The primary background sources that affect COS observations are detector dark count, earthshine, zodiacal light, and airglow emission; neither of the COS detectors suffers from read noise. For more information on COS background performance see Section 7.4 of the COS Instrument Handbook. For information on high S/N observations of the geocoronal Lyman-alpha line, please see the COS Airglow page.
Detector Dark Count Earthshine Zodiacal Light Airglow Emission
COS is considerably more sensitive than STIS and earlier-generation HST instruments at comparable spectral resolutions, particularly in the far ultraviolet. At FUV wavelengths, the peak effective area is ~3000 cm2 near 1300 Å with the G130M grating. At NUV wavelengths, the peak effective area is expected to reach ~850 cm2 near 2250 Å with the G230L grating.
For further information, see Section 5.1.1 of the COS Instrument Handbook.
FUV High Voltage History
The COS FUV detector is a high-gain, cross-delay line microchannel plate (MCP) detector. In the following link, we present the high voltage history of each segment.