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 1.6m than at 0.4m. 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 m 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.1 page(s