Since its first light at the Very Large Telescope Interferometer (VLTI), GRAVITY has reached new regimes in optical interferometry, in terms of accuracy as well as sensitivity.<sup>1</sup> GRAVITY is routinely doing phase referenced interferometry of objects fainter than K > 17 mag, which makes for example the galactic center black hole Sagittarius A*<sup>2</sup> detectable 90 % of the times. However from SNR calculations we are confident that even a sensitivity limit of K ~ 19 mag is possible. We therefore try to push the limits of GRAVITY by improving the observations as well as the calibration and the data reduction. This has further improved the sensitivity limit to K > 18 mag in the beginning of this year. Here we present some work we are currently doing in order to reach the best possible sensitivity.
The use of optical fibers in astronomical instrumentation has been becoming more and more common. High transmission, polarization control, compact and easy routing are just a few of the advantages in this respect. But fibers also bring new challenges for the development of systems. During the assembly of the VLTI beam combiner GRAVITY different side effects of the fiber implementation had to be taken into account. In this work we summarize the corresponding phenomena ranging from the external factors influencing the fiber performance, like mechanical and temperature effects, to inelastic scattering within the fiber material.
The VLTI instrument GRAVITY combines the beams from four telescopes and provides phase-referenced imaging as well as precision-astrometry of order 10 μas by observing two celestial objects in dual-field mode. Their angular separation can be determined from their differential OPD (dOPD) when the internal dOPDs in the interferometer are known. Here, we present the general overview of the novel metrology system which performs these measurements. The metrology consists of a three-beam laser system and a homodyne detection scheme for three-beam interference using phase-shifting interferometry in combination with lock-in amplifiers. Via this approach the metrology system measures dOPDs on a nanometer-level.
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.
GRAVITY is the four-beam, near-infrared, AO-assisted, fringe tracking, astrometric and imaging instrument for the Very Large Telescope Interferometer (VLTI). It is requiring the development of one of the most complex instrument software systems ever built for an ESO instrument. Apart from its many interfaces and interdependencies, one of the most challenging aspects is the overall performance and stability of this complex system. The three infrared detectors and the fast reflective memory network (RMN) recorder contribute a total data rate of up to 20 MiB/s accumulating to a maximum of 250 GiB of data per night. The detectors, the two instrument Local Control Units (LCUs) as well as the five LCUs running applications under TAC (Tools for Advanced Control) architecture, are interconnected with fast Ethernet, RMN fibers and dedicated fiber connections as well as signals for the time synchronization. Here we give a simplified overview of all subsystems of GRAVITY and their interfaces and discuss two examples of high-level applications during observations: the acquisition procedure and the gathering and merging of data to the final FITS file.
The GRAVITY Instrument Software (INS) is based on the common VLT Software Environment. In addition to the basic Instrument Control Software (ICS) which handles Motors, Shutters, Lamps, etc., it also includes three detector subsystems, several special devices, field bus devices, and various real time algorithms. The latter are implemented using ESO TAC (Tools for Advanced Control) and run at a frequency of up to 4 kHz. In total, the instrument has more than 100 ICS devices and runs on five workstations and seven vxWorks LCUs.
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 VLTI instrument GRAVITY will provide very powerful astrometry by combining the light from four tele- scopes for two objects simultaneously. It will measure the angular separation between the two astronomical objects to a precision of 10 μas. This corresponds to a differential optical path difference (dOPD) between the targets of few nanometers and the paths within the interferometer have to be maintained stable to that level. For this purpose, the novel metrology system of GRAVITY will monitor the internal dOPDs by means of phase- shifting interferometry. We present the four-step phase-shifting concept of the metrology with emphasis on the method used for calibrating the phase shifts. The latter is based on a phase-step insensitive algorithm which unambiguously extracts phases in contrast to other methods that are strongly limited by non-linearities of the phase-shifting device. The main constraint of this algorithm is to introduce a robust ellipse fitting routine. Via this approach we are able to measure phase shifts in the laboratory with a typical accuracy of λ=2000 or 1 nm of the metrology wavelength.
