GNOSIS: the first instrument to use fibre Bragg gratings for OH suppression

TL;DR

GNOSIS is a pioneering instrument using fibre Bragg gratings for atmospheric OH suppression, demonstrating high suppression in laboratory and on-sky tests, but not yet improving interline background or sensitivity.

Contribution

First implementation of fibre Bragg gratings for OH suppression in an astrophotonic instrument, showcasing design, laboratory, and on-sky performance results.

Findings

High throughput and effective skyline suppression in tests.

No significant reduction in interline background observed.

Sensitivity comparable to existing spectrograph without OH suppression.

Abstract

GNOSIS is a prototype astrophotonic instrument that utilizes OH suppression fibres consisting of fibre Bragg gratings and photonic lanterns to suppress the 103 brightest atmospheric emission doublets between 1.47-1.7 microns. GNOSIS was commissioned at the 3.9-meter Anglo-Australian Telescope with the IRIS2 spectrograph to demonstrate the potential of OH suppression fibres, but may be potentially used with any telescope and spectrograph combination. Unlike previous atmospheric suppression techniques GNOSIS suppresses the lines before dispersion and in a manner that depends purely on wavelength. We present the instrument design and report the results of laboratory and on-sky tests from commissioning. While these tests demonstrated high throughput and excellent suppression of the skylines by the OH suppression fibres, surprisingly GNOSIS produced no significant reduction in the interline background and the sensitivity of GNOSIS and IRIS2 is about the same as IRIS2. It is unclear whether the lack of reduction in the interline background is due to physical sources or systematic errors as the observations are detector noise-dominated. OH suppression fibres could potentially impact ground-based astronomy at the level of adaptive optics or greater. However, until a clear reduction in the interline background and the corresponding increasing in sensitivity is demonstrated optimized OH suppression fibres paired with a fibre-fed spectrograph will at least provide a real benefits at low resolving powers.