Space Telescope Imaging Spectrograph: Projected Performance through 2013

1. Space Telescope Imaging Spectrograph: Projected Performance through 2013 Paul Goudfrooij STScI Part of procedure to address necessity of ASCS installation CCD and MAMA Performance Estimates with/without ASCS Science Impact example

2. CCD Performance after SM4 • Main contributor of effective throughput loss is increase of Charge Transfer Inefficiency (CTI = 1–CTE) with time (due to accumulated radiation damage) • Performance degradation continues in current Safe mode • Functional dependence of CTI on source counts, background level, and elapsed on-orbit time determined in 2003 (Goudfrooij & Bohlin) • CCD spectroscopy mode unique to STIS and hence deemed most relevant for current exercise • Issues related to growth of number of hot pixels similar to those for ACS CCDs (cf. Marco’s talk), but can be mitigated by dithering • On-orbit increase of dark current typically has less impact than that of CTI for faint sources • No positive impact of ASCS/STIK expected on CCD performance • Effect of higher temp’s on CTI would need to be calibrated in flight

3. Science Impact of CTI Increase • Example of popular use of STIS CCD: Measurement of Black Hole Mass in center of typical bulge in low-mass galaxy (plenty of medium-and high-mass galaxies already done). • SETUP: • G750M/8561 Å • 52x0.2 slit • CCD binning 2x1 • galaxy with V = 16 mag/arcsec2

4. NUV-MAMA on NUV-MAMA off FUV-MAMA Performance after SM4 • Dark Current Limited: Exp time needed  Dark Counts • FUV-MAMA dark in “glow” region generally increasing with tube temp, but no clear relation between the two. • relation between dark in glow region and time since HV turn-on • If tube temp will rise significantly after SM4, might consider constraining use to only few hours after every HV turn-on Echelle data F1 apertures

5. SM3B Solar Max SM3A FUV-MAMA Performance after SM4 • Past Long-term Evolution of FUV-MAMA Temperature • Overall increase with time, but rate of increase declining • Temp stayed ~ constant with time since 2002 Note: Tube Temp is ~ 4 ºC lower than Charge Amp Temp (latter shown on left)

6. NUV-MAMA Performance after SM4 • NUV-MAMA Dark Current • Due to phosphorescence in impurity sites in MgF2 window, where e– can be in metastable level just below level decaying to emit  • Dark level strongly dependent on thermal environment • Long-term history of dark current (and temp) shows leveling off Plot by James Davies SM3B Solar Max SM3A

7. NUV-MAMA Performance after SM4 • Dark can be substantially reduced with ASCS/STIK • Temp’s (and hence dark and exp time for dark-limited obs.) down by nearly factor of 2 • Dark down extra factor ~2 if cooling campaigns established • Would open up whole new field of applications • Echelle spectroscopy of absorption lines in faint, distant QSOs • NUV imaging of distant, low surface brightness galaxies Model of dark during cooling campaign Approx. current dark level

8. Summary of potential impact of ASCS on STIS performance