We present in this paper the design and characterisation of a new sub-system of the VLTI 2<sup><i>nd</i></sup> generation instrument GRAVITY: the Calibration Unit. The Calibration Unit provides all functions to test and calibrate the beam combiner instrument: it creates two artificial stars on four beams, and dispose of four delay lines with an internal metrology. It also includes artificial stars for the tip-tilt and pupil guiding systems, as well as four metrology pick-up diodes, for tests and calibration of the corresponding sub-systems. The calibration unit also hosts the reference targets to align GRAVITY to the VLTI, and the safety shutters to avoid the metrology light to propagate in the VLTI-lab. We present the results of the characterisation and validtion of these differrent sub-units.
We present the installed and fully operational beam stabilization and fiber injection subsystem feeding the 2nd generation VLTI instrument GRAVITY. The interferometer GRAVITY requires an unprecedented stability of the VLTI optical train to achieve micro-arcsecond astrometry. For this purpose, GRAVITY contains four fiber coupler units, one per telescope. Each unit is equipped with actuators to stabilize the telescope beam in terms of tilt and lateral pupil displacement, to rotate the field, to adjust the polarization and to compensate atmospheric piston. A special roof-prism offers the possibility of on-axis as well as off-axis fringe tracking without changing the optical train. We describe the assembly, integration and alignment and the resulting optical quality and performance of the individual units. Finally, we present the closed-loop performance of the tip-tilt and pupil tracking achieved with the final systems in the lab.
The laser metrology system in the GRAVITY instrument plays a crucial role in an attempt at high-precision narrow-angle astrometry. With a design goal of achieving 10 microarcseconds precision in astrometry, the system must measure the optical path difference between two beam combiners within GRAVITY to an accuracy of better than 5nm. However in its current design, some parts of the optical paths of the metrology system are not common to the optical paths of starlight (the science path) which it must measure with high accuracy. This state of the design is true for most but not all the baselines which will be used by the GRAVITY instrument. The additional non-common optical paths could produce inaccurate path length measurements and consequently inaccurate measurements of the differential phase between fringe packets of two nearby celestial objects, which is the main astrometric observable of the instrument. With reference to the stability and the sensitivity of the non-common paths, this paper describes the impact of a biased differential phase measurement on the narrowangle astrometry and the image reconstruction performance of the GRAVITY instrument. Several alternative designs are also discussed.
GRAVITY is a new generation beam combination instrument for the VLTI. Its goal is to achieve microarsecond astrometric accuracy between objects separated by a few arcsec. This 10<sup>6</sup> accuracy on astrometric measurements is the most important challenge of the instrument, and careful error budget have been paramount during the technical design of the instrument. In this poster, we will focus on baselines induced errors, which is part of a larger error budget.
GRAVITY is the second generation VLT Interferometer (VLTI) instrument for high-precision narrow-angle astrometry and phase-referenced interferometric imaging. The laser metrology system of GRAVITY is at the heart of its astrometric mode, which must measure the distance of 2 stars with a precision of 10 micro-arcseconds. This means the metrology has to measure the optical path difference between the two beam combiners of GRAVITY to a level of 5 nm. The metrology design presents some non-common paths that have consequently to be stable at a level of 1 nm. Otherwise they would impact the performance of GRAVITY. The various tests we made in the past on the prototype give us hints on the components responsible for this error, and on their respective contribution to the total error. It is however difficult to assess their exact origin from only OPD measurements, and therefore, to propose a solution to this problem. In this paper, we present the results of a semi-empirical modeling of the fibered metrology system, relying on theoretical basis, as well as on characterisations of key components. The modeling of the metrology system regarding various effects, e.g., temperature, waveguide heating or mechanical stress, will help us to understand how the metrology behave. The goals of this modeling are to <i>1)</i> model the test set-ups and reproduce the measurements (as a validation of the modeling), <i>2)</i> determine the origin of the non-common path errors, and <i>3)</i> propose modifications to the current metrology design to reach the required 1nm stability.
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.
