Single-aperture spectro-interferometry in the visible at the Subaru telescope with FIRST: First on-sky demonstration on Keho`oea (alpha-Lyrae) and Hokulei (alpha-Aurigae

Aims. FIRST is a spectro-interferometer combining, in the visible, the techniques of aperture masking and spatial filtering thanks to single-mode fibers. By turning a monolithic telescope into an interferometer, this instrument aims to deliver high contrast capabilities at spatial resolutions that are inaccessible to classical coronagraphic instruments. Methods. The technique implemented in the FIRST instrument is called pupil remapping: the telescope pupil is divided into subpupils by a segmented deformable mirror conjugated to a micro-lens array injecting light into single-mode fibers. The fiber outputs are rearranged in a nonredundant configuration, allowing simultaneous measurement of all baseline fringe patterns. The fringes are also spectrally dispersed, increasing the coherence length and providing precious spectral information. The optical setup of the instrument has been adapted to fit onto the SCExAO platform at the Subaru Telescope. Results. We present the first on-sky demonstration of the FIRST instrument at the Subaru telescope. We used eight subapertures of the 8.2-meter diameter pupil, each with a diameter of about 1 m. Closure phase measurements were extracted from the interference pattern to provide spatial information on the target. We tested the instrument on two types of targets : a point source (Keho’oea - α Lyrae, m R = 0.1) and a binary system (Hokulei − α Aurigae, m R = −0.52, and a semi-major axis = 56.4 mas). An average accuracy of 0.6° is achieved on the closure phase measurements of Keho‘oea, with a statistical error of about 0.15° at best. We estimate that the instrument can be sensitive to structures down to a quarter of the telescope spatial resolution. We measured the relative positions of Hokulei Aa and Ab with an accuracy ≲1 mas. Conclusions. FIRST opens new observing capabilities in the visible wavelength range at the Subaru Telescope. With SCExAO being a testing platform for high contrast imaging instrumentation for future 30-meter class telescopes, the successful demonstration and exploitation of FIRST is an important stepping stone for future interferometric instrumentation on extremely large telescopes.