SPACE TELESCOPE SCIENCE INSTITUTE Operated for NASA by AURA COS Monthly Status Review 18 January 2007

Keyes – 18 January 2007 Slide 2 of 19 Agenda  Assess past scientific usage of STIS NUV MAMA detector –Include usage statistics from cycles 9-13 –Science programs include cool stars, hot stars, black hole accretion, ISM, IGM, galaxies, and QSOs  Establish common ground for comparison of STIS and COS modes  Comparison of STIS and COS efficiency and sensitivity  Assess impacts of COS performance degradation on typical science

Keyes – 18 January 2007 Slide 3 of 19 STIS Characteristics  STIS NUV Usage (cycles 9-13) –E230M is main M mode component by exposure time usage –E230M usage is approximately 6% of HST exposure time –We will not consider G230M (R~10000) in subsequent comparisons  FUV/NUV MAMA usage ratio –59:41 split averaged over cycles Spectral Element Percent of total NUV usage (cycles 9-13) E230H16.6% E230M34.7% G230L33.5% G230M7.6% other7.5%

Keyes – 18 January 2007 Slide 4 of 19 STIS Characteristics  E230M Physical characteristics –R=30,000 (2 pixels) –Typical extraction height = 7 pixels –Resel to compare with COS (R=20,000): >3 pixels x 7 pixels = 21 pixels –STIS NUV/MAMA Dark (on-orbit value) >Dark per pixel = 1.26 e-3 counts/sec/pixel >dark per COS-comparison resel (3x7) = 2.65 e-2 cts/sec/resel >WORST-CASE COS dark assumed = STIS on-orbit dark –Typical aperture for E230M is 0.2x0.2 >Aperture throughput is approximately 75%

Keyes – 18 January 2007 Slide 5 of 19 COS Characteristics  Estimate 1000 orbits of COS usage as upper limit –What is FUV/NUV ratio? We estimate 60:40  G185M, G225M, G285M characteristics – Å per pixel for G185M and G225M –0.04 Å per pixel for G285M –In comparisons use native dispersions in 3-pixel resel  COS/NUV MAMA resel and extraction box (21 pixels) –3 pixels along dispersion –assume 7 pixel extraction height  COS/NUV MAMA Dark –Dark per pixel = 2.10 e-4 counts/sec/pixel (ground values = best-case) –Dark per 3x7 extraction box = 4.41e-3 cts/sec/box (ground values) –WORST-CASE COS dark assumed = STIS on-orbit dark: >1.26e-3 cts/sec/pixel >2.65 e-2 cts/sec/extracted resel (3x7 pixels)  Typical aperture is PSA: assumed throughput is 100%

Keyes – 18 January 2007 Slide 6 of 19 The Degradation Scenario (from D. Leckrone)  Linear extrapolation of observed degradation to 9/11/2008 launch date at ~ 0.5% per month yields additional ~11% loss of sensitivity before COS gets to orbit; compared to TV I values: –G285M will have lost ~25% throughput waiting to fly –G225M will have lost ~20% throughput waiting to fly  In comparisons to follow, we have assumed 25% degradation (i.e., launch throughput = 0.75 x TV I throughput)

Keyes – 18 January 2007 Slide 7 of 19 Variances in measured COS sensitivities in vacuum 2006/2003

Keyes – 18 January 2007 Slide 8 of 19 Sensitivity Comparison: COS TV I vs STIS (no dark included)  STIS aperture throughput included  No COS degradation included  No dark included

Keyes – 18 January 2007 Slide 9 of 19 Sensitivity Ratio: COS TV I vs STIS (no dark included)  STIS aperture throughput included  No COS degradation included  No darks included

Keyes – 18 January 2007 Slide 10 of 19 Observing Efficiency Comparison: COS vs STIS – darks included

Keyes – 18 January 2007 Slide 11 of 19 Observing Efficiency Comparison: COS vs STIS – darks included

Keyes – 18 January 2007 Slide 12 of 19 Impacts – Single COS grating setting used  Single COS grating setting used: –Bright limit (ignore background): simply increase science exposures to compensate for sensitivity loss: x to achieve same S/N >In bright limit: STIS/COS(no-loss) exposure ratio ~3x; so COS S λ must degrade to 0.33 TV I level for COS=STIS efficiency –Faint “limit”: >For a 40-orbit COS observation to achieve S/N=10 at 2500 Ǻ.: No-loss case with ground dark: F λ =2.0 e-16No-loss case with ground dark: F λ =2.0 e-16 No-loss case with worst-case dark: F λ =4.0 e-16No-loss case with worst-case dark: F λ =4.0 e-16 With 25% degradation and ground dark: F λ =2.5 e-16 ; however, this flux requires 26 orbits in no-loss case (1.5x longer with degradation)With 25% degradation and ground dark: F λ =2.5 e-16 ; however, this flux requires 26 orbits in no-loss case (1.5x longer with degradation) With 25% degradation and worst-case dark: F λ =5.5 e-16 ; however, this flux requires 24 orbits in no-loss case (1.7x longer with degradation)With 25% degradation and worst-case dark: F λ =5.5 e-16 ; however, this flux requires 24 orbits in no-loss case (1.7x longer with degradation) –the limiting flux for a STIS 40-orbit observation is 1.2e-15

Keyes – 18 January 2007 Slide 13 of 19 Impacts – Multiple COS grating settings used  Multiple COS grating settings used: –COS-to-STIS efficiency advantage scales as ratio of COS-to-STIS sensitivity advantage to the number of COS exposures required. >If COS is ~3x as efficient as STIS, then STIS is more efficient if more than 3 COS exposures are needed to map a spectral region –Approximately 15 COS exposures are needed to record the entire Ǻ region, which requires 2 STIS exposures –Approximately 8 COS exposures are needed to record the entire spectral region for a single STIS G230M exposure

