Performance of the Space Telescope Imaging Spectrograph after SM4 Charles R. Proffitt, A. Aloisi, K. A. Bostroem, C. R. Cox, R. I. Diaz, W. V. Dixon, P. Goudfrooij, T. R. Gull, P. Hodge, M. E. Kaiser, M. D. Lallo, D. Lennon, D. J. Lindler, S. Niemi, R. V. Osten, I. Pascucci, E. Smith, M. A. Wolfe, B. E. Woodgate, B. York, & W. Zheng July 21,

STIS Overview  3 independent detectors (can use only one at a time) –2 Multi-Anode Microchannel Array (MAMA) detectors >1024 x 1024 pixels, ~ 25” x 25” FOV, ~ 0.025” pixels FUV CsI ~ ÅFUV CsI ~ Å NUV Cs 2 Te ~ ÅNUV Cs 2 Te ~ Å –CCD detector ~ ,000 Å >1024 x 1024 pixels, 52” x 52” FOV, ~ ” pixels  A wide variety of gratings and apertures –1 st order gratings with long slits for spatially resolved spectroscopy –FUV & NUV cross dispersed echelle modes for high spectral resolution  A few imaging modes, notably including –Time-tag imaging in FUV and NUV –Coronagraphic mask with unfiltered CCD 2  STIS is a complex instrument with a large number of modes and options

STIS History  Replaced GHRS in Axial Bay 1 on Feb 14, 1997 during SM2  STIS Side 1 electronics failed on May 16, 2001 –4.25 years and ~ 42,000 hours of operation –Probable short in tantalum capacitor - completely disabled side 1  Resumed operation using Side 2 electronics on July 7, 2001 –~ 1 e − RMS “herringbone” read-noise appears in CCD data –No longer able to directly measure or control CCD temperature >CCD dark current fluctuates with aft-shroud temperature variations  STIS Side 2 electronics failed on August 3, 2004 –3.25 years and ~ 27,000 hours of operation –Failure in converter that supplied power to move mechanisms  Repaired Side 2 on May 17, 2009 during SM4 by replacing LVPS-2 circuit board containing failed component. –Thanks to astronauts Michael Good and Mike Massimino 3

Post-repair performance changes relative to 2004  Most internal & external alignments, focus, & mechanical functioning very similar to that seen during previous side-2 operations –Some small zero-point offsets are compensated for by normal acquisition and wavecal procedures –Significant fading of wavelength calibration lamps at shortest λ >See calibration workshop poster S2 by Pasccuci et al  Radiation damage to CCD continues  Some modest decreases in channel throughputs  NUV detector dark rate much higher than previously –Only slowly declining 4

CCD changes  Read noise ~ 0.3 e − higher for all gains & amplifiers –1 e − RMS “herringbone noise” unchanged  Radiation damage continues to accumulate –increased dark rate >Increase in scatter of corrected dark rates –More hot pixels –Increasing effects from decline in charge transfer efficiency >Loss of counts, especially at low count rates >Changes in spatial profiles and astrometry >Hot pixels and cosmic rays show long “tails” >CTE effects increase with number of transfers during readout  See calibration workshop poster S3 by M. Wolfe et al. for additional information 5

 Inefficiency in charge transfer during readout (CTE) leads to “tails” on cosmic rays and hot pixels –Tails significant source of noise –Tails size increases with # of transfers –Solution is to put source closer to readout at E1 position (4x fewer transfers)  Disadvantages of E1 position –Closer to fiducial bars & top of detector >Less space along slit –Greater vignetting near top of detector makes flux calibration less certain –Available aperture positions may affect quality of point source IR fringe flats 6 CCD dark current & CTE effects E1 Standard Position

Sensitivity Changes  For most modes, current throughputs are close to what would have been expected from extrapolation of previous trends. 7

Echelle Changes  E140H has shown some anomalous behavior –Initial observations of monitor target in Sep 2009, ~ % low, –Appears to have recovered by late Dec. –E140H has previously shown occasional anomalous throughput –Other echelle gratings do not appear to show such a throughput anomaly  All STIS echelles show changes in blaze function alignment –~ 5% changes over individual orders (see poster S5 by Bostroem et al). 8

Sensitivity changes  Other than these echelle issues, for a given detector, overall sensitivity changes appear to be ~ consistent across gratings  Improved reference files with post-SM4 adjustments to the time dependence sensitivity correction (TDS) have been delivered –CCD modes TDS reference files delivered Sep 2009 –MAMA modes, pipeline TDS reference files not delivered until July 13,

10 Aug 20, 2009 NUV Window Phosphorescence Meta stable state Can’t radiatively decay Ground state 1) Excitation via cosmic ray impact 2) Thermal excitation 3) Immediate radiative decay UV photon emitted EE NUV MAMA dark current dominated by de-excitation of meta-stable states in detector window. NUV MAMA dark current dominated by de-excitation of meta-stable states in detector window. –Cosmic ray impacts during SAA populate these states

NUV Dark Rate After SM4 11  Initial measures >> old model (orange)  Red line is empirical fit assuming two exponential time scales  ~ 40 days & 600 days  Last few points falling above this fit  Time scale still increasing?  Dark current still about 3 X pre-SM4 values  cnts/pix/s vs in 2004  Unclear if population of phosphorescent states have changed, or if previous single state model was always too simple  See workshop poster by Zheng et al. for additional information and comparison with COS NUV MAMA dark current STIS NUV MAMA dark measurements since SM4 (  ), are compared with an empirical fit that assumes a short term exponential dependence on detector temperature, with 40 and 600 day decay time scales for the overall level of the dark rate (red line), and a model based on the behavior of the dark current prior to the 2004 failure of STIS (orange line).

Notable Ongoing Calibrations  Echelle flux recalibration for changes in blaze function –Previous calibration done at offset positions that are no longer used –Complete recalibration using post-SM4 data underway >See workshop poster S5 by Bostroem et al.  Improvements to CCD dark current scaling and subtraction –See also poster S4 by R. Jansen et al. on herringbone noise  Pixel-to-pixel flat fields (poster S6 by Niemi et al.) –Changes to FUV and CCD flats very small >New CCD spectroscopic p-flats delivered –data for NUV MAMA pending  Updates to wavelength calibration –A few settings need brighter lamps/longer exposures – poster S2 –Updates to echelle dispersion (see presentation by T. Ayres) 12

Conclusions  STIS performance after repair during SM4 is remarkably similar to previous side-2 operations  Most important changes for users –Continuing radiation damage to the STIS CCD –Large increase in NUV MAMA dark rate 13

STIS Spectroscopic Modes 14 (1) Resolving power of up to 200,000 may be achievable with the E140H and E230H using the 0.1x0.03 aperture and special observing and data reduction techniques 1 1