Gravity

Gravity is a four way beam combination second generation instrument for the VLTI. Its main operation mode makes use of all four 8m Unit Telescopes to measure astrometric distances between objects located within the 2” field-of-view of the VLTI. With the sensitivity of the UTs and the ~10μas astrometric precision, it will allow to measure orbital motions within the galactic center with unprecedented precision. Other modes of the instrument will allow imaging and the use of the 1.8m Auxiliary Telescopes.

Principle Investigator Frank Eisenhauer
Builders

The Gravity consortium:
Max-Planck-Institut für Exterterrestrische Physik (Garching),
LESIA, Observatoire de Paris, Section de Meudon,
Laboratoire d'Astrophysique, Observatoire de Grenoble,
Max-Planck-Institut für Astronomie (Heidelberg),
I. Physikalisches Institut, Universität zu Köln,
SIM, Faculdade de Ciências da Universidade de Lisboa

Instrument Scientist and Work Package Manager Markus Schöller
Project Manager Lieselotte Jochum
ESO Science Advistory Team Olivier Absil (Liege),
Hans-Ulrich Käufl (ESO, chair),
Mario Lattanzi (Torino),
Fabien Malbet (Grenoble),
Ralph Neuhäuser (Jena),
Huub Rottgering (Leiden)
Project Milestones Kick-Off July 2008, PDR December 2009, FDR October 2011
Location Interferometric Laboratory

Science Objectives

GRAVITY will carry out the ultimate empirical test to show whether or not the Galactic Centre harbours a black hole (BH) of four million solar masses and will finally decide if the near-infrared flares from Sgr A* originate from individual hot spots close to the last stable orbit, from statistical fluctuations in the inner accretion zone or from a jet. If the current hot-spot interpretation of the near-infrared (NIR) flares is correct, GRAVITY has the potential to directly determine the spacetime metric around this BH. GRAVITY may even be able to test the theory of general relativity in the presently unexplored strong field limit. GRAVITY will also be able to unambiguously detect intermediate mass BHs, if they exist. It will dynamically measure the masses of supermassive BHs (SMBHs) in many active galactic nuclei, and probe the physics of their mass accretion, outflow and jets with unprecedented resolution. Furthermore, GRAVITY will explore young stellar objects, their circumstellar discs and jets, and measure the properties of binary stars and exoplanet systems. In short, GRAVITY will enable dynamical measurements in an unexplored regime.

Instrument Description

GRAVITY provides high precision narrowangle astrometry and phase-referenced interferometric imaging in the astronomical K-band (2.2 μm). It combines the light from four Unit Telescopes (UTs) or Auxiliary Telescopes (ATs), measuring the interferograms from six baselines simultaneously. The instrument has three main components: the IR wavefront sensors; the beam-combiner instrument; and the laser metrology system.

The GRAVITY IR wavefront sensors will be mounted in the Coudé rooms of the UTs and will command the existing Multiple Application Curvature Adaptive Optics (MACAO) deformable mirrors. The system can work on either of the two beams (on-axis or off-axis) behind the PRIMA star separators. Any additional tip/tilt from the beam relay down to the VLTI laboratory will be corrected by a dedicated laser-guiding system. Low frequency drifts of the field and pupil will
be corrected by GRAVITY’s internal acquisition and guiding camera. The interplay of these systems will guarantee an unperturbed and seeing-corrected beam at the entrance of the beam-combiner instrument in the VLTI laboratory. The interferometric instrument will work on the 2" (for UTs) or 4" (for ATs) VLTI field of view. Both the reference star and the science object have to lie within this field of view. The light of the two objects from the four telescopes is coupled into optical fibres for modal filtering, to compensate for the differential delay and to adjust the polarisation. The fibres feed two integrated optics beam combiners and the coherently combined light is dispersed in two spectrometers. A low resolution spectrometer provides internal phase- and group-delay tracking on the reference star, and thus enables long exposure times on the science target. Three spectral resolutions with up to R~4000 are implemented in the science spectrometer, and a Wollaston prism provides basic polarimetry.

GRAVITY will measure the visibility of the reference star and the science object simultaneously for all spectral channels, and the differential phase between the two objects. This information will be used for interferometric imaging exploring the complex visibilities, and for astrometry using the differential phase and group delay. All functions of the GRAVITY beam-combiner instrument are implemented in a single cryostat for optimum stability, cleanliness, and thermal background suppression. The internal path lengths of the VLTI and GRAVITY are monitored using dedicated laser metrology. The laser light is back-propagated from the beam combiner and covers the full beam up to the telescope spider above the primary mirror. GRAVITY will make use of high-speed IR photoncountingdetector arrays in both the adaptive optics systems and the fringe tracker.These devices do not suffer from high readoutnoise, which in current IR arrays is tenor more electrons per pixel at framerates of a few hundred Hz.