METIS is a sophisticated mid-infrared imager and spectrograph covering wavelengths from 2.9 to 13.5 μm (astronomical L, M, and N bands), stands as one of the three primary science instruments at the Extremely Large Telescope (ELT). METIS will provide diffraction-limited imaging, coronagraphy, high-resolution integral field spectroscopy, and low/medium-resolution slit spectroscopy. Within the international METIS consortium, the University of Cologne is responsible for designing, manufacturing, integrating, and testing the Warm Calibration Unit (WCU). Among the other calibration functionalities, the WCU also facilitates alignment checks during Assembly, Integration, and Verification (AIT & AIV) through a visible light channel utilizing a CMOS camera, which can capture images of internal focal planes, pupil planes of the WCU, and various METIS sub-systems. This paper details the final optical design, preliminary opto-mechanical design, and analysis of alignment optics in the WCU. A laboratory test setup has been discussed, showcasing the estimation of SNR for the AIV mode of the WCU using an off-the-shelf CMOS camera and interating sphere. This paper also introduces the novel design concept of aluminum spherical mirrors employed in the main optical train of the WCU. We also present a brief overview of alignment verification procedures of the offner relay optics of the WCU sub-system. Furthermore, a detailed overview of the thermal analysis of the whole WCU will be discussed, which ensures the temperature difference between the WCU and the ELT dome will be within specified limits.
METIS is one of the first-generation instruments currently in development for the Extremely Large Telescope (ELT). As one of the partners in the project consortium, the team at University of Cologne is responsible for the development of the Warm Calibration Unit (WCU) subsystem. This subsystem is designed to deliver a selection of broadband and single wavelength sources for the calibration of the METIS instrument along with pupil/focal plane imaging optics serving as feedback for the internal alignment of the subsystems of METIS during the integration phase foreseen in Leiden. After the satisfactory Final Design Review (FDR) towards the end of 2022, the development of the WCU subsystem is currently in the manufacturing/procurement phase along with the integration of sub-assemblies. We provide an overview of the development of the WCU subsystem at this phase of the project including the progress on the manufacturing of custom components and the details on the compliance with the design principles with a primary focus on the mechanical aspects. These components include as an example: (1) a lightweight optical bench made of carbon-fiber reinforced polymer (CFRP) and with dimensions of roughly 2.5mx3m and a thickness of 20cm, satisfying compliancy both with high loads and tight tolerances (on the level of several tens of microns), (2) custom-designed six adjustable supporting links responsible for carrying the subsystem (roughly 1 tons) and its alignment to METIS focal/pupil planes within desired accuracies 50 microns with the help of laser sensors, (3) kinematic optical Aluminum mounts with a thermal invariant design (i.e. positioning of the optics are not affected by temperature). In the light of the progress, we discuss further the perspectives and the planning towards the full integration and testing of the full subsystem, foreseen to start within the second quarter of 2025.
The BlackGEM array Phase I consists of three wide field, optical telescopes, located at the ESO La Silla Observatory, Chile. Each telescope is of a modified Dall-Kirkham design, using an 0.6m primary mirror and a 110 Mpix STA1600 CCD to give a 2.7 square degrees field-of-view sampled at 0.56"/pixel. Preliminary commissioning data shows performance on-par with design specifications. Data obtained with the BlackGEM prototype MeerLICHT highlights the capabilities of the design with a 5-sigma limiting magnitude of mAB=22.2 in 300s of integration under dark-sky conditions. Extrapolation to the 1" seeing-conditions expected at La Silla shows that the main goal of BlackGEM to probe down to mAB=23 in 300s can be met. The project suffered a 2-year COVID-19 delay. Commissioning of the array has currently been resumed and science operations are expected to start in Q3/Q4 of 2022. The science programs include the follow-up of gravitational wave alerts from LIGO/Virgo/KAGRA, a six-filter Southern Sky Survey, a Fast Synoptic Survey on selected fields, a Local Universe intra-night monitoring program and a inter-night single-band monitoring for slower transients.
