformation theory of elasticity, and advanced techniques in optical fabrication. The final performances of the VCM are reviewed. As these performances had an important influence ont he design of the other systems in the interferometric laboratory, the trade-off between the instruments requirements and the VCM capabilities is presented.
This paper presents the optical layout of the REGAIN beam combiner including the optical delay line LAROCA with its variable curvature mirror, the field rotator devices, the image and pupil tracking systems and the dedicated visible spectrography. Preliminary studies of foreseen improvements, such as adaptive optics, IR spectrograph and addition of a third telescope, will be discussed.
The operation of stellar interferometers suffers from turbulence- induced random fluctuations of optical pathlength difference between collected fields. Active compensation needs an error signal which is provided by a fringe sensor. A phase A study for a fringe sensor for the ESO Very Large Telescope Interferometer (VLTI) has been conducted at OCA, leading to a proposition for the completion of a prototype. In this article, the goals and the principle of the sensor are recalled (see Gay and Rabbia, preceding paper, in this symposium). Its optimal working wavelength is discussed. Results of a numerical simulation of the sensor operation are reported, comprising sensitivity estimates. The proposed setup is then described in the details, emphasis being put on monomode optical fiber related items. Finally, current plans for the testing and the future use of the prototype are outlined.
A new focal instrumentation for the Grand Interferometre a 2 Telescopes (GI2T) called REGAIN (REcombinaison pour GrAnd INterferometre) is under study at the Observatoire de la Cote d'Azur (OCA) and the Laboratoire d'Astronomie Spatiale (LAS) in Marseille, France. The objectives of the REGAIN project are multiple. Priority number 1 is a more efficient astrophysical exploitation of the GI2T. Next is the possibility for observing simultaneously at visible and near-infrared wavelengths. Finally REGAIN should ensure the test of the OVLA prototype telescope added to the present GI2T. Therefore, the resulting GI3T could be used for phase-closure imaging with 1.5-m apertures. At the same time reservations will be made for implementing adaptive optics units for each telescope whilst the VLT interferometer fringe- sensor currently studied at the OCA, should be tested on the GI3T.
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