The near-infrared airglow continuum conundrum

Context. The airglow continuum in the near-infrared is challenging to quantify due to its faintness and the grating-scattered light from atmospheric hydroxyl (OH) emission lines. Despite its faintness, the airglow continuum sets fundamental limits for ground-based spectroscopy of faint targets and accounts for the difference between ground- and space-based observations in the interline regions between atmospheric emission lines. Aims. We aim to quantify the level of airglow continuum radiance in the visible- to near-infrared wavelength range observable with silicon photodetectors at the Observatorio del Roque de los Muchachos, in such a way that our measurement is not biased by the grating-scattered light. We aim to do this by measuring the airglow continuum radiance with minimal and controlled contamination from the broad instrumental scattering wings caused by the bright atmospheric OH lines. Methods. We measured the airglow continuum radiance using a long-slit λ/Δλ ~ 4000 spectrograph in ~100 Å-wide narrow bandpasses centered at 6720, 7700, 8700, and 10 500 Å (corresponding to the R, I, and Z broadbands) with the 2.5-meter Nordic Optical Telescope under photometric dark-sky conditions. The bandpasses were chosen to be as free as possible from atmospheric absorption and OH line emission, thereby minimizing the radiation reaching the grating surface. Results. We observe the zenith-equivalent airglow continuum to be 22.5 mag arcsec−2 at 6720 Å and 22 mag arcsec−2 at 8700 Å. We derive upper limits of 22 mag arcsec−2 at 7700 Å, due to difficulties in finding a clean part of the spectrum for measurement, and 20.8 mag arcsec−2 at 10 500 Å, due to low system sensitivity. Within measurement errors and the natural variability expected for airglow emission, our results for the Observatorio del Roque de los Muchachos are comparable to the values reported for other major observatory sites. With our medium-resolution spectra, we are unable to comment on the origin of the radiance, which could still be due to faint unresolved spectral lines or the true (pseudo)continuum. The measurement uncertainty on the zenith-scaled continuum radiance is dominated by detector effects, assumptions on atmospheric scattering, and the choice of zodiacal light model. Conclusions. We conclude that the airglow continuum radiance is not due to instrumental effects in our bandpasses and measure it to be two to four times brighter than the zodiacal light toward the ecliptic poles, the darkest foreground available for both ground- and space-based observatories. While the level is not negligible, it is dark enough to encourage further investigations into novel optical technologies and apply known stray light reduction techniques to future near-infrared and short-wave infrared spectroscopic instrumentation.