The two COS detectors are photon-counting devices that convert light focused on their photosensitive front surfaces into streams of digitized photon coordinates. A discussion of the detector characteristics and performance of each detector is provided in Chapter 4 of the COS Instrument Handbook.
The FUV detector is a windowless, crossed delay-line MCP stack optimized for the 1150 to 1775 Å bandpass. The active front surface of the detector is curved to match the focal surface radius of curvature of 826 mm. To achieve the length required to capture the entire projected COS spectrum, two detector segments are placed end to end with a small gap between them. The two detector segments are independently operable; loss of one segment does not compromise the independent operation of the other. Each detector segment has an active area of 85 x 10 mm digitized to 16384 x 1024 pixels.
The NUV detector is a multi-anode microchannel array (MAMA) optimized for spectroscopic observations in the 1700-3200 Å bandpass. Target acquisitions that are not performed with dispersed light in the FUV channel are performed in the NUV channel with this detector. The COS MAMA is similar to the NUV MAMA used on STIS - the COS detector is the backup for the STIS NUV MAMA flight unit. The MAMA high-resolution (pixel sub-sampling) mode available with STIS will not be supported for COS.