COS Background Performance

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 Å^-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. 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. The expected on-orbit background level is ~34 cnt s^-1 cm^-2 (~2.1x10^-4 cnt s^-1 pix^-1), which is about a factor of 4 lower than the background level found for the STIS flight MAMA (Ball SER COS-NUV-001). Low energy thermal ions are excluded from the COS detectors through the use of ion repeller grids. Scattered light is not expected to be a significant source of background on either COS detector since the COS first-order diffraction grating rulings have very low scatter (<2x10^-5 / Å). Full characterization of the intrinsic pre-flight backgrounds and on-orbit backgrounds for both COS detectors will be performed to assess the impact of the detector and scattered light backgrounds on observations of faint objects.

Detector Background Count Rates (per second)

Detector:	FUV XDL	NUV MAMA
Dark rate (counts sec-1)	0.5 per cm2 7.2 × 10-7 per pixel 4.3 × 10-5 per resel	60 per cm2 3.7 × 10-4 per pixel 3.3 × 10-3 per resel
Read noise	0	0

Sky Background
The sources of sky background which will affect COS observations include:

• Earthshine,
• Zodiacal light, and
• Geocoronal emission.

For more information on COS background performance see section 10.3 of the COS Instrument Handbook.