L’-band AGPM vector vortex coronagraph’s first light on VLT/NACO

Discovery of a late-type companion at two beamwidths from an F0V star

¹ European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Vitacura, Santiago 19, Chile
e-mail: dmawet@eso.org
² Département d’Astrophysique, Géophysique et Océanographie, Université de Liège, 17 Allée du Six Août, 4000 Liège, Belgium
³ European Southern Observatory Headquarters, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁴ Department of Engineering Sciences, Ångström Laboratory, Uppsala University, Box 534, 751 21 Uppsala, Sweden
⁵ LESIA, Observatoire de Paris, 5 pl. J. Janssen, 92195 Meudon, France
⁶ GDTech s.a., LIEGE Science Park, rue des Chasseurs Ardennais, 4031 Liège, Belgium

Received: 18 February 2013
Accepted: 1 April 2013

Abstract

Context. High contrast imaging has thoroughly combed through the limited search space accessible with first-generation ground-based adaptive optics instruments and the Hubble Space Telescope. Only a few objects were discovered, and many non-detections reported and statistically interpreted. The field is now in need of a technological breakthrough.

Aims. Our aim is to open a new search space with first-generation systems such as NACO at the Very Large Telescope, by providing ground-breaking inner working angle (IWA) capabilities in the L′ band. The L′ band is a sweet spot for high contrast coronagraphy since the planet-to-star brightness ratio is favorable, while the Strehl ratio is naturally higher.

Methods. An annular groove phase mask (AGPM) vector vortex coronagraph optimized for the L′ band made from diamond subwavelength gratings was manufactured and qualified in the lab. The AGPM enables high contrast imaging at very small IWA, potentially being the key to unexplored discovery space.

Results. Here we present the installation and successful on-sky tests of an L’-band AGPM coronagraph on NACO. Using angular differential imaging, which is well suited to the rotational symmetry of the AGPM, we demonstrated a ΔL′ > 7.5 mag contrast from an IWA ≃ 0."09 onwards, during average seeing conditions, and for total integration times of a few hundred seconds.