Airglow and the Subaru Night Sky Spectrograph (SuNSS)

The terrestrial atmosphere glows brightly throughout the night (i-band continuum 20-22 mag/arcsec²).However, most of the emission, commonly known as airglow, does not end up as continuum, but through a highly variable forest of strong lines.This forest of lines comes from molecular emission, especially OH and O₂, and a few atomic lines.Understanding the temporal and spatial variations of this emission represents one of the major areas of concern for upcoming deep spectrographic missions such as the Prime Focus Spectrograph (PFS), since PFS targets are often many magnitudes below the intensity levels set by the airglow.Due to its importance, the PFS collaboration has constructed a small auxiliary telescope dedicated to observing the variable airglow over Maunakea, the Subaru Night Sky Spectrograph (SuNSS), which is introduced here.SuNSS has the same field of view (∼ 1.26deg²), wavelength range (380−1260 nm), resolution (R ∼ 3000), and focal ratio as the Prime Focus Instrument (PFI), the main PFS instrument.The combination of broad wavelength coverage and a reasonably large FOV allows for investigating the spatial and temporal variations in a wide range of airglow components, as well as their correlations.Variations are observed in both the overall intensity of each emitting species as well as in other properties of the emission, such as the temperature of the molecular emission, which sets the relative strength of the lines.Clear trends have been identified and will be presented.However, the airglow is not just a source of foreground, it is also useful, since it provides a ubiquitous reference set of thousands of lines which can be used for wavelength calibration.We show how explicitly considering the variability patterns observed with SuNSS can improve airglow-based wavelength calibration, and present a new code which can produce robust wavelength calibration targets (linelists) at arbitrary spectral resolution.