A method to remove residual OH emission from near-infrared spectra

I present a technique to remove the residual OH airglow emission from near-infrared spectra. Historically, the need to subtract out the strong and variable OH airglow emission lines from 1 to 2.5-mu m spectra has imposed severe restrictions on observational strategy. For integral-field spectroscopy, where the field of view is limited, the standard technique is to observe blank sky frames at regular intervals. However, even this does not usually provide sufficient compensation if individual exposure times are longer than 2-3 min due to (1) changes in the absolute flux of the OH lines, (2) variations in flux among the individual OH lines and (3) effects of instrumental flexure which can lead to 'P-Cygni' type residuals. The data processing method presented here takes all of these effects into account and serendipitously also improves background subtraction between the OH lines. It allows one, in principle, to use sky frames taken hours or days previously so that observations can be performed in a quasi-stare mode. As a result, the observing efficiency (i.e. fraction of time spent on a source) at the telescope can be dramatically increased.