 This is done because FUV reflection and transmission efficiencies are typically quite low compared to what is common at visible wavelengths.In accomplishing this, the COS FUV channel uses a single (selectable) optic to diffract the light from HST, correct for the Hubble spherical aberration, focus the diffracted light onto the FUV detector and correct for astigmatism typical of this sort of instrument.Since aberration correction is performed after the light passes into the instrument, the entrance to the spectrograph must be an extended aperture, rather than the traditional narrow entrance slit, in order to allow the entire aberrated HST image from a point source to enter the instrument.