The dark rate of the COS NUV detector has been increasing with time following an approximately linear trend. At the present time, the mean dark rate is about 4.5 x 10^-4 cts/sec/pix, but if the established trend continues (see figure 1), by April 2012, (approximately the mid-point of Cycle 19), the mean dark rate will have reached a level of about 7.3 x 10^-4 cts/sec/pix. This latter value has been adopted for use in the Cycle 19 ETC.
The COS NUV MAMA dark rate is still substantially lower than the STIS NUV MAMA dark rate, and we expect that for most users the continuing increase will have no impact on the quality of data they obtain. However, for programs targeting very faint objects, the impact could potentially be substantial. We urge all observers with active Cycle 17 and Cycle 18 programs using the COS NUV detector to evaluate whether this increasing dark rate will have an impact on their planned program, especially if a change in the execution date for your program could substantially modify that impact. If you determine that there may be an issue, please bring it to the attention of your Program Coordinator or Contact Scientist, or send an email with your concerns to firstname.lastname@example.org.
COS NUV Dark Rate
Figure 1 shows the COS NUV Dark Rate since COS was installed during HST Servicing Mission 4.