COS is optimized for observing faint UV point sources. The Primary Science Aperture (PSA) is a 2.5'' field stop located on the HST focal surface near the point of maximum encircled energy. This aperture transmits close to 97% of the light from a well-centered, aberrated stellar image delivered by the HST Optical Telescope Assembly (OTA). The PSA is expected to be used for most COS observations. An attenuating Bright-Object Aperture (BOA), also with a diameter of 2.5'', contains a neutral density (ND2) filter that permits COS to observe targets several magnitudes brighter than the Bright Object Protection limits allow through the PSA.

Because COS is a slitless spectrograph, the spectral resolution depends on the nature of the target. The high-dispersion gratings deliver resolutions R ≥ 20,000 for unresolved sources (intrinsic diameter ≤ 0.1''). However, for an extended source, for example, ~ 0.5'' in diameter, the spectral resolution is degraded to R ≈ 5000. Though not optimized for extended objects, COS can be used to detect faint, diffuse sources with degraded spectral resolution.