Beam characterization and guiding for optical/infrared interferometry

The GRAVITY instrument has been built with the goal of studying the Milky Way Galactic Center super-massive black hole. It is a powerful phase-referenced and closure phase interferometric imaging (4 milli-arcseconds) and high precise narrow-angle astrometric instrument (10 microarcseconds) and has been installed at the Very Large Telescope Interferometer (VLTI) of the ESO. There are several practical difficulties in implementing narrow-angle astrometry. The atmospheric turbulence and the error in measuring accurate baseline lengths between any two telescopes are among the major challenges. Due to these, the performance of GRAVITY can be severely degraded in two ways: a) instrument sensitivity – the efficient star’s light injection in the single mode fibers is affected in the presence of random atmospheric tip-tilts; b) astrometric errors – caused due to unwanted tip-tilts and baseline errors. As one of the subsystems required to overcome these challenges, the GRAVITY acquisition camera was built. The acquisition camera was the responsibility of the Portuguese CENTRA-SIM group where my thesis took place. In this manuscript the acquisition camera is presented from the design phase to the commissioning, focusing on the project work packages that were lead by the author or had a major contribution by him. First, the design implementing four optical functions to enable the required beam stabilization is presented : a) pupil tracking–by imaging (with a 2×2 lenslet) four pupil reference lasers which are mounted on the spiders of each telescope secondary mirror; b) field tracking – by imaging astrophysical targets; c) aberration sensing – by imaging 9×9 lenslet Shack-Hartmanns; d) pupil monitoring by imaging telescope pupil. Second, the end-to-end simulations of the acquisition camera are performed to verify the validity and the accuracy of the acquisition camera concepts. Dedicated data reduction algorithms are developed to track the beam distortions: a) pupil tracking – lateral and longitudinal pupil positions are measured by analyzing the 2×2 lenslet images; b) field tracker: the image tip-tilts are tracked from the astrophysical targets; d) aberration tracker: quasi-static wavefront aberrations are measured using 9×9 lenselet Shack-Hartmann images. It is demonstrated that the algorithms are robust and have the accuracy to measure the beam parameters within design specifications. Third, using these algorithms, the acquisition camera data reduction pipeline is developed using the ESO standard software framework. The measured parameters (with sampling of 0.7 s) are used in the : a) alignment of GRAVITY with the VLTI; b) active pupil and field stabilization; c) focus correction for the auxiliary telescopes; d) non-common path error correction between the GRAVITY-Coudé infrared adaptive optics system and GRAVITY four unit telescopes. Finally, after testing the optical components which are procured, the camera is optically aligned, assembled, tested and integrated within GRAVITY. GRAVITY was shipped to Paranal Observatory, Chile and it had its first light at the end of 2015. The characterization results confirm that by stabilizing the telescope beams, the astrometric error induced by the field and the pupil errors is minimized to less than few micro-arcseconds. Currently, the observations of the black hole environment are ongoing.