The precision aimed at by ESA’s Astrometry and Radial Velocity mission GAIA surpasses that of the successful HIPPARCOS mission by more than 2 orders of magnitude, while at the same time increasing the number of objects 10000 times. This overwhelming increase in performance (statistical weight increased by ~ 8 orders of magnitude) is achieved by insisting on a full description in terms of photon shot noise as the fundamental limiting factor. Yet such measurements refer to wave front topography to be understood to the level of better than 100 pico meters, in an optical system a few meters across. Obviously such understanding relies heavily on the expected stability, and chromatic effects also are of dominant importance, requiring stellar spectral energy distributions to be determined. It is fascinating that the experience of HIPPARCOS can indeed generate sufficient confidence for these performance specifications to be within reach. Elaborating the design specifications and tolerances I hope to convince you of GAIA’s imminent success.
A special case of optical aperture synthesis, homothetic mapping, is the topic of this paper. It allows for a wide field of view for interferometric instruments. This paper describes a testbed constructed and tested in TNO-TPD in Delft (the Netherlands). This testbed is intended as a tool to investigate the ins and outs of homothetic mapping. The homothetic mapping approach is explained, the whole setup is specified and results are shown.
Steven Bloemen, Paul Groot, Patrick Woudt, Marc Klein Wolt, Vanessa McBride, Gijs Nelemans, Elmar Körding, Margaretha Pretorius, Ronald Roelfsema, Felix Bettonvil, Harry Balster, Roy Bakker, Peter Dolron, Arjen van Elteren, Eddy Elswijk, Arno Engels, Rob Fender, Marc Fokker, Menno de Haan, Klaas Hagoort, Jasper de Hoog, Rik ter Horst, Giel van der Kevie, Stanisław Kozłowski, Jan Kragt, Grzegorz Lech, Rudolf Le Poole, Dirk Lesman, Johan Morren, Ramon Navarro, Willem-Jelle Paalberends, Kerry Paterson, Rafal Pawłaszek, Wim Pessemier, Gert Raskin, Harrie Rutten, Bart Scheers, Menno Schuil, Piotr Sybilski
We present the MeerLICHT and BlackGEM telescopes, which are wide-field optical telescopes that are currently being built to study transient phenomena, gravitational wave counterparts and variable stars. The telescopes have 65 cm primary mirrors and a 2.7 square degree field-of-view. The MeerLICHT and BlackGEM projects have different science goals, but will use identical telescopes. The first telescope, MeerLICHT, will be commissioned at Sutherland (South Africa) in the first quarter of 2017. It will co-point with MeerKAT to collect optical data commensurate with the radio observations. After careful analysis of MeerLICHT's performance, three telescopes of the same type will be commissioned in La Silla (Chile) in 2018 to form phase I of the BlackGEM array. BlackGEM aims at detecting and characterizing optical counterparts of gravitational wave events detected by Advanced LIGO and Virgo. In this contribution we present an overview of the science goals, the design and the status of the two projects.
The Multi-site All-sky CAmeRA MASCARA is an instrument concept consisting of several stations across the globe,
with each station containing a battery of low-cost cameras to monitor the near-entire sky at each location. Once all
stations have been installed, MASCARA will be able to provide a nearly 24-hr coverage of the complete dark sky, down
to magnitude 8, at sub-minute cadence. Its purpose is to find the brightest transiting exoplanet systems, expected in the
V=4-8 magnitude range - currently not probed by space- or ground-based surveys. The bright/nearby transiting planet
systems, which MASCARA will discover, will be the key targets for detailed planet atmosphere observations. We
present studies on the initial design of a MASCARA station, including the camera housing, domes, and computer
equipment, and on the photometric stability of low-cost cameras showing that a precision of 0.3-1% per hour can be
readily achieved. We plan to roll out the first MASCARA station before the end of 2013. A 5-station MASCARA can
within two years discover up to a dozen of the brightest transiting planet systems in the sky.
