Large amounts of Adaptive-Optics (AO) control loop data and telemetry are currently inaccessible to end-users. Broadening access to those data has the potential to change the AO landscape on many fronts, addressing several use-cases such as derivation of the system’s PSF, turbulence characterization and optimization of system control. We address one of the biggest obstacles to sharing these data: the lack of standardization, which hinders access. We propose an object-oriented Python package for AO telemetry, whose data model abstracts the user from an underlining archive-ready data exchange standard based on the Flexible Image Transport System (FITS). Its design supports data from a wide range of existing and future AO systems, either in raw format or abstracted from actual instrument details. We exemplify its usage with data from active AO systems on 10m-class observatories, of which two are currently supported (AOF and Keck), with plans for more.
Hexapods are general solutions that provide movement with six degrees of freedom for instrument positioning, alignment, and support. In the case of the METIS instrument, the hexapod must satisfy the following stringent requirements: a) support the 11-ton weight of an instrument; b) allow alignment and provide position stability to the instrument to within a tenth of a millimeter; c) provide an adjustment range of about 20 cm; d) support the instrument allowing for accelerations of over 3 g in all directions; e) have the lowest mass possible.
Commercial linear actuators that are generally used in such cases are designed for extended movement, include a complete set of bearings that constrain each actuator lateral displacements and a sophisticated central screw that defines only the longitudinal movement. These solutions tend to be heavy and costly if roller screws are used to avoid backslash. They encompass ranges that are a major fraction of the total length and are designed for fast movement. Both these characteristics exceed the requirements of the METIS application.
We present an optimized design for the hexapod which includes a different, lightweight, sturdy, small-range, highprecision, no backslash, earthquake-proof actuator. The design of the hexapod is such that it can be used, in general, as a mass and vibration optimized solution for precision heavy instrument alignment.
The Mid-Infrared ELT Imager and Spectrograph (METIS) is one of the first generation science instruments on ESO's 39m Extremely Large Telescope (ELT). METIS will provide diffraction-limited imaging and medium resolution slit-spectroscopy from 3 – 13 microns (L, M, and N bands), as well as high resolution (R ~ 100,000) integral field spectroscopy from 2.9 – 5.3 microns. After passing its preliminary design review (PDR) in May 2019, and the final design review (FDR) of its optical system in June 2021, METIS is now preparing for the FDR of its entire system in the fall of 2022, while the procurements of many optical components have already started. First light at the telescope is expected in 2028, after a comprehensive assembly integration and test phase. We describe the conceptual setup of METIS, its key functional components, and the resulting observing modes. Last but not least, we present the expected sensitivity, adaptive optics, and high contrast imaging performance.
This article presents the final design of the METIS/ELT warm support structure subsystem. The warm support structure provides the mechanical interface between the cryostat and the Nasmyth platform. It consists of three substructures: the elevation platform, the cryostat alignment structure (CAS), and the instrument access platform. The elevation platform is connected to the Nasmyth platform and holds the CAS. It consists of seven legs connected to three notes. The CAS is a hexapod holding the cryostat, allowing maintenance, alignment, and positioning. The instrument access platform allows human access to the cryostat, it holds the cable support system and is prepared to support the future Single Laser Adaptive Optics system. The subsystem requirements, design trade-offs, interface considerations, final design and simulation results of the substructures will be detailed as presented to the METIS Final Design Review, in 2022.
As part of the GRAVITY+ project, the near-infrared beam combiner GRAVITY and the VLTI are currently undergoing a series of significant upgrades to further improve the performance and sky coverage. The instrumental changes will be transformational, and for instance uniquely position GRAVITY to observe the broad line region of hundreds of Active Galactic Nuclei (AGN) at a redshift of two and higher. The increased sky coverage is achieved by enlarging the maximum angular separation between the celestial science object (SC) and the off-axis fringe tracking (FT) star from currently 2 arcseconds (arcsec) up to unprecedented 30 arcsec, limited by the atmospheric conditions. This was successfully demonstrated at the VLTI for the first time.
