Spectroscopy using a visible photonic lantern at the Subaru Telescope: Laboratory characterization and the first on-sky demonstration on Ikiiki (α Leo) and ‘Aua (α Ori)

¹ National Astronomical Observatory of Japan, Subaru Telescope, 650 North Aohoku Place, Hilo, HI 96720, USA
² Astrobiology Center, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan
³ LESIA, Observatoire de Paris, Universite PSL, CNRS, Sorbonne Universite, Sorbonne Paris Cite, 5 place Jules Janssen, 92195 Meudon, France
⁴ Sydney Astrophotonic Instrumentation Laboratory, School of Physics, The University of Sydney, Sydney, NSW 2006, Australia
⁵ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
⁶ College of Optical Sciences, University of Arizona, Tucson, AZ 85721, USA
⁷ California Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, USA
⁸ Korea Astronomy and Space Science Institute (KASI), Daejeon 34055, Republic of Korea
⁹ University of California, Irvine, G302 C Student Center, Irvine, CA 92697, USA
¹⁰ University of California, Los Angeles, 405 Hilgard Avenue, CA 90095, USA
¹¹ Sydney Institute for Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia
¹² AAO-USyd, School of Physics, University of Sydney, Sydney 2006, Australia
¹³ The College of Optics and Photonics, University of Central Florida, 4304 Scorpius St, Orlando, FL 32816, USA
¹⁴ Institute for Astronomy, University of Hawaii, 640 N. Aohoku Pl, Hilo, HI 96720, USA
¹⁵ The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹⁶ NAOJ, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁷ Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX 78006, USA
¹⁸ Univ. Grenoble Alpes, CNRS, IPAG, 414 Rue de la Piscine, 38400 Saint-Martin-d’Hères, France

^★ Corresponding author; vievard@naoj.org

Received: 4 April 2024
Accepted: 11 September 2024

Abstract

Context. Photonic lanterns (PLs) are waveguide devices enabling high-throughput single-mode spectroscopy and high angular resolution.

Aims. We aim to present the first on-sky demonstration of a PL operating in visible light, to measure its throughput and assess its potential for high-resolution spectroscopy of compact objects.

Methods. We used the SCExAO instrument (a double-stage extreme adaptive optics system installed at the Subaru Telescope) and FIRST mid-resolution spectrograph (R 3000) to test the visible capabilities of the PL on internal source and on-sky observations.

Results. The best averaged coupling efficiency over the PL field of view was measured at 51% ± 10%, with a peak at 80%. We also investigated the relationship between coupling efficiency and the Strehl ratio for a PL, comparing them with those of a single-mode fiber (SMF). Findings show that in the adaptive optics regime a PL offers a better coupling efficiency performance than an SMF, especially in the presence of low-spatial-frequency aberrations. We observed Ikiiki (α Leo – m_R = 1.37) and ‘Aua (α Ori – m_R = −1.17) at a frame rate of 200 Hz. Under median seeing conditions (about 1 arcsec measured in the H band) and large tip or tilt residuals (over 20 mas), we estimated an average light coupling efficiency of 14.5% ± 7.4%, with a maximum of 42.8% at 680 nm. We were able to reconstruct both star’s spectra, containing various absorption lines.

Conclusions. The successful demonstration of this device opens new possibilities in terms of high-throughput single-mode fiber-fed spectroscopy in the visible. The demonstrated on-sky coupling efficiency performance would not have been achievable with a single SMF injection setup under similar conditions, partly because the residual tip or tilt alone exceeded the field of view of a visible SMF (18 mas at 700 nm). This emphasizes the enhanced resilience of PL technology to such atmospheric disturbances. The additional capabilities in high angular resolution are also promising but still have to be demonstrated in a forthcoming investigation.