 FUV resolution is slightly lower than pre-launch predictions due to mid-frequency polishing errors on the HST primary mirror, while NUV resolution exceeds pre-launch values in all modes.Thanks to the minimal number of reflections, the G140L mode, and G130M central wavelength settings added after 2010, can observe light at wavelengths down to ~90 nm, and shorter, despite the very low reflectivity of the MgF2 coated optics at these wavelengths.The Cosmic Origins Spectrograph is designed to enable the observation of faint, point-like UV targets at moderate spectral resolution, allowing COS to observe hot stars (OB stars, white dwarfs, cataclysmic variables and binary stars) in the Milky Way and to observe the absorption features in the spectra of active galactic nuclei.