GRAVITY is a second generation VLTI instrument, combining the light of four telescopes and two objects
simultaneously. The main goal is to obtain astrometrically accurate information. Besides correctly measured stellar
phases this requires the knowledge of the instrumental differential phase, which has to be measured optically during the
astronomical observations. This is the purpose of a dedicated metrology system. The GRAVITY metrology covers the
full optical path, from the beam combiners up to the reference points in the beam of the primary telescope mirror,
minimizing the systematic uncertainties and providing a proper baseline in astrometric terms. Two laser beams with a
fixed phase relation travel backward the whole optical chain, creating a fringe pattern in any plane close to a pupil. By
temporal encoding the phase information can be extracted at any point by means of flux measurements with photo
diodes. The reference points chosen sample the pupil at typical radii, eliminating potential systematics due differential
focus. We present the final design and the performance estimate, which is in accordance with the overall requirements
We present design results of the 2nd generation VLTI instrument GRAVITY beam stabilization and light injection
subsystems. Designed to deliver micro-arcsecond astrometry, GRAVITY requires an unprecedented stability of the
VLTI optical train. To meet the astrometric requirements, we have developed a dedicated 'laser guiding system',
correcting the longitudinal and lateral pupil position as well as the image jitter. The actuators for the correction are
provided by four 'fiber coupler' units located in the GRAVITY cryostat. Each fiber coupler picks the light of one
telescope and stabilizes the beam. Furthermore each unit provides field de-rotation, polarization analysis as well as
atmospheric piston correction. Using a novel roof-prism design offers the possibility of on-axis as well as off-axis fringe
tracking without changing the optical train. Finally the stabilized beam is injected with minimized losses into singlemode
fibers via parabolic mirrors. We present lab results of the first guiding- as well as the first fiber coupler prototype
regarding the closed loop performance and the optical quality. Based on the lab results we discuss the on-sky
performance of the system and the implications concerning the sensitivity of GRAVITY.
The GRAVITY instrument’s adaptive optics system consists of a novel cryogenic near-infrared wavefront sensor to be
installed at each of the four unit telescopes of the VLT. Feeding the GRAVITY wavefront sensor with light in the 1.4 -
2.4 micrometer band, while suppressing laser light originating from the GRAVITY metrology system, custom-built
optical components are required. Here we report on optical and near-infrared testing of the silicon entrance windows of the wavefront sensor cryostat and other reflective optics used in the warm feeding optics.
We present the Fiber Coupler subsystem of the future VLTI instrument GRAVITY. GRAVITY is specifically designed
to deliver micro-arcsecond astrometry and deep interferometric imaging. The Fiber Coupler is designed to feed the light
from a science and a reference object into single-mode fibers. The Fiber Coupler consists of four independent units. The
units de-rotate the FoV. A motorized half-wave plate allows rotating the liner polarization axis. Each unit provides
actuators for fast piston actuation, tip-tilt correction and pupil stabilization for one of the beams from four VLT
telescopes. The actuators are operated in closed-loop. Together with a dedicated Laser Guiding System, this allows to
stabilize the beams and maximize the coherently coupled light. The fast piston actuator provides the crucial fringe
tracking capability at a bandwidth of >220Hz. A special roof prism design allows to either split the FoV or to serve as a
50/50 beam splitter without changing the optical path. This offers the possibility of on-axis as well as off-axis fringe
tracking. The optical train consists solely of mirrors, which ensures an achromatic behavior and maximum throughput.
The sophisticated optical design compensates for aberrations which are introduced by off-axis parabolic mirrors. This
allows to achieve Strehl ratios of >95% across the FoV.
GRAVITY is a VLTI second generation instrument designed to deliver astrometry at the level of 10 μas. The
beam transport to the beam combiner is stabilized by means of a dedicated guiding system whose specifications
are mainly driven by the GRAVITY astrometric error budget. In the present design, the beam is monitored using
an infrared acquisition camera that implements a mosaic of field, pupil and Shack-Hartmann images for each of the telescopes. Star and background H-band light from the sky can be used to correct the tip-tilt and pupil lateral position, within the GRAVITY specifications, each 10 s. To correct the beam at higher frequencies laser guiding beams are launched in the beam path, on field and pupil planes, and are monitored using position sensor detectors. The detection, in the acquisition camera, of metrology laser light back reflected from the telescopes, is also being investigated as an alternative for the pupil motion control.
Interferometric measurements of optical path length differences of stars over large baselines can deliver extremely
accurate astrometric data. The interferometer GRAVITY will simultaneously measure two objects in the field
of view of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO) and
determine their angular separation to a precision of 10 μas in only 5 minutes. To perform the astrometric
measurement with such a high accuracy, the differential path length through the VLTI and the instrument has
to be measured (and tracked since Earth's rotation will permanently change it) by a laser metrology to an even
higher level of accuracy (corresponding to 1 nm in 3 minutes). Usually, heterodyne differential path techniques
are used for nanometer precision measurements, but with these methods it is difficult to track the full beam size
and to follow the light path up to the primary mirror of the telescope. Here, we present the preliminary design of a differential path metrology system, developed within the GRAVITY project. It measures the instrumental differential path over the full pupil size and up to the entrance pupil location. The differential phase is measured by detecting the laser fringe pattern both on the telescopes' secondary mirrors as well as after reflection at the primary mirror. Based on our proposed design we evaluate the phase measurement accuracy based on a full budget of possible statistical and systematic errors. We show that this metrology design fulfills the high precision requirement of GRAVITY.
