GRAVITY-RESOLVE: A comprehensive image reconstruction tool for Galactic Center observations with GRAVITY
We aim to improve and extend the imaging capabilities of the GRAVITY beam combiner instrument of ESOs VLTI with respect to reconstruction software in the Galactic Center. We revise and develop the image reconstruction code GRAVITY-RESOLVE $\text{G}^{\text{R}}$, which has been specifically developed to reconstruct stars in the Galactic Center with GRAVITY. The main objectives are to increase the limiting magnitude in GRAVITY images, reduce its computational runtime, and allow for a joint reconstruction of multiple, spatially separate datasets/pointings on sky. We apply $\text{G}^{\text{R}}$ to recent GRAVITY data, featuring the implemented GRAVITY-faint mode as part of GRAVITY+, the upgrade of GRAVITY. We reconstruct visibility phases and amplitudes and present the first GRAVITY image reconstructed with $\text{G}^{\text{R}}$ from these quantities. We demonstrate NIR mosaicking with $\text{G}^{\text{R}}$ by jointly reconstructing nine separately acquired datasets of 2025 into a single image on a scale of $\simeq 3390\ \text{AU}\times 3390\ \text{AU}$, corresponding to $409.6\ \text{mas}\times 409.6\ \text{mas}$. With dedicated injection tests of point sources into GRAVITY data, we estimate the limiting magnitude to be $m_{\text{K}}\ \simeq\ 20.0$ within a radius of $25\ \text{mas}$ around the field center when reconstructing from closure phases and visibility amplitudes. We report on the discovery of two new faint stars in the Galactic Center: S303 and S304. Finally, we find a decrease in computational runtime by a factor of at least $\sim10$. With $\text{G}^{\text{R}}$, we demonstrate GRAVITYs capabilities for NIR imaging. We show that $\text{G}^{\text{R}}$ is a fast, precise, and deep image reconstruction tool for Galactic Center observations, enabling the discovery of even more faint stars.