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We analyse the near-infrared interline sky background, OH and O2 emission in19 hours of H band observations with the GNOSIS OH suppression unit and theIRIS2 spectrograph at the 3.9-m AAT. We find that the temporal behaviour of OHemission is best described by a gradual decrease during the first half of thenight followed by a gradual increase during the second half of the nightfollowing the behaviour of the solar elevation angle. We measure the interlinebackground at 1.520 microns where the instrumental thermal background is verylow and study its variation with zenith distance, time after sunset, eclipticlatitude, lunar zenith angle and lunar distance to determine the presence ofnon-thermal atmospheric emission, zodiacal scattered light and scatteredmoonlight. Zodiacal scattered light is too faint to be detected in the summedobservations. Scattered moonlight due to Mie scattering by atmospheric aerosolsis seen at small lunar distances (< 11 deg), but is otherwise too faint todetect. Except at very small lunar distances the interline background at aresolving power of R~2400 when using OH suppression fibres is dominated by anon-thermal atmospheric source with a temporal behaviour that resemblesatmospheric OH emission suggesting that the interline background containsinstrumentally-scattered OH. However, the interline background dims morerapidly than OH early in the night suggesting contributions from rapid dimmingmolecules. The absolute interline background is 560 +/- 120 photons s^-1 m^-2micron^-1 arcsec^-2 under dark conditions. This value is similar to previousmeasurements without OH suppression suggesting that non-suppressed atmosphericemission is responsible for the interline background. Future OH suppressionfibre designs may address this by the suppression of more sky lines using moreaccurate sky line measurements taken from high resolution spectra.

The nature of the near-infrared interline sky background using fibre Bragg grating OH suppression