FIRST, a fibered aperture masking instrument

Abstract

Aims. In this paper we present the first on-sky results with the fibered aperture masking instrument FIRST. Its principle relies on the combination of spatial filtering and aperture masking using single-mode fibers, a novel technique that is aimed at high dynamic range imaging with high angular resolution. Methods. The prototype has been tested with the Shane 3-m telescope at Lick Observatory. The entrance pupil is divided into subpupils feeding single-mode fibers. The flux injection into the fibers is optimized by a segmented mirror. The beams are spectrally dispersed and recombined in a non-redundant exit configuration in order to retrieve all contrasts and phases independently. Results. The instrument works at visible wavelengths between 600 nm and 760 nm and currently uses nine of the 30 43 cm subapertures constituting the full pupil. First fringes were obtained on Vega and Deneb. Stable closure phases were measured with standard deviations on the order of 1 degree. Closure phase precision can be further improved by addressing some of the remaining sources of systematic errors. While the number of fibers used in the experiment was too small to reliably estimate visibility amplitudes, we have measured closure amplitudes with a precision of 10% in the best case. Conclusions. These first promising results obtained under real observing conditions validate the concept of the fibered aperture masking instrument and open the way for a new type of ground-based instrument working in the visible. The next steps of the development will be to improve the stability and the sensitivity of the instrument in order to achieve more accurate closure phase and visibility measurements, and to increase the number of sub-pupils to reach full pupil coverage.

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“…Additionally, interferometric non-redundant spare aperture masking (SAM) modules such as VAMPIRES and FIRST (Huby et al 2012;Norris et al 2015) operating in the visible can reach down to 0.5λ/D (10 mas at 700 nm), albeit at more moderate contrast (∼10 −3 ). Key design choices have been made, which prioritize imaging at small angular separation from day one, for example by using a Pyramid Wavefront Sensor (PyWFS) to improve AO correction close to the star, and by implementing a coronagraphic Lyot-plane low-order wavefront sensor (LLOWFS) (Singh et al 2015).…”

FIRST, a fibered aperture masking instrument

“…As in previous observations (Huby et al 2012), the Shane adaptive optics system provided sufficient correction to stabilize the fringes, although it is optimized for the infrared wavelengths.…”

“…After the first light of the instrument obtained in 2010 (Huby et al 2012), efforts have been focused on improving the stability, sensitivity, and dynamic range achievable with FIRST. The injection part of the instrument (including the pupil imager, the Iris AO segmented mirror used to steer the beams precisely on the fiber cores that are gathered in the fiber bundle, from Fiberguide Industries) is basically unchanged; however, significant upgrades have been carried out in the recombination part of the instrument, as can be seen in Fig.…”

“…The acquisition procedure is similar to the description given in Huby et al (2012). One data set comprises fringe sequences, fiber calibration files (every fiber diffraction pattern is recorded individually), and background files.…”

“…The first light of the instrument was achieved in July 2010 at Lick Observatory with FIRST-9 mounted on the Cassegrain focus of the 3 m Shane telescope (Huby et al 2012). The instrument has since been significantly improved to increase the number of subpupils and the accuracy on the closure phase measurements, hence the achievable dynamic range.…”