For faint sources, backgrounds can be a significant source of counts detected by COS and must be considered when making exposure time estimates if flux levels are less than ~10-15 erg cm-2 s-1 е-1. The background events arise mainly from radioactive decays in the materials used to construct the microchannel plates and associated hardware. For the windowless FUV MCP detector, the on-orbit background rate is expected to be similar to the rate of ~0.5-0.8 cnt s-1 cm-2 (~7.5x10-7 cnt s-1 pix-1) measured for the FUSE MCP detectors. On-orbit measurements confirm these estimates (see table below). However, like the STIS NUV MAMA, the COS NUV MAMA suffers significantly increased background levels due to fluorescence caused by impurities in its MgF2 window.
Initial on-orbit data from SMOV indicated a much lower than expected dark rate (~75 counts/sec) in the NUV MAMA. However, this level has steadily increased. The reason for this increase is not yet understood.
Sky Background
The sources of sky background which will affect COS observations include: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 Web site