We present the testbench aimed at integrating the GRAVITY+ adaptive optics GPAO. It consists of two independent elements, one reproducing the Coudé focus of the telescope, including the telescope deformable mirror mount (with its surface facing down), and one reproducing the Coudé room opto-mechanical environment, including a downwards-propagating beam, and the telescope mechanical interfaces in order to fit in the new GPAO wavefront sensor. We discuss in this paper the design of this bench and the solutions we adopted to keep the cost low, keep the design compact (allowing it to be fully contained in a 20 sqm clean room), and align the bench independently from the adaptive optics. We also discuss the features we have set in this bench.
With the upgrade from GRAVITY to GRAVITY+ the instrument will evolve to an all-sky interferometer that can observe faint targets, such as high redshift AGN. Observing the faintest targets requires reducing the noise sources in GRAVITY as much as possible. The dominant noise source, especially in the blue part of the spectrum, is the backscattering of the metrology laser light onto the detector. To reduce this noise we introduce two new metrology modes. With a combination of small hardware changes and software adaptations, we can dim the metrology laser during the observation without losing the phase referencing. For single beam targets, we can even turn off the metrology laser for the maximum SNR on the detector. These changes lead to a SNR improvement of over a factor of two averaged over the whole spectrum and up to a factor of eight in the part of the spectrum currently dominated by laser noise.
Portugal will build the warm support and access structure (WSS) to the mid-infrared, first generation ELT instrument - METIS. The particular characteristics of METIS and the ELT pose several challenges to designing the WSS according to requirements, as well challenges to the assembly and integration of the WSS. We here provide you an overview of those challenges, as well as strategies to overcome and mitigate issues related to the mass and dimensions of the WSS.
Hexapods are very common in astronomy as a mechanism to provide a stiff mount or a precision alignment tool. Here, we present a lumped model for a general symmetric hexapod that allows us to compute the load distribution under external forces, the hexapod’s resolution, and the identification of singularity loci within the workspace. We also developed a script to analyze this parametric model, which is publicly available. We use this model to develop and design a hexapod for mid-infrared ELT imager and spectrograph, one of the extremely large telescope’s first light instruments. The designed hexapod solution can survive strict earthquake conditions that can go up to 5g, and position and align the 11 ton instrument with submillimetric and arcsecond precisions. Although the model presented is not as precise or as realistic as a finite element (FE) analysis, it provides, in a fraction of a second, a very good first approximation. Therefore, unlike FE methods, the model is able to study many geometries in a short time.
During the past years, the VLTI-instrument GRAVITY has made spectacular discoveries with phase-referenced interferometric imaging with milliarcsecond resolution and ten microarcsecond astrometry. Here, we report on the upgrade of the GRAVITY science spectrometer with two new grisms in October 2019, increasing the instrument throughput by a factor > 2. This improvement was made possible by using a high refractive index Germanium substrate, which reduces the grism and groove angles, and by successfully applying an anti-reflection coating to the ruled surface to overcome Fresnel losses. We present the design, manufacturing, and laboratory testing of the new grisms, as well as the results from the re-commissioning on sky.
Combining adaptive optics and interferometric observations results in a considerable contrast gain compared to single-telescope, extreme AO systems. Taking advantage of this, the ExoGRAVITY project is a survey of known young giant exoplanets located in the range of 0.1” to 2” from their stars. The observations provide astrometric data of unprecedented accuracy, being crucial for refining the orbital parameters of planets and illuminating their dynamical histories. Furthermore, GRAVITY will measure non-Keplerian perturbations due to planet-planet interactions in multi-planet systems and measure dynamical masses. Over time, repetitive observations of the exoplanets at medium resolution (R = 500) will provide a catalogue of K-band spectra of unprecedented quality, for a number of exoplanets. The K-band has the unique properties that it contains many molecular signatures (CO, H2O, CH4, CO2). This allows constraining precisely surface gravity, metallicity, and temperature, if used in conjunction with self-consistent models like Exo-REM. Further, we will use the parameter-retrieval algorithm petitRADTRANS to constrain the C/O ratio of the planets. Ultimately, we plan to produce the first C/O survey of exoplanets, kick-starting the difficult process of linking planetary formation with measured atomic abundances.
