In the framework of the Opticon-Radionet Pilot (ORP) programme, a joint activity programme has been designed to broaden the scientific community of the VLTI. Three projects have been selected from an open call to the optical/infrared (OIR) interferometry community via independent peer review by a panel of international experts. As described below, these three new projects will complement and further expand the existing VLTI capabilities.
GRAVITY has transformed optical interferometry with ground-breaking results on the Galactic Center, Active Galactic Nuclei, and Exoplanets. In dual-object, externally fringe-tracked mode GRAVITY has already achieved or outperformed its design specifications of achieving K = 17 mag milli-arcsec imaging and 10-30 micro-arcsec astrometry at K ~ 15 mag (Eisenhauer et al. 2008). The GRAVITY fringe-tracker is currently able to operate on-axis (using 50% of the science target’s flux for fringe tracking) or off-axis at up to 2” (on the 8.2-meter unit telescopes). Unfortunately, this is very restrictive as few Galactic sources and essentially no extragalactic source has a suitable reference star within this radius. Hence, few science cases can make use of the sensitivity and accuracy enhancements of the off-axis tracking. In the context of this activity, an off-axis mode for GRAVITY will be commissioned. New feeding optics for GRAVITY will be built to use the VLTI differential delay lines. This way, GRAVITY itself can fringe-track on targets at such large separations, thus improving the sky coverage by >200 times. This will allow observations of many faint Galactic targets as well as non-AGN galaxies and high-z extragalactic targets. Through the existing GRA4MAT interface, this development will benefit not only GRAVITY but also MATISSE and future instruments, illustrating the legacy value of this task.
The ASGARD Suite is a collection of three visitor instruments bringing novel scientific capabilities to VLTI, including high-sensitivity multi-band (H+K), low-RMS fringe tracking (Heimdallr), high-spectral resolution in YJ-band (BIFROST), and L-band high-contrast imaging (Hi-5/VIKING). Specifically, the technology for the Hi-5/VIKING instrument is a result of previous OPTICON funding under the JRA umbrella and is now fully funded by the European Research Council (ERC).
Hi-5/VIKING aims at direct exoplanet detection at separations <0.1” and L-band spectroscopy, probing the snow line where most exoplanets are located. The instrument makes use of the unique capabilities of micro-arcsecond-precision astrometry and combines it with high contrast nulling interferometry. This joint activity will support the integration of Hi-5/VIKING into the VLTI infrastructure in several ways: (1) develop the operational modes for use at the observatory, including optimisation of data recording and calibration sequences; (2) produce a commissioning and observing plan; and (3) document the technology design and facility interfaces as required by ESO. Note that Hi-5/VIKING is nominally a visitor instrument and community access will be supported by this joint activity following the “PIONIER” model (i.e., interested users will contact and collaborate with the instrument team on proposal preparation, planning, and data reduction).
In mid-infrared observations, the thermal background is the limiting factor for the sensitivity. Infrared interferometry usually measures both correlated fluxes from the coherent combination of light through multiple telescopes as well as photometric fluxes from each telescope as a reference for the degree of resolution (visibility) of the interferometric measurements. The correlated fluxes usually do not suffer from strong thermal background as it is un-correlated and, hence, disappears. However, the photometric fluxes are highly affected by the background, and the current mid-IR instrument MATISSE is limited in its capabilities to measure fainter sources (e.g. AGN) by the precision of the photometric fluxes due to the background. In this activity, a correlated flux mode will be implemented and commissioned for MATISSE. The strength of the correlated flux mode is that it does not require any single-dish measurements. It does require, however, different calibration and data processing which is why efforts are needed to develop it into a mode that is accessible for the general user.