GRAVITY Coudé Infrared Adaptive Optics (CIAO) system for the VLT Interferometer
2012
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 interferometric
capabilities. To maximise the performance of GRAVITY, adaptive optics correction will be implemented
at each of the VLT Unit Telescopes to correct for the e_ects 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 μm. This is crucially important for an e_cient 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. Bene_ting 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 400 nm at an operating frequency of 500 Hz.≤
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