Extremely Large Telescopes are considered worldwide as one of the highest priorities in ground-based astronomy, for they have the potential to vastly advance astrophysical knowledge with detailed studies of subjects including the first objects in the Universe, exoplanets, super-massive black holes, and the nature and distribution of the dark matter and dark energy which dominate the Universe. ESO is building its own Extremely Large optical/infrared Telescope, the ELT. This new telescope will have a 39 m main mirror and will be the largest optical/NIR telescope in the world, able to work at the diffraction limit. METIS, one of the first light instruments of the ELT, has powerful imaging and spectrographic capabilities on the thermal wavelengths. It will allow the investigation of key properties of a wide range of objects, from exoplanets to star forming regions, and it is highly complementary to other facilities such as the JWST. METIS is an extremely complex instrument, weighing almost 11 ton, and requiring high positioning and steering precisions. Here we present the ELT’s METIS’ Warm Support Structure. It consists on a 7 leg elevation platform, a passive hexapod capable of providing METIS with sub-millimetre and arcsecond positioning and steering resolutions, and an access platform where personnel can perform in-situ maintenance activities. The support structure weighs less than 5 ton and is capable of surviving earthquake conditions with accelerations up to 5g. The current design is supported by FEM simulations in ANSYS®, and was approved for Phase C.
The METIS consortium in Portugal will build the support and access structure (WSS) for the mid-infrared, first generation ELT instrument - METIS. The specific characteristics of the METIS instrument and the ELT pose several challenges to building the WSS according to functional requirements. In addition, the assembly of the WSS and integrating the WSS with METIS poses its own particular challenges due to the singular loads and dimensions. Transversal to all phases of assembly and integration of the WSS and METIS is the concern for the safety of the instruments and personnel involved. We here present these requirements, challenges and mitigation measures in light of the assembly and integration of the WSS, and the WSS with METIS.
On-sky testing of new instrumentation concepts is required before they can be incorporated within facility-class instrumentation with certainty that they will work as expected within a real telescope environment. Increasingly, many of these concepts are not designed to work in seeing-limited conditions and require an upstream adaptive optics system for testing. Access to on-sky AO systems to test such systems is currently limited to a few research groups and observatories worldwide, leaving many concepts unable to be tested. A pilot program funded through the H2020 OPTICON program offering up to 15 nights of on-sky time at the CANARY Adaptive Optics demonstrator is currently running but this ends in 2021. Pre-run and on-sky support is provided to visitor experiments by the CANARY team. We have supported 6 experiments over this period, and plan one more run in early 2021. We have recently been awarded for funding through the H2020 OPTICON-RADIO PILOT call to continue and extend this program up until 2024, offering access to CANARY at the 4.2m William Herschel Telescope and 3 additional instruments and telescopes suitable for instrumentation development. Time on these facilities will be open to researchers from across the European research community and time will be awarded by answering a call for proposals that will be assessed by an independent panel of instrumentation experts. Unlike standard observing proposals we plan to award time up to 2 years in advance to allow time for the visitor instrument to be delivered. We hope to announce the first call in mid-2021. Here we describe the facilities offered, the support available for on-sky testing and detail the eligibility and application process.
Instrumental polarization can have large effects on measurements with the VLTI, as it can alter measured polarization and introduce uncertainties. To understand these effects we measured and simulated the instrumental polarization of the VLTI and of GRAVITY. We are able to provide a calibration model for GRAVITY observations and quantify systematic uncertainties due to instrumental polarization. This work has shown to be crucial to measure the polarization of the galactic center black hole Sgr A* where we detect a swing in the polarization angle during flare events. While the analysis was done for GRAVITY, it also gives an important basis for the design of future near-infrared instruments at the VLTI.
The GRAVITY instrument has revolutionized optical/IR interferometry: fringe-tracking and phase-referencing allow for 30 micro-arcsecond astrometry in a dual beam mode, and for spectro-differential astrometry better than 10 micro-arcseconds. The control of systematic effects is essential to fully exploit this technological advancement. Among those systematics are static phase aberrations, introduced along the instrument's optical path, which in particular affect the inferred separation of two unresolved objects within the same FOV. Here, we present how the aberrations can be measured, characterized by low-order Zernike polynomials and, most importantly, how their impact on the astrometry is corrected. The resulting astrometry corrections are verified with calibration observations of a binary before we discuss how they affect GRAVITY's measurement of the galactic center distance.
