COS NUV Detector Dark Rate

Ground measurements of the faceplate material used in the COS NUV MAMA led to the prediction of an on-orbit dark rate about 25% of that seen by the STIS MAMAs after SM2, or 225-250 counts/sec over the full detector (~2.2e-4 counts/pixel/sec). The first on-orbit measurements of the dark rate in SMOV were much lower than this - on the order of 75 counts/sec. Since these initial measurements, however, the level has risen steadily, and as of mid-January 2010 had reached about 300 counts/sec, or 2.9e-4 counts/pixel/sec. The reason for this increase is not yet understood.

The dark rates presented in the COS Instrument Handbook and used by the ETC are lower than those presently observed, since they were based on measurements obtained early in SMOV. Both the Handbook (Table 11.1) and the ETC use a rate of 6.9e-5 counts/pixel/sec. Users should take into account these differences when planning observations of targets for which the background level is important.

The dark rate of both detectors is measured weekly and will be monitored closely during the coming months.