Various components may feature in the data from RGS observations, such as:
Four calibration sources permanently illuminate the CCD of the RFC. The emission is primarily in F-K and Al-K and care has been taken such that the energies of these lines do not coincide spatially with the equivalent energies in the diffracted spectrum. The source intensity is 0.1 counts cm-2 s-1. Using both spatial and energy information, their contribution to the celestial spectra can be estimated.
This is caused by various optical straylight components. Detected optical photons will modify the gain calibration of the CCDs through an introduction of an additional energy offset. Using diagnostic mode data, these offsets can be measured.
This is expected to cause a uniform distribution of low surface brightness background across the surface of the RFC (see § 3.4.5 for assumptions on the intensity).
At any location on the RFC, the grating equation is satisfied for a wide range of energies and celestial locations, but most photons from the diffuse background will be discriminated by the spatial/spectral filtering meachanism. Any remnant signals will manifest as a uniform low surface brightness feature (seen as in the particle background above). Given an appropriate model of the celestial background spectral distribution and assuming an isotropic spatial distribution, the response of the instrument can be satisfactorily modeled. Independently, EPIC data can be used to directly measure the background.
The CCD response includes a low energy tail due to a finite number of photons producing anomalously low energy signatures. In the case of bright emission feature being measured in the second order spectrum, the intensity of this tail may become significant, as it may produce counts in the co-located first order spectrum. These effects can be modeled and can be easily identified in the PHA dispersion plane (cf. Fig. 47, e.g., at Z = 0-110 mm).
Due to X-ray scattering off the gratings (about 20% at mid band), there is an additional tail of the LSF. While true source continuum emission follows the dispersion equation, scattered light appears as an horizontal distribution in the PHA versus dispersion plot (cf. Fig. 47). This effect can be modeled and is included in SciSim (cf. e.g. Fig. 63).