We present the successful demonstration of world's first large-separation ~30" off-axis fringe tracking with four telescopes in October 2019. With this technique we increase the sky-coverage for optical interferometry by orders of magnitude compared to current technology. Following the early work at the Palomar Testbed Interferometer, the first demonstration of off-axis fringe tracking at the Keck Interferometer and with PRIMA at the ESO Very Large Telescope Interferometer, and the breakthrough with the GRAVITY Galactic Center observations, we enhanced the VLTI infrastructure for GRAVITY to take advantage of the PRIMA Star separators and Differential Delay Lines for off-axis fringe tracking. In our presentation we give an introduction to the subject, present the enhancements of the VLTI, and present our results from the first on-sky operation in October 2019, with observations of the Orion Trapezium Cluster, a field brown dwarf, and a high redshift quasar.
The Mid-Infrared ELT Imager and Spectrograph (METIS) is one of three first light instruments on the ELT. It will provide high-contrast imaging and medium resolution, slit-spectroscopy from 3 – 19um, as well as high resolution (R ~ 100,000) integral field spectroscopy from 2.9-5.3µm. All modes observe at the diffraction limit of the ELT, by means of adaptive optics, yielding angular resolutions of a few tens of milliarcseconds. The range of METIS science is broad, from Solar System objects to active galactic nuclei (AGN). We will present an update on the main science drivers for METIS: circum-stellar disks and exoplanets. The METIS project is now in full steam, approaching its preliminary design review (PDR) in 2018. In this paper we will present the current status of its optical, mechanical and thermal design as well as operational aspects. We will also discuss the challenges of building an instrument for the ELT, and the required technologies.
We present a solution to the challenges of interfacing the ELT’s METIS to the telescope using a steerable hexapod structure. To guide the architectural choices, lumped physical models were derived from inverse kinematics in order to address the load distribution in each arm. Complete FE Analysis is carried on the optimal solutions of these models. The hexapod arms, which are high precision heavy duty linear actuators enduring forces in the excess of 30 tons, are designed using standard components whenever possible. An overall fully functional support structure design, satisfying the ESO/ELT and METIS requirements, is described.
OPTICON currently supports a Joint Research Activity (JRA) dedicated to providing easy to use image reconstruction algorithms for optical/IR interferometric data. This JRA aims to provide state-of-the-art image reconstruction methods with a common interface and comprehensive documentation to the community. These tools will provide the capability to compare the results of using different settings and algorithms in a consistent and unified way. The JRA is also providing tutorials and sample datasets to introduce the principles of image reconstruction and illustrate how to use the software products. We describe the design of the imaging tools, in particular the interface between the graphical user interface and the image reconstruction algorithms, and summarise the current status of their implementation.
GRAVITY acquisition camera implements four optical functions to track multiple beams of Very Large Telescope Interferometer (VLTI): a) pupil tracker: a 2×2 lenslet images four pupil reference lasers mounted on the spiders of telescope secondary mirror; b) field tracker: images science object; c) pupil imager: reimages telescope pupil; d) aberration tracker: images a Shack-Hartmann. The estimation of beam stabilization parameters from the acquisition camera detector image is carried out, for every 0.7 s, with a dedicated data reduction software. The measured parameters are used in: a) alignment of GRAVITY with the VLTI; b) active pupil and field stabilization; c) defocus correction and engineering purposes. The instrument is now successfully operational on-sky in closed loop. The relevant data reduction and on-sky characterization results are reported.
The GRAVITY Acquisition Camera was designed to monitor and evaluate the optical beam properties of the four ESO/VLT telescopes simultaneously. The data is used as part of the GRAVITY beam stabilization strategy. Internally the Acquisition Camera has four channels each with: several relay mirrors, imaging lens, H-band filter, a single custom made silica bulk optics (i.e. Beam Analyzer) and an IR detector (HAWAII2-RG). The camera operates in vacuum with operational temperature of: 240k for the folding optics and enclosure, 100K for the Beam Analyzer optics and 80K for the detector. The beam analysis is carried out by the Beam Analyzer, which is a compact assembly of fused silica prisms and lenses that are glued together into a single optical block. The beam analyzer handles the four telescope beams and splits the light from the field mode into the pupil imager, the aberration sensor and the pupil tracker modes. The complex optical alignment and focusing was carried out first at room temperature with visible light, using an optical theodolite/alignment telescope, cross hairs, beam splitter mirrors and optical path compensator. The alignment was validated at cryogenic temperatures. High Strehl ratios were achieved at the first cooldown. In the paper we present the Acquisition Camera as manufactured, focusing key sub-systems and key technical challenges, the room temperature (with visible light) alignment and first IR images acquired in cryogenic operation.
