KEYWORDS: Astronomical interferometry, Image restoration, Observatories, Education and training, Fourier transforms, Astronomy, Radio optics, Radio interferometry, Equipment, Data modeling
The optical/infrared interferometry community is a vibrant, active community. Nonetheless, the interferometric technique is not without its challenges. The VLTI Expertise Centers were created precisely to fulfill this shortcoming in optical/infrared interferometry expertise and to maximize the impact of VLTI instrumentation. In this talk I will address the various activities in which we try to engage and stimulate the optical/infrared interferometry community through user support and training, as well as activities aimed at wider audiences, namely complementary observing communities, the general public, stakeholders and policy makers.
The WSS is a subsystem being designed and manufactured by the CENTRA team (Portugal) for the ESO ELT first light instrument METIS. The WSS consists of three substructures – the support system (ELP), the alignment system (CAS), and the access and maintenance system (RIG). In total, the WSS dimensions are approximately 6 × 6 × 6 meters. In order to fully assemble, integrate, and test such a large structure, an integration hall of at least 2.5 times the WSS volume would be required to accommodate the necessary lateral and vertical clearance around WSS. Such integration halls are not readily available or accessible. In order to overcome this challenge, we have devised a 3-step strategy to assemble, integrate, and test the WSS at three different locations in three different configurations.
METIS is a first light mid-infrared instrument for the new ESO/ELT telescope. It includes a cryostat with a mass of more than ten tons that must face the telescope optical beam placed 6 m above the telescope Nasmyth instrument mounting platform. To overcome this height and allow the overall alignment of the field and pupil of the entrance beam, a large size structure has been designed, analyzed and optimized and is now being manufactured. To minimize the overall mass, the beam attachments do not fit in a vertical/horizontal grid but are oblique in a 3D structure. In building this structure one needs to combine 3D CNC machined parts of reasonable dimensions with several welded structural tube structures. Due to the sizes involved, these subsystems must be manufactured accurately within an angle tolerance of 0.1 degrees, imposing challenges on the welding process. The overall approaches to the main problems, together with the structures needed for the assembly, integration and transport are discussed.
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.
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.
METIS, the Mid-infrared E-ELT Imager and Spectrometer, is being designed for the Extremely Large Telescope (ELT) and is currently expected to arrive at the telescope early 2028. As part of the design of the instrument, we are developing the Assembly, Integration and Verification strategy for METIS. Although the sub-systems will be largely qualified at their respective institutes, only once all components come together at system level will it be possible to verify all the interfaces, full system thermal characteristics and full instrument performance. Although one of the smaller instruments for the ELT, the fully integrated METIS will still be more than 7 meters high, with a footprint in excess of 15 square meters and a weight of the order of 10 tons. This paper describes the system level assembly, integration and verification of METIS, both in Europe as well as once delivered to the telescope.
The VLTI Expertise Centres were created to fulfill a shortcoming in optical interferometry expertise and to maximize the impact of the VLTI instrumentation. We here present the various activities in which the VLTI Expertise Centres are engaged, with a particular focus on providing user support and expanding the VLTI community.
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.
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.
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.
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.
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.
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.
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.
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