The NOVA Fringe Tracker (NFT) is a proposed solution to the call by ESO for a second generation fringe tracking facility.
This instrument at the VLTI will enable the cophasing of up to 6 telescopes simultaneously. Using broad band optics with
detection from 1.2 to 2.4 microns, a unique configuration is employed that eliminates so-called “photometric crosstalk.”
This refers to imbalance in the beam combiner which results in fluctuations of the incoming wavefronts and the proportion
of power accepted by a spatial filter masquerading as a visibility, a common problem afflicting previous interferometric
instruments and fringe trackers. Also proposed for use in “science instruments” (for the measurement of visibility), the
“Polarization-Based Collimated Beam Combiner,” with its achievement of photometric symmetry in hardware, is particularly
suited for combined use of the smaller AT (1.8 meter) telescopes with the UT (8 meter) telescopes involving a 20:1
intensity ratio of the interfering beams, and also for fringe tracking using highly resolved sources having a very small visibility.
Recent enhancements to the proposed fringe tracker include selectable modes which detect only a single quadrature
phase, both quadrature phases, or an uneven combination of the two. Optimization of partial spatial filtering using pinholes
has been performed using a wavefront simulator and simulated tracking loop. Aiming for an instrument achieving the
best limiting sensitivity, analysis and simulations predict that reliable cophasing will be obtained using the 1.8 meter AT
telescopes tracking on an unresolved reference star with a K magnitude of 10
The Polarization-Based Collimated Beam Combiner efficiently produces pairwise interference between beams from multiple
telescopes. An important feature is achieving "Photometric Symmetry" whereby interference measurements have
no first-order sensitivity to wavefront perturbations (or photometric variations following spatial filtering) which otherwise
entail visibility measurements with increased error, bias, and nonlinearity in phase determination. Among other proposed
applications, this topology has been chosen as the basis for the design of the NOVA Fringe Tracker (NFT), a proposed 4
or 6 telescope second-generation fringe tracker for the VLTI. The NFT takes advantage of the photometric symmetry thus
achieved making it capable of tracking on stars resolved beyond the first visibility null, as well as interfering a telescope
beam with one which is 20 times brighter, a design goal set by ESO. By not requiring OPD modulation for interferometric
detection, the detector exposure time can be increased without performance reduction due to time skew nor is sensitivity
reduced by removing optical power for photometric monitoring, and use of two-phase interferometric detection saves one
half of the photons being diverted for detection of the other two (mainly) unused quadrature phases. The topology is also
proposed for visibility measuring interferometers with configurations proposed for the achievement of balanced quadrature
or 3-phase interferometric detection. A laboratory demonstration confirms >>100:1 rejection of photometric crosstalk in a
fringe tracking configuration where atmospheric OPD fluctuations were simulated using a hair dryer. Tracking with a 30:1
intensity ratio between the incoming beams was performed while rejecting large introduced photometric fluctuations.
A special case of optical aperture synthesis, homothetic mapping, is the topic of this paper. It allows for a wide field of view for interferometric instruments, interesting for astrometric measurments of wide objects. This paper describes a testbed constructed and tested in TNO-TPD in Delft (the Netherlands). This testbed is intended as a tool to investigate the ins and outs of homothetic mapping. The homothetic mapping approach is explained, the whole setup is specified and results are shown.
The PRIMA facility will implement dual-star astrometry at the VLTI. We have formed a consortium that will build the PRIMA differential delay lines, develop an astrometric operation and calibration plan, and deliver astrometric data reduction software. This will enable astrometric planet surveys with a target precision of 10μas. Our scientific goals include determining orbital inclinations and masses for planets already known from radial-velocity surveys, searches for planets around stars that are not amenable to high-precision radial-velocity observations, and a search for large rocky planets around
nearby low-mass stars.