The so-called “phase delay tracking” attempts to estimate the effects of the turbulence on the phase of the interferograms in order to numerically cophase the measured complex visibilities and to coherently integrate them. This is implemented by the “coherent fringe analysis” of MIDI instrument1 but has only been used for high SNR data. In this paper, we investigate whether the sensitivity of this technique can be pushed to its theoretical limits and thus applied to fainter sources. In the general framework of the maximum likelihood and exploiting the chromatic behavior of the turbulence effects, we propose a global optimization strategy to compute various estimators of the differential pistons between two data frames. The most efficient estimators appear to be the ones based on the phasors, even though they do not yet reach the theoretical limits.
The acquisition camera for the GRAVITY/VLTI instrument implements four functions: a) field imager: science field imaging, tip-tilt; b) pupil tracker: telescope pupil lateral and longitudinal positions; c) pupil imager: telescope pupil imaging and d) aberration sensor: The VLTI beam higher order aberrations measurement. We present the dedicated algorithms that simulate the GRAVITY acquisition camera detector measurements considering the realistic imaging conditions, complemented by the pipeline used to extract the data. The data reduction procedure was tested with real aberrations at the VLTI lab and reconstructed back accurately. The acquisition camera software undertakes the measurements simultaneously for all four AT/UTs in 1 s. The measured parameters are updated in the instrument online database. The data reduction software uses the ESO Common Library for Image Processing (CLIP), integrated in to the ESO VLT software environment.
Two simulated astronomical objects (a star cluster, and a young stellar object) were mock observed with the VLTI for different array configurations and instruments, and their images reconstructed and compared. The aim of the work is to infer when/if phase referencing with less telescopes is a better choice over closure phases with more telescopes. Three scenarios were put under scrutiny: Phase Referencing (PhR) with 2 telescopes vs Closure Phase (CPh) with 3 telescopes, PhR with 3 telescopes vs CPh with 4 telescopes, and PhR with 4 telescopes vs CPh with 6 telescopes. The number of nights is kept fixed for a given PhR vs CPh configuration. The UV -coverage was improved for the PhR case, by uniformly paving the (u, v) plane while keeping fixed the total number of sampled spatial frequencies. For the majority of the configurations, the results point to comparable performances of phase referencing and closure phases, when the UV-space is judiciously chosen.
The GRAVITY acquisition camera has four 9x9 Shack-Hartmann sensors operating in the near-infrared. It measures the slow variations of a quasi-distorted wavefront of four telescope beams simultaneously, by imaging the Galactic Center field. The Shack-Hartmann lenslet images of the Galactic Center are generated. Since the lenslet array images are filled with the crowded Galactic Center stellar field, an extended object, the local shifts of the distorted wavefront have to be estimated with a correlation algorithm. In this paper we report on the accuracy of six existing centroid algorithms for the Galactic Center stellar field. We show the VLTI tunnel atmospheric turbulence phases are reconstructed back with a precision of 100 nm at 2 s integration.
The GRAVITY acquisition camera measurements are part of the overall beam stabilization by measuring each second
the tip-tilt and the telescope pupil lateral and longitudinal positions, while monitoring at longer intervals the full
telescope pupil, and the VLTI beam higher order aberrations.
The infrared acquisition camera implements a mosaic of field, pupil, and Shack Hartman type images for each telescope.
Star light is used to correct the tip-tilt while laser beacons placed at the telescope spiders are used to measure the pupil
lateral positions. Dedicated optimized algorithms are applied to each image, extracting the beam parameters and storing
them on the instrument database.
The final design is built into the GRAVITY beam combiner, around a structural plane where the 4 telescope folding
optics and field imaging lenses are attached. A fused silica prism assembly, kept around detector temperature, is placed
near to the detector implementing the different image modes.
