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.
Gravity is a 2nd generation interferometric instrument for VLTI. It will combine 4 telescopes in dual feed in the K band
to study general relativity effects around the Galactic Center black hole. The concept of Gravity is based on two
equivalent beam combiner instruments: the scientific one fed by the science target (Sgr A*) and the fringe tracker fed by
a bright reference star (See Gillessen et al.<sup>1</sup>). Both beam combination instruments are based on silica on silicon
integrated optics (IO) component glued to fluoride glass fiber array. The beam combiners are implemented in a
cryogenic vessel cooled at 200°K and back-illuminated by a high power laser used for metrology (Bartko et al.<sup>2</sup>). This
paper is dedicated to the description of the development of the integrated beam combiner assembly.
A two stage blocking system is implemented in the GRAVITY science and the fringe tracking spectrometer optical
design. The blocking system consists of a dichroic mirror and a long wave band-pass filter with the top level
requirements of high transmission of the science light in the K-Band (1.95 - 2.5 μm) region and high blocking power
optical density (OD) ≥ 8 for the metrology laser wavelength at 1.908 μm. The laser metrology blocking filters have been
identified as one critical optical component in the GRAVITY science and fringe tracker spectrometer design.
During the Phase-B study of GRAVITY we procured 3 blocking filter test samples for demonstration and qualification
tests. We present the measurements results of an effective blocking of the metrology laser wavelength with a long wave
band-pass filter at OD=12.
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.
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.
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.
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.
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.