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