The GRAVITY Coud\'e Infrared Adaptive Optics (CIAO) system for the VLT Interferometer

TL;DR

The paper describes the design and expected performance of the adaptive optics system for the GRAVITY instrument on the VLT Interferometer, enhancing near-infrared imaging and astrometry capabilities.

Contribution

It introduces the design of wavefront sensors and real-time control systems for adaptive optics correction in the near-infrared, tailored for challenging regions like the Galactic Centre.

Findings

Expected residual wavefront error a4400 nm at 500 Hz

Use of advanced SAPHIRA detectors for low-noise, fast readout

Enhanced adaptive optics performance in infrared wavelengths

Abstract

GRAVITY is a second generation instrument for the VLT Interferometer, designed to enhance the near-infrared astrometric and spectro-imaging capabilities of VLTI. Combining beams from four telescopes, GRAVITY will provide an astrometric precision of order 10 micro-arcseconds, imaging resolution of 4 milli-arcseconds, and low and medium resolution spectro-interferometry, pushing its performance far beyond current infrared interfero- metric capabilities. To maximise the performance of GRAVITY, adaptive optics correction will be implemented at each of the VLT Unit Telescopes to correct for the effects of atmospheric turbulence. To achieve this, the GRAVITY project includes a development programme for four new wavefront sensors (WFS) and NIR-optimized real time control system. These devices will enable closed-loop adaptive correction at the four Unit Telescopes in the range 1.4-2.4 {\mu}m. This is crucially important for an efficient adaptive optics implementation in regions where optically bright references sources are scarce, such as the Galactic Centre. We present here the design of the GRAVITY wavefront sensors and give an overview of the expected adaptive optics performance under typical observing conditions. Benefiting from newly developed SELEX/ESO SAPHIRA electron avalanche photodiode (eAPD) detectors providing fast readout with low noise in the near-infrared, the AO systems are expected to achieve residual wavefront errors of \leq400 nm at an operating frequency of 500 Hz.