KEYWORDS: Visibility, Signal to noise ratio, Deconvolution, Interferometry, Calibration, Data modeling, Spectral resolution, Spectrographs, Telescopes, Analog electronics
In this communication an extraction procedure that takes into account the spectral dispersion function (the
spectral analog of the PSF) is presented. The method is named least-squares deconvolution. It allows the
recovery of the relative line-to-continuum visibility amplitude ratio and the relative line to continuum visibility
phase difference. The method only uses as input the AMBER data making the sole hypotheses that the spectral
broadening of the spectra in the photometric channel is the same as that of the interferometric data. A subset
of this hypothesis is the case of unresolved lines. It is extremely robust being able to recover line to continuum
visibility and phase at very low signal-to-noise ratio. It is shown that it is superior to other differential visibility
and phase methods presented in the literature, which in certain conditions are biased. The method can be
trivially generalized to similar instruments as those available at CHARA and Keck-I.
Least squares deconvolution opens the possibility of delivering legacy quality measurements from the AMBER
archive without relying on visibility calibration or environmental effects such as vibrations. It is a key tool for
the astrophysical exploitation of this instrument.
Integrated optics is a well established technology that finds its main applications in the fields of optical communication
and sensing. However, it is expanding into new areas, and in the last decade application in astronomical interferometry
has been explored. In particular, several examples have been demonstrated in the areas of beam control and combination.
In this paper, different examples of application integrated optics devices for fabrication of beam combiners for
astronomical interferometry is given. For the multiaxial beam combiners, a UV laser direct writing unit is used for mask
fabrication. The operation principles of the coaxial combiners fabricated in hybrid sol-gel were validated using an
interferometric set-up. These results demonstrate that hybrid sol-gel technology can produce quality devices, opening the
possibility of rapid prototyping of new designs and concepts.
Astrophotonics offers a solution to some of the problems of building instruments for the next generation of telescopes
through the use of photonic devices to miniaturise and simplify instruments. It has already proved its worth in
interferometry over the last decade and is now being applied to nightsky background suppression. Astrophotonics offers
a radically different approach to highly-multiplexed spectroscopy to the benefit of galaxy surveys such as are required to
determine the evolution of the cosmic equation of state. The Astrophotonica Europa partnership funded by the EU via
OPTICON is undertaking a wide-ranging survey of the technological opportunities and their applicability to high-priority
astrophysical goals of the next generation of observatories. Here we summarise some of the conclusions.
The hydrogen emission line is a defining characteristic of young stellar objects probing the planet forming regions
of the disks. The limiting sensitivity of current interferometers has precluded it's detailed study. We'll review
our current understanding of hydrogen emission, recent results and project the science that can be achieved with
sensitive interferometers such as the PRIMA off-axis mode and GRAVITY.
We compare the quality of interferometric image reconstructions for two different sets of data: square of the
visibility plus closure phase (e.g. AMBER like case) and square of the visibility plus visibility phase (e.g.
PRIMA+AMBER or GRAVITY like cases). We used the Multi-aperture image Reconstruction Algorithm for
reconstructions of test cases under different signal-to-noise ratios and noisy data (squared visibilities and phases).
Our study takes into account noise models based on the statistics of visibility, phase and closure phase. We
incorporate the works developed by Tatulli and Chelly (2005) on the noise of the power-spectrum and closure
phase in the read-out and photon noise regimes,1 and by Colavita (1999) on the signal-to-noise ratio of the
visibility phase.2 The final images were then compared to the original one by means of positions and fluxes,
computing the astrometry and the photometry. For the astrometry, the precision was typically of tens of
microarcseconds, while for the photometry, it was typically of a few percent. Although both cases are suitable
for image restorations of real interferometric observations, the results indicate a better performance of phase
referencing (V2 + visibility phase) in a low signal-to-noise ratio scenario.
Hybrid sol-gel technology was used for fabrication of prototypes of coaxial two, three and four telescopes beam
combiners for astronomical applications. These devices were designed for the astronomical J-band and have been
characterized using an optical source with emission centered at 1265 nm and with a spectral FWHM of 50 nm.
Interferometric characterization of the two, three and four beam combiners, showed average contrasts respectively
higher than 98%, 96% and 95%. Interferometric spectral analysis of the beam combiners revealed that the chromatic
differential dispersion is the main contributor to the observed contrast decay in the latter cases. The laser direct writing
technique was used for fabrication of a coaxial two beam combiner on sol-gel material; it showed a contrast of 95%. The
measured high contrast fringes confirm that the procedures used lead to performant IO beam combiners. These results
demonstrate the capabilities of the hybrid sol-gel technology for fast prototyping of complex chip designs for astronomical applications.
