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

We analyse the near-infrared interline sky background, OH and O2 emission in 19 hours of H band observations with the GNOSIS OH suppression unit and the IRIS2 spectrograph at the 3.9-metre Anglo-Australian Telescope. The observations cover a range of sky positions with varying lunar conditions throughout the night. We find that the temporal behaviour of OH emission is best described by a gradual decrease during the first half of the night followed by a gradual increase during the second half of the night following the behaviour of the solar elevation angle. We measure the interline background at 1.520µm where the instrumental thermal background is very low and exploit its variation with zenith distance, time after sunset, lunar conditions, and ecliptic latitude to determine the presence of non-thermal atmospheric emission, zodiacal scattered light and scattered moonlight. Zodiacal scattered light is too faint to be detected in the summed observations. Our data are consistent with a contribution from moonlight that is �30 times greater than expected from a model based on extrapolated V band measurements. Demanding near-infrared observations during low lunar phase angles (|α| & 30deg) should be made at lunar distances ρ & 80deg if the background contribution is not to exceed 100photonss 1 m 2 µm 1 arcsec 2 . Although moonlight may dominate the interline background at very small lunar distances, the background at a resolving power of R � 2400 when using OH suppression fibres is otherwise dominated by light from an atmospheric source that fades gradually throughout the night. As such, it bears a resemblance to atmospheric OH emission suggesting that the interline background contains OH, which likely comes from unsuppressed lines which are scattered into the interline regions by the diffraction grating of the spectrograph. However, the interline background linearly correlates with both OH and O2 emission, which have different temporal behaviours suggesting it contains contributions from multiple molecular species. The absolute interline background is 560 ± 130photonss 1 m 2 µm 1 arcsec 2 at 1.520µm under dark conditions. This value is similar to previous measurements without OH suppression suggesting that the suppression with the current grating design does not affect the interline background, likely as a result of poor design due to inaccurate skyline models. Future OH suppression fibre designs will address this issue using more accurate skyline measurements taken from high resolution spectra.