With ESO's Phase Referenced Imaging and Micro-arcsecond Astrometry (PRIMA) facility well into its procurement phase expectations are made about the astrometric performance. It appears that in almost all respects the instrumentally induced errors are expected to have Power Spectral densities well below those due to the atmosphere.
However for the target performance to be achieved, some effects must reduce by averaging by some 4 orders of magnitude. The most serious worry foreseen is lack of thermal stability in the air-filled Delay Line tunnel, and it is recommended that outside wind influence be impeded.
Homothetic mapping is an aperture synthesis technique that allows interferometric imaging over a wide field-of-view. A laboratory experiment was set up to demonstrate the feasibility of this technique. Here, we present the first static experiments on homothetic mapping that have been done on the Delft Testbed for Interferometry (DTI). Before a changeable telescope configuration is provided, we first took a fixed telescope configuration and tested the algorithms for their ability to provide an exit pupil configuration before beam combination, that was an exact copy of this telescope configuration. By doing so, we created a homothetic imaging system. This is an imaging system that acts as a masked aperture monolithic telescope, but consists of (in our case) three telescopes of which the light follow their own optical trains.
Knowledge of the dispersion due to (humid) air in the light path of the Very Large Telescope Interferometer (VLTI) is crucial to obtaining good science data from MIDI, PRIMA and GENIE. To calculate the refraction due to air at infra red wavelengths in the ducts and delay line tunnel, the temperature and humidity has to be monitored during observations. To accomplish these measurements an easy to use and reliable system was assembled, based on commercially available components. In-house calibration of four humidity and temperature sensors of the system was done in Leiden. A test and calibration program was carried out to make sure that they work reliably and accurately and to determine the sensor characteristics. For this purpose a calibration box was designed which isolates the sensors from the environment so that there is no exchange of air with the outside environment. Using constant humidity salt solutions, the humidity in the box can be controlled. This allows the calibrations to be carried out for typical values of relative humidity and temperature at Cerro Paranal. Calibration of the sensors includes: 1. Reducing the systematic relative humidity differences between the sensors to less than 0.1 % and 2. Reducing the systematic temperature differences between the sensors to less than 0.01 K. In this paper we will present the outcome of the calibrations and the future of the sensors at Paranal.
A search for extrasolar planets using the ESO VLTI PRIMA facility
will become feasible in 2007. An astrometric accuracy of 10 micro-arcseconds will allow us to detect sub-Uranus mass planets around the
most nearby stars, as well as to conduct a planet search around stars of different ages. Most of the PRIMA hardware subsystems are currently being developed by industry. At the same time a scientific Consortium has formed that will deliver the differential delay lines and astrometric software for PRIMA to ESO.
In this paper we describe the planned efforts by the Consortium
related to the "PRIMA astrometry operations and software". These
activities include an overall "PRIMA astrometry error budget", a
"PRIMA astrometry calibration and observation strategy", the "PRIMA astrometry observation preparation tools" and the "PRIMA astrometry data reduction tools". We describe how all these components fit together in an overall approach to the flow of knowledge within the project. First by quantifying the fundamental limits of the VLTI infrastructure and the astronomical sources under study. Followed by elimination or suppression of the errors through either a hardware change to the system, software control of the system, or a proper calibration and observation strategy.
The ultimate goal is being able to calibrate all PRIMA astrometric data acquired over the full lifetime of PRIMA (5 to 10 years) to a uniform accuracy of 10 micro-arcseconds. This will allow identification of long-term trends in the astrometric parameters due to planetary companions around nearby stars and to determine the distances and proper motions for the selected sources.
