Gravity is a second generation instrument for the VLT interferometer. At its heart, the instrument combines the light of four telescopes and produces interferometric fringes. The instrument will be offered to the community for regular science programs in P98 for the first time. Note however that a call for science verification in P97 is issued (see the SV page). For these calls, Gravity will work in its so-called imaging mode; its astrometric mode by means of phase referencing is still under commissioning. 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.
Gravity operates in the K-band and can be used in P98 and for the science verification in two different spectral setting, viz. medium resolution (R~500) or high resolution (R~4000). Make note of the fact that the full K-band (2.05 to 2.45 micron) is delivered by each spectral setting. Fringe-track data are delivered to the user together with the science data. Dispersed fringe-tracker data have a resolution of approximately 20.
Detector integration times
Preliminary operations with the instrument show that the fringe-tracker is reliable allowing integration time 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 that includes 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.