The SNR is computed from the detected photons of the source and background, and from the instrumental noise characteristics. Each of the components specify a transmission or related function giving the fraction of incident light transmitted as a function of some parameter or parameters. The data files may also include parameters, pointers to other tables, or defaults for the task parameters. The files are searched first in the users working directory and then using a list of directories. Thus, users may place files in their work area to override system supplied files and observatories can organize the data files in a database directory tree.
Many spectrographs provide a wide variety of wavelength regions and dispersions.
If the transmission function is given as an interpolation table this would require data files for each setup of each disperser. However, it is also possible to specify the grating and spectrograph parameters and have the task predict the grating efficiency in any particular setup. In many cases it may be easier to use the calculated efficiencies rather than measure them. Depending on the level of accuracy desired this may be adequate or deviations from the analytic blaze function can be accounted for in another component. Also there will usually be many tables for different gratings and filters.