Provides the fraction of light that the RGA intercepts of the total light that exits the telescope. This is the fraction that is collected by the surface of the gratings, which is used for the dispersion.
Note that the transmission to EPIC is not the complement of this function, as in both cases also the passive volume of the RGA support structure has to be taken into account. The fraction of light transmitted to EPIC is available in CAL_getRGAObscure.