ESA's DARWIN will be an interferometric mission carrying out high-resolution astrophysical observations as well as the detection/characterization of earthlike exoplanets. In this paper, the current status and development perspectives of the Darwin imaging mode are discussed. First, overall system aspects are addressed including expected sensitivity, and baseline reconfiguration needs. Subsequently, the current instrumental concept is reviewed. This is based on a phase-referencing architecture supporting simultaneous observation of the science object, and an off-axis reference target for OPD stabilization purposes. The reference and science beams are wavelength-multiplexed and propagate along a common path through the interferometer. The viability of the cophasing approach is discussed, with emphasis on crosstalk control for multiplexed beam transfer, real-time compensation of the astrometric OPD, and associated metrology requirements. Studies have shown that imaging capabilities can be implemented within the current nulling beam combiner concept, which avoids the complexity and cost of developing a dedicated imaging beam combiner spacecraft. However, this approach has important drawbacks for the imaging mission
A wideband interferometer is sensitive to the effects of longitudinal dispersion which affect the interfering light beams unequally. At shorter wavelengths the major effect of dispersion is from dry air itself, while at mid infrared wavelengths the effect of water vapor is dominant. MIDI, the future 10 micron instrument of the VLTI, will experience significant effects from the imbalances in the water vapor content affecting the paths of the two interfering beams. This imbalance will include terms due to the unbalanced air paths in the delay line, random atmospheric humidity fluctuations in the lines-of-sight to the star, and random humidity variations inside of the VLTI delay line tunnels.
Large amounts of dispersion, if not monitored, can reduce the accuracy of measured visibility amplitudes. Measurements of the visibility phase as a function of wavelength will be highly sensitive
to dispersion. This will then become a source of noise in results dependent on the phase of interference, such as imaging of non-symmetric objects, or detection of faint companions. In addition
to dispersion over the 7 - 14 micron region detected by MIDI, observations using phase tracking with detection in the near IR, could be catastrophically affected by differential phase delays between the 2 micron and 10 micron bands.
Dispersion measurements from VINCI observing in the K band, both due to dry air and to water vapor, are presented. Combining VINCI results with published data from millimeter wave measurements leads us to expect atmospheric differential water vapor fluctuations to exceed 1 mole/m2 rms over typical baselines. Specific effects from such a level of unmonitored dispersion variations are presented, which demand corrective action. Various solutions to monitor water vapor dispersion in realtime are
considered.
With the imaging and astrometric facilities coming up as the next major leaps forward in interferometry at the VLT we examine options for beam combination of multiple beams, both in the Science Channel and in the Reference Channel. We look at the science we expect to become feasible as soon as phase referencing becomes available, and the hopes for the future of this facility with the advent of more ATs and next-generation beam combiners. Contrary to "conventional wisdom," the fringe tracking accuracy for a multi-element array is not worse than for a single-baseline interferometer with equal telescope size. We discuss applications of phase referencing to observations of faint targets, and a double-differential phase technique for spectroscopy of extrasolar planets. We point out that phase-referenced methods have some important advantages compared with the closure-phase technique for imaging of extended sources.
The start of NEVEC was initiated by the opportunity in the Netherlands to reinstate instrumental efforts in astronomy through a funding program for 'Top Research Schools,’ which brought about the creation of NOVA. The fact that considerable experience exists in Radio Astronomical imaging through interferometry (the Westerbork Synthesis Radio Telescope started in 1970), and the relatively small size at the time of ESO's VLTI Team made it opportune to aim for a win-win situation through collaboration. So presently an MOU between ESO and NOVA is in force, which stipulates that 10 out of the 18 man-years funded by NOVA for NEVEC until 2005 [new personnel, in university setting (Leiden) but on project money] shall be used on tasks that are mutually agreed between NOVA and ESO.
The tasks presently are found in the domain of observing modes, calibration and modeling, as well as contributing to the commissioning of new instruments and thinking about future instruments. Another task, outside these 10 FTE, has been the data handling and analysis software for MIDI, and again contributing to its commissioning. Delivery of the first operational version in Heidelberg has just taken place (summer 2002) contributing to the successful Preliminary Acceptance in Europe for MIDI on September 10, 2002. The actual state of 'products and deliveries' and the future outlook are reviewed.
