The COS Instrument Definition Team (Principal Investigator, James Green,
University of Colorado) was responsible for the ground testing and ground calibration of COS. Most of the ground test data was obtained in 2003 and 2006 during thermal vacuum testing at Ball Aerospace and Goddard Space Flight Center, respectively. These tests characterized the basic properties of the optics, the detectors, and the mechanisms. While some measurements (e.g., FUV full-detector flat-field images) cannot be repeated in orbit, most of the ground-test data have been superseded by on-orbit measurements obtained during SMOV4.