SPACE TELESCOPE SCIENCE INSTITUTE

1. SPACE TELESCOPE SCIENCE INSTITUTE Operated for NASA by AURA The On-Orbit Performance of the Cosmic Origins Spectrograph Alessandra Aloisi(for the COS STScI team and the COS IDT)21 July 2010

2. Highlights • Quick Overview of the Instrument • Sensitivities and their Time Dependence • Gain Sag of FUV Detector • On-orbit Line Spread Function • Darks and their Evolution • Flat Fields • Wavelength Calibration • COS-to-FGS Alignment • COS Documentation Aloisi – 21 July 2010 Slide 1 of 15

3. Instrument Overview • FUV spectroscopy: R = 3,000 - 20,000 [1150 - 2050 Å] • NUV spectroscopy: R = 2,000 - 20,000 [1700 - 3200 Å] • Some NUV imaging capabilities (mostly used for TA) Aloisi – 21 July 2010 Slide 2 of 15

4. COS Sensitivity • Superb in FUV • Comparable to STIS in NUV • NUV sensitivity in G225M & G285M is as expected at launch from grating degradation • Within 20% of ground values New COS & STIS on-orbit sensitivities Aloisi – 21 July 2010 Slide 3 of 15

5. COS has Sensitivity down to the Lyman Limit • Mode currently supported in Cycle 18 (G140L @ 1280 Å Segment B) • Additional F &  characterization in Cycle 17 Calibration program 12081 • Two new G130M settings at R ~ 5,000 (1055 Å & 1096 Å) S. Osterman’s talk McCandliss et al. 2010, ApJ, 709, L183 McCandliss et al. 2009 ApJL, submitted G140L Mode R ~ 2,500 Aloisi – 21 July 2010 Slide 4 of 15

6. Time Dependence of COS Sensitivity • COS sensitivity is regularly monitored as part of Cycle 17 & 18 Calibration Programs • NUV Sensitivities - gratings keep following pre-launch trends • Bare-Al gratings G225M and G285M continue to show grating-dependent but wavelength-independent decline on orbit at a rate similar or higher than pre-launch rates (up to ~ 10% per year ) • MgF2-coated gratings G185M and G230L continue to be stable • FUV Sensitivities – non localized wavelength-dependent decline up to ~ 12% per year at longer λ similar for all gratings suggestive of CsI photocathode aging (with each segment of the XDL detector having different rates) • COS TDS trends already included in ETC (v18.2) and in CALCOS See R. Osten’s talk fore more details Aloisi – 21 July 2010 Slide 5 of 15

7. Gain Sag of COS FUV Detector • FUV cumulative images and maps of pulse height distributions as a function of time now available (fully implemented into pipeline for OPUS build 2010.4) • Evidence of detector gain sag (localized effect) • Gain after launch higher than on the ground • HV lowered during SMOV • Evolution of gain sag faster than expected • Consistent with on-orbit gas contamination around launch time • We are probably re-scrubbing the detector on-orbit • Gain sag is already slowing down in most heavily exposed areas • Very localized loss up to a few % in most heavily affected areas (geocoronal Lyα) • In the process to identify mitigativeactions See D. Sahnow’s poster C2 Aloisi – 21 July 2010 Slide 6 of 15

8. COS On-Orbit FUV LSF FUV spectral resolution not quite as expected due to MFWFEs — 1150 Å (G130M) — 1450 Å (G130M) — 1750 Å (G160M) — Gaussian 6.5 pixel FWHM (no MFWFEs) • LSF power is distributed from core to wings • Effect is wavelength dependent, being most extreme in the FUV • R ~ 16,000 vs 20,000 @ 1300 Å See P. Ghavamian’s poster C5 Aloisi – 21 July 2010 Slide 7 of 15

9. COS On-Orbit NUV LSF NUV spectral resolution at  > 2500 Å as expected • Contribution of MFWFE present also in NUV • Almost negligible longward of 2500 Å Aloisi – 21 July 2010 Slide 8 of 15

10. COS FUV Darks • FUV dark rate away from SAA measured during SMOV was as expected from pre-launch estimates, i.e. ~ 1.5-2 x 10-6 cts/sec/pixel, when pulse-height filtering (4-30) is applied • FUV dark rate up to ~ 30 times higher close to the SAA boundary (before SAA model adjustment in May 2010) • Several weak features found for Segment B (Segment A is featureless) • TIME-TAG: most of the features removed by pulse-height filtering (4-30) during ground processing (already implemented into CALCOS) • ACCUM: suspect regions flagged; features too weak to affect very bright objects observed in this mode • FUV dark rate monitored weekly • No evolution with time • See D. Sahnow’sposter C2 G130M @ 1309 Å (Segment B) Aloisi – 21 July 2010 Slide 9 of 15

