The STIS NUV MAMA, while otherwise performing well, appears to at least temporarily have a value for the dark current several times higher than that which had been predicted and which was adopted for use in the STIS Cycle 17 exposure-time calculator. For Cycle 17 planning purposes, we had adopted a mean rate of 0.0013 counts/s/pixel. As of early September, 2009, the measured mean value was about 0.011 counts/s/pixel.
This excess dark current is slowly declining, but neither the rate of its future decline nor its eventual minimum can be predicted with confidence. At best, it likely will be several months before the dark current declines to anything close to what had been expected.
For programs observing sufficiently bright targets, the impact will be minimal, but observations of targets with a count rate comparable to the background can be significantly impacted. For example, with a dark current of 0.0013 c/pixel/s, an E230H echelle observation at 2300 Å of a source with a flux of 1X10^-13 ergs/cm2/Å /s would yield a source count rate in the extraction region about 2.5 times higher than the dark current and would require about 4500 s to reach a S/N of 10 per resolution element. However, with a dark current of 0.011 c/pixel/s, the source counts in the extraction region would only be about 30% as large as the dark counts, and an exposure time greater than 14000 s would be required to achieve the same S/N. For a target ten times brighter, the required exposure time would increase by about 30%, from about 330 s to 430 s.
Observers using time-tag mode with the STIS NUV-MAMA may also be impacted. As of October 1, 2009, the global rate for the STIS NUV-MAMA dark current has still been occasionally reaching values as high as 13500 counts/s. This high dark current will force use of a signficantly smaller buffer time than would be otherwise needed, and in many cases may push the count rate high enough to make time-tag observations impractical.
The NUV MAMA dark current is dominated by a phosphorescent glow from the detector window. Impurities in the window have meta-stable states that are populated by cosmic-ray impacts. These states are then depopulated by thermal excitation to states that decay quickly by emitting a UV photon. Since the rate at which the meta-stable states depopulate is sensitive to temperature, it was expected that there would be a temporary increase in the NUV MAMA dark current as the excess population of meta-stable states that had built up over the years that STIS was inoperative and cold readjusted to the higher temperature of a fully-powered STIS. (This strong temperature dependence also causes the dark rate to vary by a factor of two over several hours. See STIS ISR 1999-02: http://www.stsci.edu/hst/stis/documents/isrs/199902.pdf for further details of our model for the expected behavior of the NUV MAMA dark current.)
The previous behavior of the NUV MAMA dark current led us to expect an initial dark rate of 2500-3500 counts/s over the whole detector, which should then have declined with an e-folding time of a week or so, reaching the predicted range of between 900 and 1800 counts/s within about a month of the initial detector turn-on. This modeling led to a predicted mean NUV MAMA dark rate for Cycle 17 of 0.0013 counts/s/pixel.
However, we instead found for the NUV MAMA detector an initial global dark rate of between 9000 and 18000 counts/s (a mean of 0.013 counts/s/pixel). We suspect that a new group of meta-stable states was populated during the 4.5 years that STIS was inoperative and cold. This new excess dark current appears to be declining, but with an apparent time scale much longer than that of the states we have seen before. Initial measurements suggest an e-folding time of about 100 days, but since we do not understand the detailed physics of these new states or how many different time scales might be involved, the future evolution of the NUV MAMA dark current remains highly uncertain.