Classically, optical and near-infrared interferometry have relied on closure phase techniques to produce images.
Such techniques allow us to achieve modest dynamic ranges.
In order to test the feasibility of next generation optical interferometers in the context of the VLTI-spectro-imager
(VSI), we have embarked on a study of image reconstruction and analysis. Our main aim was to test the
influence of the number of telescopes, observing nights and distribution of the visibility points on the quality of
the reconstructed images. Our results show that observations using six Auxiliary Telescopes (ATs) during one
complete night yield the best results in general and is critical in most science cases; the number of telescopes is
the determining factor in the image reconstruction outcome.
In terms of imaging capabilities, an optical, six telescope VLTI-type configuration and ~200 meter baseline
will achieve 4 mas spatial resolution, which is comparable to ALMA and almost 50 times better than JWST will
achieve at 2.2 microns. Our results show that such an instrument will be capable of imaging, with unprecedented
detail, a plethora of sources, ranging from complex stellar surfaces to microlensing events.
We present the work developed within the science team of the Very Large Telescope Interferometer Spectro-Imager (VSI) during the Phase A studies. VSI aims at delivering ~ 1 milliarcsecond resolution data cubes
in the near-infrared, with several spectral resolutions up to 12 000, by combining up to 8 VLTI telescopes. In
the design of an instrument, the science case plays a central role by supporting the instrument construction
decision, defining the top-level requirements and balancing design options. The overall science philosophy of
VSI was that of a general user instrument serving a broad community. The science team addressed themes
which included several areas of astrophysics and illustrated specific modes of operation of the instrument: a)
YSO disks and winds; b) Multiplicity of young stars; c) Exoplanets; d) Debris disks; e) Stellar surface imaging;
f) The environments of evolved stars; g) AGN tori; h) AGN's Broad Line Region; i) Supermassive black-holes;
and j) Microlensing. The main conclusions can be summarized as follows: a) The accessible targets and related
science are extremely sensitive to the instrument limiting magnitude; the instrument should be optimized for
sensitivity and have its own fringe tracker. b) Most of the science cases are readily achievable with on-axis fringe
tracking, off-axis fringe tracking enabling extra science. c) In most targets (YSOs, evolved stars and AGNs), the
interpretation and analysis of circumstellar/nuclear dust morphology requires direct access to the gas via spectral
resolved studies of emission lines, requiring at least a spectral resolution of 2 500. d) To routinely deliver images
at the required sensitivity, the number of telescopes in determinant, with 6 telescopes being favored. e) The
factorial increase in the number of closure phases and visibilities, gained in a single observation, makes massive
surveys of parameters and related science for the first time possible. f) High dynamic range imaging and very
high dynamic range differential closure phase are possible allowing the study of debris disks and characterization
of pegasides. g) Spectro-imaging in the near-infrared is highly complementary to ALMA, adaptive optics and
interferometric imaging in the thermal infrared.
KEYWORDS: Telescopes, Stars, Spectral resolution, Spatial resolution, Interferometry, Integrated optics, Space telescopes, Visibility, Image restoration, Signal to noise ratio
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; fainter targets can be accessed if a suitable reference is
available. 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 astrophysics. The high level specifications of the instrument are
derived from a detailed science case based on the capability to obtain, for the first time, milliarcsecond-resolution
images of a wide range of targets 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.
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.
Integrated optics is a mature technology with standard applications to telecommunications. Since the pioneering work of
Berger et al. 1999 beam combiners for optical interferometry have been built using this technology. Classical integrated
optics device production is very expensive and time consuming. The rapid production of devices using hybrid sol-gel
materials in conjunction with UV laser direct writing techniques allows overcoming these limitations. In this paper this
technology is tested for astronomical applications. We report on the design, fabrication and characterization of multiaxial
two beam combiners and a coaxial beam combiner for astronomical interferometry. Different multiaxial two beam combiner designs were tested and high contrast (better than 90%) was obtained with a 1.3 μm laser diode and with an
SLD ( λ0 = 1.26 μm, FWHM of 60 nm). High contrast fringes were produced with 1.3 μm laser diode using the coaxial two beam combiner. These results show that hybrid sol-gel techniques produce devices with high quality, allowing the
rapid prototyping of new designs and concepts for astronomy.
