First Light for GRAVITY Wide: Large Separation Fringe Tracking for the Very Large Telescope Interferometer (2206.00684v2)

Abstract: GRAVITY+ is the upgrade of GRAVITY and the Very Large Telescope Interferometer (VLTI) with wide-separation fringe tracking, new adaptive optics, and laser guide stars on all four 8~m Unit Telescopes (UTs), for ever fainter, all-sky, high contrast, milliarcsecond interferometry. Here we present the design and first results of the first phase of GRAVITY+, called GRAVITY Wide. GRAVITY Wide combines the dual-beam capabilities of the VLTI and the GRAVITY instrument to increase the maximum separation between the science target and the reference star from 2 arcseconds with the 8 m UTs up to several 10 arcseconds, limited only by the Earth's turbulent atmosphere. This increases the sky-coverage of GRAVITY by two orders of magnitude, opening up milliarcsecond resolution observations of faint objects, and in particular the extragalactic sky. The first observations in 2019 - 2022 include first infrared interferometry of two redshift $z\sim2$ quasars, interferometric imaging on the binary system HD 105913A, and repeated observations of multiple star systems in the Orion Trapezium Cluster. We find the coherence loss between the science object and fringe-tracking reference star well described by the turbulence of the Earth's atmosphere. We confirm that the larger apertures of the UTs result in higher visibilities for a given separation due to larger overlap of the projected pupils on sky and give predictions for visibility loss as a function of separation to be used for future planning.

• The paper presents the first-phase results of GRAVITY Wide, significantly expanding the VLTI’s ability to track fringes over wider separations.
• It employs dual-beam interferometry to extend fringe tracking from 2 arcseconds to several arcseconds, effectively mitigating atmospheric turbulence impacts.
• Observations of quasars, binary systems, and star clusters validate the upgrade and open new avenues for high-resolution astronomical imaging.

Analysis of the GRAVITY Wide Project for VLTI

The paper "First Light for GRAVITY Wide: Large Separation Fringe Tracking for the Very Large Telescope Interferometer" presents the first phase results of the GRAVITY+ upgrade implemented at the Very Large Telescope Interferometer (VLTI). This initiative, conducted by the GRAVITY+ Collaboration, outlines the enhancements on the GRAVITY instrument aimed at expanding its observational capabilities. Notably, the GRAVITY Wide mode leverages dual-beam capabilities to achieve wider fringe tracking separations, thus broadening the interferometer's sky coverage significantly, up to two orders of magnitude.

Key Developments and Outcomes

1. Instrumental Advancements: The GRAVITY Wide mode extends the separation capability between the science target and reference star from 2 arcseconds to several arcseconds, mainly constrained by atmospheric turbulence rather than by instrumentation. This extension is facilitated by overlaying dual beams from VLTI's Unit Telescopes (UTs), enhancing the system's capabilities in capturing high-contrast, milliarcsecond (mas) images across larger separations.
2. Observational Achievements: The initial results from the GRAVITY Wide include successful infrared interferometry of quasars at redshift $z \sim 2$, imaging of binary systems, and observations of multiple star systems in the Orion Trapezium Cluster. The larger baselines of UTs contribute to improving resolution and signal quality.
3. Atmospheric Considerations and Anisoplanatism: The paper confirms the coherence loss between science objects and fringe-tracking reference stars correlating well with Earth's atmospheric turbulence, described through anisoplanatism models. Understanding these dynamics is crucial for planning observations that demand high precision and expanded separations.
4. Empirical Validation and Future Potentials: The GRAVITY Wide's efficacy is validated through observed data sets matching the predictions by atmospheric models. The acquisition of data at greater separations than allowed by previous setups opens new vistas in observing faint extragalactic objects and phenomena previously restricted by narrow sky coverage.

Practical and Theoretical Implications

From a practical standpoint, the enhancements introduced by GRAVITY Wide make VLTI a more versatile and sensitive astronomical tool, notably facilitating the paper of distant quasars, binary star systems, and events near supermassive black holes. These advancements also enable astrophysical inquiries that rely on high-resolution imagery and precise astrometric measurements, thus broadening the scope of potential scientific discoveries. Theoretically, the project pushes forward our understanding of observational limitations imposed by atmospheric conditions, contributing to the refinement of computational models that predict visibility loss.

Speculative Future Directions

Looking ahead, the success of the GRAVITY Wide is a stepping stone towards full GRAVITY+ capabilities, which will further refine adaptive optics, potentially employ laser guide stars, and enhance vibration control mechanisms. These developments promise to enable universal sky coverage, define new limits on observable magnitudes, and expand the temporal resolution of dynamic cosmic phenomena. The upgrades are expected to allow fringe tracking of stars as faint as $m_K = 13$ and science observations with a magnitude up to $m_K = 22$.

In conclusion, the GRAVITY Wide project showcases a series of methodical enhancements to the VLTI infrastructure, presenting a transformation in the interferometer's ability to explore the universe. These advances not only improve current observational competencies but also open avenues for future research addressing some of the fundamental questions in astronomy and cosmology.