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Space Telescope Imaging Spectrograph
STIS-20

ACTIVITY TITLE: NUV Dark Monitor

ID: STIS-20

APPLICABLE SMOV REQUIREMENTS: L.10.4.5.4.1

DESCRIPTION: The STIS NUV-MAMA dark current is dominated by a phosphorescent glow from the detector window. Meta-stable states in this window are populated by cosmic ray impacts, which, days later, can be thermally excited to an unstable state from which they decay, emitting a UV photon. The equilibrium population of these meta-stable states is larger at lower temperatures; so warming up the detector from its cold safing will lead to a large, but temporary, increase in the dark current.

To monitor the decay of this glow, and to determine the equilibrium dark current for Cycle 17, four 1380s NUV-MAMA ACCUM mode darks should be taken each week during the SMOV period. The observations should be done in pairs, with the two observations of each pair separated by 4 to 7 orbits to ensure that they are taken at different parts of the same SAA-free block. Each pair should be taken 3 to 4 days after the preceding pair. Once the observed dark current has reached an approximate equilibrium with the mean detector temperature, the frequency of this monitor can be reduced to one pair of darks per week.

IMPLEMENTATION METHOD: Stored commanding

DEPENDENCIES: Begin monitor observations within 36 hours after the completion of STIS-18, “NUV-MAMA HV Recovery”.

DURATION: 4 x 1800 s internal visits per week

DATA REQUIREMENTS: 67 Mbits per visit x 4 visits per week.

ANALYSES & EXPECTED RESULTS: The mean dark rate in each exposure will be tabulated and compared with predictions of a model for the NUV window glow. These results will be used to estimate the mean NUV MAMA dark rate expected for Cycle 17, and to determine when the window glow has declined enough to allow GO observations that are sensitive to the amount of dark current to proceed.

COMMENTS: This activity may be subsumed into the Cycle 17 NUV Dark Monitor Calibration Program.

AUTHOR/telephone/email: Charles Proffitt/410-338-4938/proffitt@stsci.edu

DATE: September 24, 2007