The Relative Spectral Response Function (RSRF) is the total throughput of the system. It arises from the convolution of the detector response as a function of wavelength, the filter response as a function of wavelength and takes such effects as geometrical considerations (e.g. slit width, source structure) into account.
Fringes are a modulation with wavelength of the light falling on the detectors. The fringes arise due to constructive/destructive interference in either in the detector material itself (between the front and back of the detector) or similarly in the filter material. To some extent all detector bands suffer from fringes. Bands 1, 2 & 4 are only lightly affected, whereas the fringes are more pronounced in band 3. An example of fringes in the band 3D spectrum of a star can be seen in figure 6.1.
Figure 6.1: Example of fringe effects in band 3
An attempt is made to remove fringes during the OLP processing pipeline. However, because the fringes may shift in wavelength space depending on where the source is in the slit (especially in the dispersion direction) and whether the objects being observed is a point source or extended, fringe removal may not be perfect. This may cause problems for observations of faint sources. Any faint features should always be checked against the relevant Relative Spectral Responsivity File, SWS Cal-G 25_x , to ensure they are not artifacts.