Performance of the Space Telescope Imaging Spectrograph after SM4

Charles R. Proffitt, A. Aloisi, K. A. Bostroem, Charles R. Proffitt, A. Aloisi, K. A. Bostroem, C. R. Cox, R. I. Diaz, W. V. Dixon, P. C. R. Cox, R. I. Diaz, W. V. Dixon, P. Goudfrooij, T. R. Gull, P. Hodge, M. E. Kaiser, Goudfrooij, T. R. Gull, P. Hodge, M. E. Kaiser, M. D. Lallo, D. Lennon, D. J. Lindler, S. Niemi, M. D. Lallo, D. Lennon, D. J. Lindler, S. Niemi, R. V. Osten, I. Pascucci, E. Smith, M. A. Wolfe, R. V. Osten, I. Pascucci, E. Smith, M. A. Wolfe, B. E. Woodgate, B. York, & W. Zheng B. E. Woodgate, B. York, & W. Zheng

July 21, 2010July 21, 2010

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STIS Overview

3 independent detectors (can use only one at a time)3 independent detectors (can use only one at a time)– 2 Multi-Anode Microchannel Array (MAMA) detectors2 Multi-Anode Microchannel Array (MAMA) detectors

> 1024 x 1024 pixels, ~ 25” x 25” FOV, ~ 0.025” pixels1024 x 1024 pixels, ~ 25” x 25” FOV, ~ 0.025” pixels• FUV CsI ~ 1150 - 1700 Å FUV CsI ~ 1150 - 1700 Å

• NUV CsNUV Cs22Te ~ 1600 - 3200 ÅTe ~ 1600 - 3200 Å

– CCD detector CCD detector ~ 1650 - 11,000 Å ~ 1650 - 11,000 Å

> 1024 x 1024 pixels, 52” x 52” FOV, ~ 0.0507” pixels 1024 x 1024 pixels, 52” x 52” FOV, ~ 0.0507” pixels

A wide variety of gratings and aperturesA wide variety of gratings and apertures– 11stst order gratings with long slits for spatially resolved spectroscopy order gratings with long slits for spatially resolved spectroscopy

– FUV & NUV cross dispersed echelle modes for high spectral resolutionFUV & NUV cross dispersed echelle modes for high spectral resolution

A few imaging modes, notably includingA few imaging modes, notably including– Time-tag imaging in FUV and NUVTime-tag imaging in FUV and NUV

– Coronagraphic mask with unfiltered CCDCoronagraphic mask with unfiltered CCD

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STIS is a complex instrument with a large number of STIS is a complex instrument with a large number of modes and optionsmodes and options

STIS History

Replaced GHRS in Axial Bay 1 on Feb 14, 1997 during SM2Replaced GHRS in Axial Bay 1 on Feb 14, 1997 during SM2 STIS Side 1 electronics failed on May 16, 2001STIS Side 1 electronics failed on May 16, 2001

– 4.25 years and ~ 42,000 hours of operation4.25 years and ~ 42,000 hours of operation

– Probable short in tantalum capacitor - completely disabled side 1Probable short in tantalum capacitor - completely disabled side 1

Resumed operation using Side 2 electronics on July 7, 2001Resumed operation using Side 2 electronics on July 7, 2001– ~ 1 e~ 1 e− − RMS “herringbone” read-noise appears in CCD data RMS “herringbone” read-noise appears in CCD data

– No longer able to directly measure or control CCD temperatureNo longer able to directly measure or control CCD temperature> CCD dark current fluctuates with aft-shroud temperature variationsCCD dark current fluctuates with aft-shroud temperature variations

STIS Side 2 electronics failed on August 3, 2004STIS Side 2 electronics failed on August 3, 2004– 3.25 years and ~ 27,000 hours of operation3.25 years and ~ 27,000 hours of operation

– Failure in converter that supplied power to move mechanismsFailure in converter that supplied power to move mechanisms

Repaired Side 2 on May 17, 2009Repaired Side 2 on May 17, 2009 during SM4 by replacing LVPS-2 during SM4 by replacing LVPS-2 circuit board containing failed component.circuit board containing failed component.