One of the aims of next generation optical interferometric instrumentation is to be able to make use of information
contained in the visibility phase to construct high dynamic range images.
Radio and optical interferometry are at the two extremes of phase corruption by the atmosphere. While in
radio it is possible to obtain calibrated phases for the science objects, in the optical this is currently not possible.
Instead, optical interferometry has relied on closure phase techniques to produce images. Such techniques allow
only to achieve modest dynamic ranges. However, with high contrast objects, for faint targets or when structure
detail is needed, phase referencing techniques as used in radio interferometry, should theoretically achieve higher
dynamic ranges for the same number of telescopes.
Our approach is not to provide evidence either for or against the hypothesis that phase referenced imaging
gives better dynamic range than closure phase imaging. Instead we wish to explore the potential of this technique
for future optical interferometry and also because image reconstruction in the optical using phase referencing
techniques has only been performed with limited success.
We have generated simulated, noisy, complex visibility data, analogous to the signal produced in radio interferometers,
using the VLTI as a template. We proceeded with image reconstruction using the radio image
reconstruction algorithms contained in aips imagr (clean algorithm). Our results show that image reconstruction
is successful in most of our science cases, yielding images with a 4 milliarcsecond resolution in K band.
We have also investigated the number of target candidates for optical phase referencing. Using the 2MASS
point source catalog, we show that there are several hundred objects with phase reference sources less than 30
arcseconds away, allowing to apply this technique.
VLTi Spectro-Imager (VSI) is a proposition for a second generation VLTI instrument which is aimed at providing
the ESO community with the capability of performing image synthesis at milli-arcsecond angular resolution. VSI
provides the VLTI with an instrument able to combine 4 telescopes in a baseline version and optionally up to
6 telescopes in the near-infrared spectral domain with moderate to high spectral resolution. The instrument
contains its own fringe tracker in order to relax the constraints onto the VLTI infrastructure. VSI will do
imaging at the milli-arcsecond scale with spectral resolution of: a) the close environments of young stars probing
the initial conditions for planet formation; b) the surfaces of stars; c) the environment of evolved stars, stellar
remnants and stellar winds, and d) the central region of active galactic nuclei and supermassive black holes. The
science cases allowed us to specify the astrophysical requirements of the instrument and to define the necessary
studies of the science group for phase A.
AMBER is a 3 beam combiner for the Very Large Telescope Interferometer (VLTI). It will soon add to VLTI tremendous angular resolution, sensitivity and spectral resolution (λ/Δλ) up to 10,000. This combination opens important new opportunities for the study of the close environment of pre-main-sequence stars.
In order to understand star formation and its evolution, one needs to solve the problem of ejection and collimation mechanisms in jets from young stars. The importance of jets in pre-main-sequence stars relies on the fact that they regulate its angular momentum. By measuring the jet opening angle at the ejection region we can test models for jet origin. In particular, AMBER will provide crucial information on the mechanisms of mass loss and collimation observed in the most active objects. It will allow, for the first time, the differentiation of competing models for jet origin and collimation, namely the X-wind model of Shu and the disk-wind model of Blandford & Payne. In this paper we compare different jet models presented in the literature.
The science objectives of VITRUV is to investigate the morphology of compact astrophysical objects in optical wavelengths like the environment of AGN, star forming regions, stellar surfaces. This instrument will take full advantage of the VLTI site with 4 very large telescopes and 4 auxiliary telescopes. The instrument concept is to built aperture synthesis images like the millimeter-wave radiointerferometer of the IRAM Plateau de Bure. VITRUV coupled to the VLTI will have similar and even better resolution than ALMA. The astrophysical specifications although not yet finalized will be a temporal resolution of the order of 1 day, spectral resolution from 100 to 30,000, image dynamic from 100 to 1,000, a field of view of 1 arcsec for an initial wavlength coverage from 1 to 2.5 microns that could be extended from 0.5 to 5 microns. The technology that is contemplated at this stage is integrated optics.
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