GRAVITY is an adaptive optics assisted Beam Combiner for the second generation VLTI instrumentation. The
instrument will provide high-precision narrow-angle astrometry and phase-referenced interferometric imaging in the
astronomical K-band for faint objects. We describe the wide range of science that will be tackled with this instrument,
highlighting the unique capabilities of the VLTI in combination with GRAVITY. The most prominent goal is to observe
highly relativistic motions of matter close to the event horizon of Sgr A*, the massive black hole at center of the Milky
Way. We present the preliminary design that fulfils the requirements that follow from the key science drivers: It includes
an integrated optics, 4-telescope, dual feed beam combiner operated in a cryogenic vessel; near-infrared wavefrontsensing
adaptive optics; fringe-tracking on secondary sources within the field of view of the VLTI and a novel metrology
concept. Simulations show that 10 μas astrometry within few minutes is feasible for a source with a magnitude of
m<sub>K</sub> = 15 like Sgr A*, given the availability of suitable phase reference sources (m<sub>K</sub> = 10). Using the same setup, imaging of m<sub>K</sub> = 18 stellar sources in the interferometric field of view is possible, assuming a full night of observations and the corresponding UV coverage of the VLTI.
The Phase-Referenced Imaging and Micro-arcsecond Astrometry (PRIMA) facility is scheduled for installation
in the Very Large Telescope Interferometer observatory in Paranal, Chile, in the second half of 2008. Its goal
is to provide an astrometric accuracy in the micro-arcsecond range. High precision astrometry can be applied
to explore the dynamics of the dense stellar cluster. Especially models for the formation of stars near super
massive black holes or the fast transfer of short-lived massive stars into the innermost parsec of our galaxy can
be tested. By measuring the orbits of stars close the the massive black hole one can probe deviations from a
Keplerian motion. Such deviations could be due to a swarm of dark, stellar mass objects that perturb the point
mass solution. At the same time the orbits are affected by relativistic corrections which thus can be tested. The
ultimate goal is to test the effects of general relativity in the strong gravitational field. The latter can be probed
with the near infrared flares of SgrA* which are most likely due to accretion phenomena onto the black hole.
We study the expected performance of PRIMA for astrometric measurements in the Galactic Center based on
laboratory measurements and discuss possible observing strategies.
The Fringe Sensor Unit (FSU) is the central element of the dual-feed facility PRIMA at the VLT Interferometer
(VLTI). Two identical FSU fringe detectors deliver real-time estimates of phase delay, group delay and signal-to-noise ratio for the two observed targets. They serve both as the scientific instrument for astrometry with
PRIMA and as sensor for the fringe tracking system of the interferometer. Prior to its installation at the VLTI
scheduled for mid-2008, the FSU is going through an extensive laboratory test phase. It is therefore embedded in
a semi-realistic environment, involving a VLTI-like control system and a laser metrology. This allows us to probe
the system response to atmospheric piston jitter, tip-tilt disturbances and higher order aberrations, as they are
expected at the observatory. We report on the system test results, outline the optimisation of the calibration
procedure and we evaluate the FSU fringe tracking performance under realistic conditions. Finally, we compare
the obtained performances to the scientific and technical requirements.
We present the second-generation VLTI instrument GRAVITY, which currently is in the preliminary design phase.
GRAVITY is specifically designed to observe highly relativistic motions of matter close to the event horizon of Sgr A*,
the massive black hole at center of the Milky Way. We have identified the key design features needed to achieve this
goal and present the resulting instrument concept. It includes an integrated optics, 4-telescope, dual feed beam combiner
operated in a cryogenic vessel; near infrared wavefront sensing adaptive optics; fringe tracking on secondary sources
within the field of view of the VLTI and a novel metrology concept. Simulations show that the planned design matches
the scientific needs; in particular that 10µas astrometry is feasible for a source with a magnitude of K=15 like Sgr A*,
given the availability of suitable phase reference sources.