 The BOA introduces a neutral density filter to the optical path that attenuates the light by approximately a factor of one hundred (five astronomical magnitudes).Both apertures are oversized (2.5 arc second clear aperture) permitting more than 95% of the light from a point source to enter the spectrograph.After passing through the PSA or BOA the light travels to one of the optics on the first of two optic select wheels, either one of the three FUV diffraction gratings or the first of the NUV collimation mirrors (table 1), depending on whether an FUV, NUV, or target acquisition channel is selected.