– Thanks to astronauts Michael Good and Mike MassiminoThanks to astronauts Michael Good and Mike Massimino

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Post-repair performance changes relative to 2004

Most internal & external alignments, focus, & mechanical Most internal & external alignments, focus, & mechanical functioning very similar to that seen during previous side-2 functioning very similar to that seen during previous side-2 operationsoperations– Some small zero-point offsets are compensated for by normal Some small zero-point offsets are compensated for by normal

acquisition and wavecal proceduresacquisition and wavecal procedures

– Significant fading of wavelength calibration lamps at shortest λSignificant fading of wavelength calibration lamps at shortest λ> See calibration workshop poster S2 by Pasccuci et alSee calibration workshop poster S2 by Pasccuci et al

Radiation damage to CCD continuesRadiation damage to CCD continues Some modest decreases in channel throughputsSome modest decreases in channel throughputs NUV detector dark rate much higher than previouslyNUV detector dark rate much higher than previously

– Only slowly decliningOnly slowly declining

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CCD changes

Read noise ~ 0.3 eRead noise ~ 0.3 e−− higher for all gains & amplifiers higher for all gains & amplifiers

– 1 e1 e− − RMSRMS “herringbone noise” unchanged“herringbone noise” unchanged Radiation damage continues to accumulateRadiation damage continues to accumulate

– increased dark rateincreased dark rate> Increase in scatter of corrected dark ratesIncrease in scatter of corrected dark rates

– More hot pixelsMore hot pixels

– Increasing effects from decline in charge Increasing effects from decline in charge transfer efficiencytransfer efficiency

> Loss of counts, especially at low count ratesLoss of counts, especially at low count rates

> Changes in spatial profiles and astrometryChanges in spatial profiles and astrometry

> Hot pixels and cosmic rays show long “tails”Hot pixels and cosmic rays show long “tails”

> CTE effects increase with number of transfers CTE effects increase with number of transfers during readoutduring readout

See calibration workshop poster S3 by M. Wolfe et al. for See calibration workshop poster S3 by M. Wolfe et al. for additional informationadditional information

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Inefficiency in charge transfer Inefficiency in charge transfer during readout (CTE) leads to during readout (CTE) leads to “tails” on cosmic rays and hot “tails” on cosmic rays and hot pixelspixels

– Tails significant source of noiseTails significant source of noise

– Tails size increases with # of transfersTails size increases with # of transfers

– Solution is to put source closer to Solution is to put source closer to readout at E1 position (4x fewer readout at E1 position (4x fewer transfers)transfers)

Disadvantages of E1 positionDisadvantages of E1 position– Closer to fiducial bars & top of detectorCloser to fiducial bars & top of detector

> Less space along slitLess space along slit

– Greater vignetting near top of detector Greater vignetting near top of detector makes flux calibration less certainmakes flux calibration less certain

– Available aperture positions may affect Available aperture positions may affect quality of point source IR fringe flatsquality of point source IR fringe flats

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CCD dark current & CTE effects

E1

Standard Position

Sensitivity Changes For most modes, current throughputs are close to what would have For most modes, current throughputs are close to what would have

been expected from extrapolation of previous trends.been expected from extrapolation of previous trends.

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Echelle Changes

E140H has shown some anomalous E140H has shown some anomalous behaviorbehavior

– Initial observations of monitor target in Initial observations of monitor target in Sep 2009, ~10 -20 % low,Sep 2009, ~10 -20 % low,

– Appears to have recovered by late Dec.Appears to have recovered by late Dec.

– E140H has previously shown occasional E140H has previously shown occasional anomalous throughput anomalous throughput

– Other echelle gratings do not appear to Other echelle gratings do not appear to show such a throughput anomalyshow such a throughput anomaly

All STIS echelles show changes in All STIS echelles show changes in blaze function alignment blaze function alignment

– ~ 5% changes over individual orders ~ 5% changes over individual orders (see poster S5 by Bostroem et al).(see poster S5 by Bostroem et al).