Keyes – 18 January 2007 Slide 14 of 19 Impacts: Ratios of exposure to achieve same S/N with degraded sensitivity ObjectFlux COS Exposure ratio to reach S/N = 10 25%-degraded S λ vs no-loss COS S λ with COS ground dark (with worst-case on-orbit dark) COS/STIS Exposure ratio to reach S/N = 10 25%-degraded COS S λ vs STIS S λ with COS ground dark (with worst-case on-orbit dark) 1.e (1.34) 0.43 (0.44) 1.e (1.41) 0.29 (0.36) 1.e (1.64) 0.09 (0.24) 5.e (1.69) 0.07 (0.23) 2.e (1.74) [>40 orbits] 0.05 (0.22) [>40 orbits] 1.e (1.76) [>40 orbits] 0.04 (0.21) [>40 orbits] 0.04 (0.21) [>40 orbits]

Keyes – 18 January 2007 Slide 15 of 19 Impacts: Large STIS programs using E230M  with ProgramID orbits / No. Targets F λ [S/N] Title/Comment 8111 (C. Sneden) 57 / 1 1.e-14 [S/N~40] CS : A Rosetta Star for the Age and Early History of the Galaxy (ultra-metal poor Halo star) 8471 (E. Jenkins) 40 / e-15 [S/N<10] D/H in Quasar Absorption Line Systems 9040 (D. Reimers) 38 / 2 4. e-15 [S/N~15] Baryons in Intermediate Redshift (z > 1) O VI Absorbers (even split between targets) 9173 (J. Bechtold) 30 / 1 2. e-15 [S/N~10] The Pattern of Heavy Element Abundances in a Damped LyAlpha Galaxy 9186 (J. Webb) 45 / 2 5.e-15 [S/N~25] D/H in Lyman Limit 9359 (R. Cayrel) 48 / 1 3.e-14 [S/N=50] The Old Star CS , the Age of the Universe, and the Nature of the r-process (halo uranium star) 8673 (B. Jannuzi) 42 / 3 4. e-15 [S/N~15] The Properties of Lyman-Alpha Absorbers at Redshifts Between 0.9<z< (cy 13 planned) (J. Howk) 130 / 6 1. e-14 Testing the Warm-Hot IGM Paradigm (6 targets / even split between FUV and NUV)

Keyes – 18 January 2007 Slide 16 of 19 Impacts: COS Orbits Required to Execute Large STIS programs using E230M (worst-case COS dark)  with ProgramID STIS orbits / No. Targets F λ [S/N] COSsettings COS orbits no-loss S λ Total orbits needed COS orbits degraded S λ Total orbits needed 8111 (C. Sneden) 57 / 1 1.e-14 [S/N~40] 8 15/setting12020/setting (E. Jenkins) 40 / e-15 [S/N<10] * 8 6/setting4810/setting (D. Reimers) 38 / 2 4. e-15 [S/N~15] 3 8/setting2412/setting (J. Bechtold) 30 / 1 2. e-15 [S/N~10] 3 5/setting158/setting (J. Webb) 45 / 2 5.e-15 [S/N~25] 8 10/setting8014/setting (R. Cayrel) 48 / 1 3.e-14 [S/N=50] 8 15/setting12020/setting (B. Jannuzi) 42 / 3 4. e-15 [S/N~15] 8 9/setting7213/setting (J. Howk) 130 / 6 1. e /setting6647/setting94 *=at STIS detection limit

Keyes – 18 January 2007 Slide 17 of 19 Impacts  E230M used in programs requiring 1573 orbits over 7 cycles (excluding snaps and HDF-S) –Corresponds to ~225 orbits per cycle or 6-7% of HST observations per cycle  There were long E230M observations of faint targets: 430 of 1573 orbits were in programs requiring more than 15 orbits per E230M grating setting –About half of these are at the STIS effective faint limit (1-5 e-15) –Most of remainder are brighter targets (~1.e-14) requiring higher S/N (25-50). >These brighter targets are in the flux range where COS exposures are not seriously impacted by background considerations; hence COS exposure will scale with inverse of COS sensitivity degradation.  For the flux-level of faintest STIS targets (~1.e-15) COS exposures would be increased by 50-60% due to the assumed 25% sensitivity degradation: –However, as noted in table on slide 14, at these flux levels a 25%-degraded COS is still 4x more efficient than STIS.

Keyes – 18 January 2007 Slide 18 of 19 Impacts: Summary and Questions  For observing the fainter targets with degraded S λ, two considerations important: –Brighter limiting flux for observation at a particular S/N –Increase of exposure time to reach a target at a specific S/N  For most STIS targets the modest difference in COS limiting flux due to the degradation does not appear to be an important consideration  Targets with the faintest fluxes attempted by STIS (~1.e-15) in 40 orbits are important, but with 25% degradation and worst-case background would require ~12-13 orbits for a single grating setting with COS (or ~7-8 orbits if no degradation). –Question: is 8 versus 12 orbits significant for science at this flux level? >Depends on number of targets and/or COS grating settings required >In most cases where multiple COS grating settings are needed; COS probably would not be chosen as the SI of choice  The difference in limiting flux between 25% degradation and no degradation (for S/N=10 at 2500 Å and worst-case background) is 6. e-16 versus 4. e-16 (or for best-case dark, 2. e-16) –Is there any significant new science in this regime that might be achievable with an un-degraded COS? >Most new discovery space expected to be with FUV channel

Keyes – 18 January 2007 Slide 19 of 19 Impacts: Summary  For the degradation scenario considered here, our assessment of the impact on likely GO science yields no compelling reason for NUV grating change