GNOSIS: An OH suppressing near-infrared spectrograph for astronomy using fibre Bragg gratings

Astronomical near-infrared spectroscopy is made difficult by the extremely bright and variable night sky background. The night sky surface brightness is more than a thousand times brighter at 1.6μm than at 0.4μm. Furthermore the brightness of the sky changes by factors of -10% on time-scales of minutes. Background subtraction is therefore frustrated by high Poisson noise from the extreme brightness, and by systematic noise from the variability. Between 1.0 and 1.8 μm almost all of this background results from the rotational and vibrational de-excitiation of hydroxyl molecules located at ~90 km in the atmosphere. The hydroxyl emission lines are intrinsically very bright, but very narrow. Between the OH lines the sky should be very dark; it is expected that the interline continuum is dominated by the zodiacal scattered light. Therefore selectively filtering the OH lines would enable deep near-infrared observations.GNOSIS is an instrument that uses fibre Bragg gratings to filter the light from hundreds of night-sky OH emission lines. These filters are placed before the dispersive optics, significantly reducing the background due to light scattered by the spectrograph. System models predict that the average level of suppression of the atmospheric background within the target bandwidth will be a factor of greater than 20.