Methods for multiple telescope beam imaging and guiding in the near infrared

Abstract

Atmospheric turbulence and precise measurement of the astrometric baseline vector between any two telescopes are two major challenges in implementing phase referenced interferometric astrometry and imaging. They limit the performance of a fibre-fed interferometer by degrading the instrument sensitivity and astrometric measurements precision and by introducing image reconstruction errors due to inaccurate phases. A multiple beam acquisition and guiding camera was built to meet these challenges for a recently commissioned four beam combiner instrument, GRAVITY, at the ESO Very Large Telescope Interferometer. For each telescope beam it measures: a) field tip-tilts by imaging stars in the sky; b) telescope pupil shifts by imaging pupil reference laser beacons installed on each telescope using a $2\times 2$ lenslet; c) higher order aberrations using a $9\times 9$ Shack-Hartmann. The telescope pupils are imaged for a visual monitoring while observing. These measurements enable active field and pupil guiding by actuating a train of tip-tilt mirrors placed in the pupil and field planes, respectively. The Shack-Hartmann measured quasi-static aberrations are used to focus the Auxiliary Telescopes and allow the possibility of correcting the non-common path errors between the Unit Telescopes adaptive optics systems and GRAVITY. The guiding stabilizes light injection into single-mode fibres, increasing sensitivity and reducing the astrometric and image reconstruction errors. The beam guiding enables to achieve astrometric error less than $50\mu$as. Here, we report on the data reduction methods and laboratory tests of the multiple beam acquisition and guiding camera and its performance on-sky.

12 pages, 20 figures and 7 tables. Accepted for publication in MNRAS

References in corpus (10)

• First Light for GRAVITY: Phase Referencing Optical Interferometry for the Very Large Telescope Interferometer
• Resolved astrometric orbits of ten O-type binaries
• Reaching micro-arcsecond astrometry with long baseline optical interferometry; application to the GRAVITY instrument
• A high-mass protobinary system with spatially resolved circumstellar accretion disks and circumbinary disk
• The fiber coupler and beam stabilization system of the GRAVITY interferometer
• Accretion-ejection morphology of the microquasar SS433 resolved at sub-au scale
• NGC 1068: No change in the mid-IR torus structure despite X-ray variability
• GRAVITY: the Calibration Unit
• GRAVITY: beam stabilization and light injection subsystems
• The metrology system of the VLTI instrument GRAVITY

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• Betelgeuse scope: Single-mode-fibers-assisted optical interferometer design for dedicated stellar activity monitoring