Dark count rates in the STIS MAMA

The dark count rates in the STIS MAMA detectors have been monitored. This report covers the period since the Servicing Mission 4 of May 2009. We find both long-term and short-term variations which for the NUV side we express as a function of date and temperature. The NUV dark rate has declined significantly from its surprisingly high initial rate of 0.014 counts/pixel/s that was seen immediately after SM4. By October, 2012 it had dropped to an average value of about 0.002 counts/pixel/sec The behavior and characteristics of the FUV dark rate remain very similar to that seen in 2004, prior to the STIS side-2 failure and subsequent repair. Introduction Proposals 12415 and 12776 which ran in cycles 18 and 19 were designed to monitor the dark rates in the NUV and FUV MAMA detectors. One motive is to look for evidence of change in the dark rate, indicative of detector problems developing, but more routinely, we wish to measure the dark rate to support calibrating science images. If the dark rate is significant compared with the signal and can be accurately predicted, a normalized dark image can then be subtracted from the raw image. Normally, a pair of 1300 second exposures were taken for the NUV and FUV MAMAs at two-week intervals. Each pair of observations was timed to span a single SAA-free period, the aim being to separate long term temporal variability from short term temperature dependent changes. For both detectors, additional blocks of exposures were taken once a year. These were groups of three 1314 s TIME-TAG darks for each of the MAMA detectors, distributed over a single SAA free interval. This gives more information on the value of the FUV MAMA dark current as a function of the amount of time that the HV has been on, and for the NUV MAMA a better measure of the short term temperature dependence. Operated by the Association of Universities for Research in Astronomy, Inc., for the National Aeronautics and Space Administration STIS was repaired during Servicing Mission 4 (SM4) which took place in May 2009. Before that STIS had not been operable for about six years. The current analysis has been performed on all measurements since then, starting from June 2009. NUV MAMA Each image is collected as a 2048 by 2048 array. The header reports a value GOODMEAN which is, in fact, the median value of counts in the pixels not flagged as bad. To match previous reporting (Zheng, 2011) the values have been normalized to a 1024 by 1024 array, simply multiplying the raw numbers by 4. Figure 1: Dark count rate in the NUV MAMA measured from SM4 to present. Rates are counts per pixel per second on a 1024 by 1024 array. Figure 1 shows the count rates as a function of time. Several features of the data should be noted. In the initial period between June and October 2009, measurements were taken at a higher frequency than in subsequent times. In this period the rate fluctuated more than at other times. Secondly the values at any time can usually be separated into a high and low value. The lower values correspond to the measurements taken at initial turn on while the high value is found at the end of the SAA-free period, usually about nine hours later. Thirdly there is a gap between October and November 2011 during which STIS was not turned on. This was because problems with HST’s control and data handling electronics were under study. After this hiatus the dark rate resumed at a higher rate than when it was turned off and then decayed as before. In view of this, Instrument Science Report STIS 2013-01