GRAVITY is the first second generation instrument for the VLT interferometer. At its heart, the instrument combines the light of four telescopes and produces interferometric fringes in two seperate interferometers, the science and the fringe-tracking channel. The instrument was offered to the community for regular science programs in P98 for the first time. Science verification was successfully performed in P97 (see the corresponding SV page). GRAVITY works for so-called imaging observations as well as for high-precision astrometric observations by means of phase referencing. The latter technique is still being commissioned and developed. GRAVITY observations are always performed in conjunction with the dedicated GRAVITY fringe-tracker (FT); observations without FT are not possible. The FT can be used on-axis (or 'single-field' mode in which FT star and science star are the same) and off-axis (or 'dual-field' mode). The off-axis FT case allows for observations of faint targets that are accompanied by relatively bright objects within the 4" field of view of an AT or the 2" of a UT.
Publications based on GRAVITY observations should cite the following reference paper: Gravity Collaboration 2017, A&A, 602, 94
GRAVITY operates in the K-band and is offered in three different spectral setting, viz. low resolution (R~20), medium resolution (R~500) and high resolution (R~4000). The low resolution mode is only offered in dual field mode, because the dispersed fringe-tracker data also have a resolution of approximately 20 and are delivered to the user together with the science data. Note that the full K-band (2.05 to 2.45 micron) is delivered by each spectral setting.
Detector integration times
Preliminary operations with the instrument show that the fringe-tracker is reliable, allowing integration times on the science of up to 30 seconds under average coherence time and seeing conditions. This stability will allow for background limited observations in the K-band. Further experience with the instrument are necessary to provide optimum science detector integration time as function of brightness and conditions. As is the case with FINITO, the GRAVITY FT settings are not controlled by the user.
The policy on execution times is similar to that of AMBER and PIONIER with standard slots of 1h which include a calibrator and a science observation. The GRAVITY ETC is available and provides visibility calculations and feasibility calculations for an observation with a given target geometry, magnitude, and baseline configuration. The calculation of the S/N on the science spectra is not yet fully verified. It is expected that in upcoming periods detailed integration times can be calculated to reach a certain S/N.
More information can be found on these pages and in the GRAVITY user manual.
Contact addresses related to the GRAVITY operations team can be found on this webpage.