ORP Joint Activities (2021-2025)
In the framework of the Opticon-Radionet Pilot (ORP) programme, a joint activity programme has been designed to both enhance the VLTI capabilities (Joint Research Activities, JRA) and to broaden the VLTI scientific community (Telescope Access, TA). Three JRA and two TA activities have been selected from an open call to the optical/infrared (OIR) interferometry community via independent peer review by a panel of international experts. The results of these activities are summarized below:
Joint Research Activities
GRAVITY has transformed optical interferometry with ground-breaking results on the Galactic Center, Active Galactic Nuclei, and Exoplanets. In dual-object mode with off-axis fringe tracking, GRAVITY had 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 was 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). This was very restrictive as only a few Galactic sources and essentially no extragalactic sources have a suitable reference star within this radius. Hence, a few science cases could make use of the sensitivity and accuracy enhancements of the off-axis tracking. In the context of this ORP activity, the off-axis mode of GRAVITY was extended to 30”, thus improving the sky coverage by >200 times. The new extended off-axis mode has been offered to the community since 2022, and the first results have been published (see e.g., GRAVITY+ collaboration 2022). In 2023, designs for further improvements and upgrades continue with new faster differential delay lines (DDLs). The DDLs were commissioned in 2024. Several technical and peer-reviewed papers have already emerged, including one in Nature (GRAVITY+ collaboration, 2024). Through the existing GRA4MAT interface, this development not only benefits GRAVITY but also MATISSE and future instruments, illustrating the legacy value of this task.
The ASGARD Suite (Martinod et al. 2024) 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 (NOTT). Specifically, the technology for the NOTT instrument is a result of previous OPTICON funding under the JRA umbrella and is now fully funded by the European Research Council (ERC). Using nulling interferometry, NOTT aims at direct exoplanet detection at separations <0.1” and L-band spectroscopy, probing the snow line where most exoplanets are located (Defrère et al., 2022, 2024). Within this joint research activity, the NOTT team, in collaboration with the Asgard consortium, developed and submitted to ESO: (i) the science case and technical feasibility document in 2022, (ii) a commissioning plan in 2023, and (iii) the full Interface Control Document (ICD) in 2024. Several technical and peer-reviewed papers were published, including one on the full Asgard instrument (Martinod et al. 2023).
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 uncorrelated 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, the MATISSE correlated flux mode was developed and is now part of ESO’s call proposal since 2022. An essential ingredient for correlated-flux observations is the availability of a calibrator source with a reliable flux. To this end, a calibrator database and Python tools for faint calibrator targets were developed.
Telescope Access
The goal of this activity was twofold: (i) build a simplified model of performances to define the best observing strategy. Eventually, this model would be embedded inside the Aspro2 software; (ii) develop a calibration and reduction software with documentation for GRAVITY. In 2023, different modes of GRAVITY+ AO (GPAO, Boudarot et al. 2025) were deployed on the LIRA bench to define the performance model and understand its limitations. In 2024, the analytical model was validated on the AO simulator and implemented in Aspro2 (link). The GRAVITY pipeline and python tools were updated (link). Several technical and peer-reviewed publications resulted from this activity (see e.g., Berdeu et al. 2024).
The goal of this activity was twofold: (i) release and curate a data reduction/calibration software package with associated documentation for Asgard/NOTT. (ii) Deliver a software tool to predict the best configuration and observing time for NOTT. The data reduction software and documentation were released in 2024 (Martinod et al. 2025, GitHub: link) and the final version of the observing tool was released in early 2025 (GitHub: link).