The field of view for the first generation VLTI instruments
(VINCI, MIDI, AMBER etc.) is limited to the diffraction limited
beam of a single telescope (for a 8.2-meter telescope at 2-micron
this is 0.6 arcseconds). However, the VLTI infrastructure with its
main delay lines, transfers a 2 arcseconds beam from the
telescopes to the interferometric laboratory. When we discuss wide
field imaging in the context of this paper, we refer to these 2
arcseconds. Although most current optical interferometers use
Michelson pupil plane beam-combination there is a convincing
scientific justification for wider field imaging capabilities to
allow astrometry, photometry and ultimately spectroscopy of
crowded fields (e.g. galactic center, globular clusters),
binaries, stellar disks and dust shells. In this paper we present
a newly developed tool to model a fringe tracker for a Fizeau
interferometer by retrieving the piston of each individual
telescope from the combined point spread function. This study
shows that the pistons can be retrieved. Detecting the signal in
at least two wavelength channels with an energy sensitive detector
(e.g. Superconducting Tunneling Junction) allows compensation for
the atmospheric piston and successively coherent integration of a
science channel. The examples provided in this paper are compliant
with the VLTI infrastructure and are part of a larger project to
study the feasibility of a wide field imager for the VLTI as a
third generation instrument. Finally we present a conceptional
design for a wide field imaging for the VLTI, referred to as
VLTI-WIDY.
We describe principles, design and present status of MIDI, the mid-infrared interferometric instrument for the VLTI, which is planned to come into operation at the ESO Very Large Telescope Interferometer during the second half of 2001.
In order to improve the efficiency with which SCUBA (see elsewhere in this conference) operates on the JCMT, a new and innovative data acquisition system has been developed and will be implemented shortly. The fundamental innovation is in the operation of the telescope secondary mirror, merging the function of chopping for sky-elimination, and 'jiggling' to sub-pixel positions to Nyquist sample the image as seen by the arrays themselves. This eliminates the need to sample 'empty sky' for half the time, thus doubling the time spent 'on target.' Additional expected advantages are: (1) improved sky suppression and; (2) increased dynamic range in the resulting image. It is hoped that the target of shotnoise limited performance will prove within reach.
PRIMA (instrument for Phase-Referenced Imaging and Microarcsecond Astrometry) is a conceptual study for a single-baseline dual-feed instrument for the Very Large Telescope Interferometer (VLTI), which is under construction by the European Southern Observatory on Cerro Paranal in Chile. The goals of PRIMA include narrow-angle astrometry with a precision of 10 μas over an arc of 10", and imaging of faint sources with the full sensitivity of the 8 m telescopes in the VLT array. Key scientific programs that can be carried out with PRIMA in imaging mode include observations of active galactic nuclei, the Galactic Center, stars, and circumstellar matter. Scientific drivers for the astrometry are searches for planets and low-mass stellar companions, binary stars, dynamics of clusters, and (relative) parallaxes. We list the main performance requirements for PRIMA, present system architectures for the dual-beam system, and discuss limitations of the interferometric field-of-view.
We describe a concept for an interferometric space mission dedicated to global (wide-angle) astrometry. The GAIA satellite contains two small (baseline APEQ 3 m) optical interferometers of the Fizeau type, mechanically set at a large and fixed angle to each other. Each interferometer has a field of view of about one degree. Continuous rotation of the whole satellite provides angular connections between the stars passing through the two fields of view. Positions, absolute parallaxes and annual proper motions can be determined with accuracies on the 20 micro-arcsec level. The observing programme may consist of all objects to a limiting magnitude around V = 15-16, including 50 million stars. The GAIA concept, which has been proposed for a Cornerstone Mission within the European Space Agency's long-term science programme, is based on the same general principles as the very successful ESA Hipparcos mission, but takes advantage of the much higher resolution and efficiency permitted by interferometry and modern detector techniques.
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