5.4 Apertures
The PSA aperture will be used for most COS targets. The BOA aperture should be used only for those targets too bright to be observed with the PSA aperture. The BOA aperture degrades the spectral resolution by a factor of three or more from nominal design levels. The WCA aperture is used only for user-specified target=WAVE wavelength calibration observations. Also note that the BOA cannot be used for TIME-TAG mode observations with FLASH=YES ("TAGFLASH" mode) because the WCA is blocked when the BOA is in position.
See Section 3.2.2 for more information.
