Chapter 3: Description and Performance of the COS Optics 3.1 The Optical Design of COS 3.1.1 External Shutter 3.1.2 The Apertures and Aperture Mechanism 3.1.3 Gratings and Mirrors: The Optics Select Mechanisms 3.2 Size of a Resolution Element 3.3 The COS Line-Spread Function 3.3.1 Non-Gaussianity of the COS LSF 3.3.2 Quantifying the Resolution 3.3.3 Impact on Equivalent Width Measurements 3.3.4 Extended Wings of the COS LSF 3.3.5 Enclosed Energy of COS LSF