A Primer on SOXS Observation Modes

The glow of thermal emission from the earth’s atmosphere, and even the telescope itself, start to dominate photon counts at NIR wavelengths, seriously contaminating signal from target sources. This thermal emission glow is typically dubbed ‘sky background’ (despite the fact it originates in the foreground of the target source).

The sole reason SOXS has multiple observation modes is to provide astronomers with flexibility and a choice of methods they can employ to attempt to isolate and remove the noise generated by the sky-background from their science frames.

The 3 main observation modes are (click links for more detail):

1. Stare mode where the on-source sky-background is modelled, fitted and removed with software. Although removal of the background is not as accurate as the other observation modes, the time lost in overheads is lower.

2. Nodding mode employs an observational technique to measure the sky-background of an empty patch of sky near in time and spatially close to the source. This spectral measurement of the sky-background flux is simply removed from the on-source spectrum. Although generating more overhead time than stare-mode, unlike offset-mode, this technique allows for source and sky to be observed on the same frames.

3. Offset mode employs the same basic ‘observe and subtract’ method to remove the sky-background as nod mode, but is typically favoured for extended sources where it is not possible to observe a blank patch of sky and the source within the same 11 arcsec slit. As sky and source are observed in separate frames this observation mode has the highest fraction of time lost to overheads.