 The 2.5 arc second diameter entrance aperture allows ≈ 95% of the light from compact sources to enter COS, yielding high sensitivity at the design resolution for compact sources.Post launch performance closely matched expectations.Instrument sensitivity is close to pre-launch calibration values, and detector background is exceptionally low (0.16 counts per resolution element per 1000 seconds for the FUV detector, and 1.7 counts per resolution element per 100 seconds for the NUV detector).