Gravity Instrument Description

The Gravity system

Gravity is not a monolithic instrument. It is a collection of sub-systems that aims to precisely control the wavefront of the incoming light and its path through the VLT-I system before the actual combination of beams takes place and interference fringes are created.  A unique aspect of Gravity, and the first time this is ever realized, is its ability to interfere the light coming from either a single astronomical source (single-field mode or on-axis) or from two sources simultaneously (dual-field mode or off-axis). In dual field, the Gravity system can perform phase referenced observations supported by accurate knowledge of the path length which is assessed by a laser system. In this mode of observation, the interferometric phase of the primary star is calibrated to that of the secondary against the detrimental influence of the atmosphere. This enables highly accurate angle measurements on sky and is the basis for Gravity's astrometric mode. In Gravity's imaging mode, dual-field observations allow to observe relatively faint targets and use a brighter star as the fringe-track star. The difference between the two Gravity modes is the observation strategy and the use of laser metrology to measure the light-path.

The Gravity sub-systems include:

  • the IR wavefront sensing system CIAO. CIAO is located in each of the UT coude room and it will operate with the deformable mirror of the MACAOs.
  • a polarization control system to counteract polarization effects in the VLTI. Gravity can work either in a split or a combined polarization mode.
  • an active pupil guide system including LED sources mounted on each of the telescope spiders.
  • a field-guide system to track the position of the source and ensure proper injection into mono-mode fibers

The overall Gravity system interacts and works in harmony with the VLT-I system of telescopes and delay lines. The operation of various Gravity sub-systems is transparent to the user. Detailed description of the working principle of Gravity with an example on how the system is arranged for on-sky observations can be found in Section 3  of the User Manual.

The single and dual field modes

The primary unit of Gravity is the beam combining instrument (BCI) that performs the acquisition process and provides the interferometric fringes. The Gravity BCI is cryogenically cooled and physically located in the VLT-I laboratory. Within the BCI cryostat. a field is separated and two stars find their way into either the science channel or the fringe-tracking channel. In single-field the FT channel and SC channel receive light from the same star which is split 50%-50%. The Gravity fringe-tracker forms an integral part of the observational approach, i.e. Gravity science observations are always done with active fringe-tracking. The FT fringe position is used in real-time to correct for the atmospheric and instrumental piston (i.e. a residual optical path difference between beams) by modulating piezo mounted mirrors within the instrument. The FT star thus allows longer detector integration times in the science channel (up to 60 seconds) without compromising the fringe pattern’s contrast. In dual field mode (astrometric or imaging), the fringes of one star are formed in the SC channel, and those of the other star in the FT channel. Gravity delivers spectrally dispersed interference fringes that allow stellar interferometry.  The fringes give access to interferometric quantities like absolute and differential visibility, spectral differential phase and closure phase. These quantities provide information of physical phenomena at a spatial resolution that can reach 2 mas (depending on VLT-I baseline), as well as time-resolved differential astrometry at the exquisite accuracy of a few tens of μas.

The Beam Combiner Instrument

Obtaining fringe data in dual-field observations, the two astronomical sources are required to have an angular separation less than 2 arcsec when observing with the UTs, or less than 4 arcsec with the ATs. Fringes are produced in two separate channels, the fringe-track (FT) channel and the science (SC) channel. The science channel records the entire K-band at each of the three implemented spectral resolutions of R ∼ 22, 500 and 4000. The FT spectrometer always operates at low spectral resolution (R ∼ 22). When observing in dual-field, one of the two sources is injected into the FT channel and its fringe pattern analyzed at a frequency of approximately a kHz. The FT fringe position is used in real-time to correct for the atmospheric and instrumental piston (i.e. a residual optical path difference between beams) by modulating piezo mounted mirrors within the instrument.

Characteristics and performances

Gravity's expected performance for P100 are found below in the two tables. From the experience obtained during the commissioning the following limiting magnitudes are adopted covering three seeing regimes, of which the excellent weather regime (<0.6") is new for P100.

Table 1. K-band limiting correlated magnitudes on ATs
  <0.6" 0.6"< seeing <1" >1"
single field 7.0m 6.0m 5.0m
FT dual field 7.5m 6.5m 5.5m
SC dual field 7.5m+3m 6.5m +3m 
5.5m +3m  
Table 2. K-band limiting correlated magnitudes on UTs
  <0.6" 0.6" < seeing <1" >1"
single field 10m
9.0m 8.0m
FT dual field 10.5m
9.5m 8.5m
SC dual field 10.5m +3m
9.5m +3m 
8.5m +3m  

Tables 1 and 2 list the limiting K-band correlated magnitudes for observations with ATs and UTs in P100. The following restrictions apply:

  • Low spectral resolution is offered only in dual field and single field. However,  users are advised not to use low spectral resolution in single field, since the FT already provides low resolution data on the science target.
  • To be able to track all six baselines, the correlated magnitude has to fulfill the above criteria on at least three baselines that do not form a triangle (e.g. 13/23/24, or 12/13/14).  Additionally, in service mode the visibility should not drop below 5% for these three baselines and not below 1% in visitor mode.
  • Observations with Gravity should ask for CLR sky conditions, with the exception of bright objects, where users could ask for THN.
  • In dual field the magnitude for the science target is at most 3 magnitudes fainter than that of the FT star. The signal to noise in the science channel depends on the FT performance.
  • On ATs in dual field, the separation of the two targets must be at least 1.5" and less than 4".
  • On UTs in dual field, the separation of the two targets must be at least 0.4" and less than 2".
  • While the limiting correlated magnitudes are independent of spectral resolution setting of the science channel, please consult the Gravity ETC to ensure sufficient SNR in absorption lines.
  • The science channel will reach saturation for the minimum DIT at a K-band magnitude of -1 for medium spectral resolution. At high spectral resolution and in split polarization the limiting brightness is K=-4. These brightness values apply for observations with the ATs.
  • For UTs, the estimated brightness limits are K=+4 and +1, respectively. In practice, the values depend on the strehl ratio achieved by MACAO.
  • For VLT-I specific limiting magnitudes, as for example for STRAP the primary star's V magnitudes need to be between -1.7 and 11 for proper Coude guiding at the telescope.
  • The source's altitude needs to be higher than 40 deg above horizon.

More detailed information on the VLTI facility is available in the VLTI user manual.