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Sensitivity changes

Other than these echelle issues, for a given Other than these echelle issues, for a given detector, overall sensitivity changes appear to be ~ detector, overall sensitivity changes appear to be ~ consistent across gratingsconsistent across gratings

Improved reference files with post-SM4 Improved reference files with post-SM4 adjustments to the time dependence sensitivity adjustments to the time dependence sensitivity correction (TDS) have been deliveredcorrection (TDS) have been delivered– CCD modes TDS reference files delivered Sep 2009CCD modes TDS reference files delivered Sep 2009

– MAMA modes, pipeline TDS reference files not MAMA modes, pipeline TDS reference files not delivered until July 13, 2010delivered until July 13, 2010

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10Aug 20, 2009

NUV Window Phosphorescence

Meta stable stateCan’t radiatively decay

Ground state

1) Excitation viacosmic ray impact

2) Thermal excitation

3) Immediate radiative decayUV photon emitted

E

• NUV MAMA dark current dominated by de-excitation NUV MAMA dark current dominated by de-excitation of meta-stable states in detector window.of meta-stable states in detector window.– Cosmic ray impacts during SAA populate these statesCosmic ray impacts during SAA populate these states

NUV Dark Rate After SM4

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Initial measures >> old model (Initial measures >> old model (orangeorange) ) Red line is empirical fit assuming two Red line is empirical fit assuming two

exponential time scalesexponential time scales ~ 40 days & 600 days~ 40 days & 600 days

Last few points falling above this fitLast few points falling above this fit Time scale still increasing?Time scale still increasing?

Dark current still about 3Dark current still about 3XX pre-SM4 values pre-SM4 values 0.0035 cnts/pix/s vs 0.0013 in 20040.0035 cnts/pix/s vs 0.0013 in 2004

Unclear if population of phosphorescent Unclear if population of phosphorescent states have changed, or if previous single states have changed, or if previous single state model was always too simplestate model was always too simple

See workshop poster by Zheng et al. for See workshop poster by Zheng et al. for additional information and comparison additional information and comparison with COS NUV MAMA dark currentwith 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 functionEchelle flux recalibration for changes in blaze function– Previous calibration done at offset positions that are no longer usedPrevious calibration done at offset positions that are no longer used

– Complete recalibration using post-SM4 data underwayComplete recalibration using post-SM4 data underway

> See workshop poster S5 by Bostroem et al.See workshop poster S5 by Bostroem et al.

Improvements to CCD dark current scaling and subtractionImprovements to CCD dark current scaling and subtraction– See also poster S4 by R. Jansen et al. on herringbone noiseSee also poster S4 by R. Jansen et al. on herringbone noise

Pixel-to-pixel flat fields Pixel-to-pixel flat fields (poster S6 by Niemi et al.)(poster S6 by Niemi et al.)

– Changes to FUV and CCD flats very smallChanges to FUV and CCD flats very small> New CCD spectroscopic p-flats deliveredNew CCD spectroscopic p-flats delivered

– data for NUV MAMA pending data for NUV MAMA pending

Updates to wavelength calibrationUpdates to wavelength calibration– A few settings need brighter lamps/longer exposures – A few settings need brighter lamps/longer exposures – poster S2poster S2

– Updates to echelle dispersion (see presentation by T. Ayres)Updates to echelle dispersion (see presentation by T. Ayres)12

Conclusions

STIS performance after repair during SM4 is STIS performance after repair during SM4 is remarkably similar to previous side-2 operationsremarkably similar to previous side-2 operations

Most important changes for usersMost important changes for users– Continuing radiation damage to the STIS CCDContinuing radiation damage to the STIS CCD

– Large increase in NUV MAMA dark rateLarge increase in NUV MAMA dark rate

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STIS Spectroscopic Modes

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(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

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