11. COS NUV Darks • NUV dark rate away from SAA measured during SMOV was 6x10-5 cts/sec/pixel, significantly below prelaunch predictions (~ 20x10-5 cts/sec/pixel) • NUV dark rate up to ~ 15 times higher close to the SAA boundary until SAA model adjustment in May 2010 (no long-term phosphorescence in MAMA window after SAA passage) • NUV dark exposures relatively featureless • NUV dark rate monitored weekly • Approximately linear increase with time, once temperature fluctuations are taken into account • Currently slightly higher than prelaunch estimates, i.e. ~ 35x10-5 cts/sec/pixel • Still several times lower than current STIS NUV MAMA values See W. Zheng’s poster S1 Aloisi – 21 July 2010 Slide 10 of 15

12. COS NUV Flat • NUV on-orbit flat (internal lamp) confirmed consistency with ground flat at higher S/N (alignment better than 1 pixel) • NUV flat field currently used • Combination of ground and on-orbit flats • Includes on-orbit vignetting (depression up to 15-20%) in first ~ 200 pixels • Vignetting currently implemented separately for M (G185M, G225M, G285M) and L (G230L) gratings • NUV flat field correction currently applied in CALCOS • NUV flat allows to achieve S/N > 100 per resel when combined with 4 different FP-POS positions with high S/N data NUV ground flat NUV flat for on-orbit data Aloisi – 21 July 2010 Slide 11 of 15

13. COS FUV Flat “Berkeley” Flat of Segment A (whole detector, no grid wires) • 2D ground flats (Berkeley + TV03) still to be validated with on-orbit data • No 2D on-orbit flat with adequate S/N currently available • FUV flat field correction currently not applied in CALCOS • CALCOS currently omits grid wires and other defects when creating x1dsum files • 1D iterative technique combined with 4 different FP-POS positions looks promising and allows to achieve S/N > 100 per resel with high S/N data • Implementation of 1D flats on extracted spectra underway • New 1D flats for all FUV gratings to be obtained in Cycle 17 Calibration program 12086 • See T. Ake’s talk for more details on 1D flats Sum of the 8 rows indicated above Vallerga et al. 2001, SPIE 4498, 141 Aloisi – 21 July 2010 Slide 12 of 15

14. COS On-Orbit Wavelength Calibration • TV03 demonstrated thatdispersion solutions of PSA and WCA differ from each other • PSA/WCA wavelength dispersion solutions obtained in TV03 • TV06 demonstrated that dispersion solution difference between WCA and PSA depends on optical alignment • SMOV data used to: • infer new on-orbit wavecal templates • measure on-orbit zeropoints compared to ground calibration • New CALCOS reference files that update wavelength calibration for most settings delivered in January 2010 (G185M still to be completed) • On-orbit wavelength scales for most settings meet requirements • 15 km/s for the M gratings • 150 km/s (175 km/s) for G140L (G230L) gratings • In a few cases, extra offsets of up to 10 pixels in FUV appear in part of the spectrum (nature & frequency of such localized distortions still under investigation) • Cycle 17 monitoring indicates that PSA to WCA on-orbit offsets are stable with time See C. Oliveira’s poster C4 Aloisi – 21 July 2010 Slide 13 of 15

15. COS-to-FGS Alignment and TA • COS-to-FGS alignment optimized during SMOV and Cycle 17; SIAF tables updated • Figure includes centering slews in all TA sequences of GTO/GO programs since update of SIAF table in March 2010 • Alignment now appears to be accurate to within ~ 0.1” • Initial pointing now limited by accuracy of GSC2 catalogue (~ 0.2”) • Reconsidered the need for an ACQ/Search at start of each visit for Cycle 18 Phase II • Change will save time, especially for acquisitions of very faint targets • For faint targets, ACQ/Search can have problems with variable background • Coordinates need to be accurate within 0.4” Aloisi – 21 July 2010 Slide 14 of 15

16. COS Documentation • On-orbit COS performance summarized in 10 published ISRs and 6 soon-to-be-published ISRs: http://www.stsci.edu/hst/cos/documents/isrs • Preliminary SMOV updates included into the Instrument Handbook for Cycle 18 (published in January 2010): http://www.stsci.edu/hst/cos/documents/handbooks/current/cos_cover.html • SMOV and Cycle 17 Calibration updates currently implemented into the Instrument Handbook for Cycle 19 (for publication in December 2010) • Visit our web pages at: http://www.stsci.edu/hst/cos Aloisi – 21 July 2010 Slide 15 of 15