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Temperature change for ACS CCDs: initial study on scientific performance

Temperature change for ACS CCDs: initial study on scientific performance. M. Sirianni, T. Wheeler, C.Cox, M. Mutchler, A. Riess, K. Sembach, R. Doxsey. Introduction. We have been asked to predict the impact of variations in operating temperature for WFC and HRC.

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Temperature change for ACS CCDs: initial study on scientific performance

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  1. Temperature change for ACS CCDs: initial study on scientific performance M. Sirianni, T. Wheeler, C.Cox, M. Mutchler, A. Riess, K. Sembach, R. Doxsey

  2. Introduction We have been asked to predict the impact of variations in operating temperature for WFC and HRC. The current operating temperature is -77 C for WFC -81 C for HRC Variation of the CCD temperature can affect the following aspects: • Read noise • Dark Current • Hot pixel population • Quantum Efficiency (and Flat Field) • Charge Transfer Efficiency On average ~ 80% of the ACS usage is with WFC The temperature range investigated is -74 to -80 (WFC) -77 to -84 (HRC)

  3. Dark Current Variation • Dark Current changes with Temperature: D(T) = C T1.5 exp(-Eg/2kT) Ground Test: Flight build (and similar devices) tested from -100 C to -55 C On-orbit test: Tests at warmer temperature (-71.5 and -66.7C) were executed on March 2003 (Proposal 9097 Cox et al - ISR 2003-04) • On-orbit,dark current increases with time due to radiation damage : • ~ 2.0 e-/pix/hr/yr for WFC1 • ~ 1.6 e-/pix/hr/yr for WFC2 • ~ 2.1 e-/pix/hr/yr for HRC

  4. Dark variation with temperature Using -77 C as a reference: At -74 C the dark rate increases by 71% At -81 C the dark rate decreases by 55 %

  5. On-orbit dark variation due to radiation damage Mean dark current doubles every ~ 4 years

  6. Dark variation: prediction Flight data at 1 year Radiation Damage Ground Test data Temperature • A change to -81 C in 2008 would bring back the dark current at the same level after 1 year on orbit. • A change to -74 C in 2008 would bring the dark current at the level we • would reach after 18 years on orbit at -77 C.

  7. Dark variation : scientific impact T exp= 628s CONCLUSION: An increase in Dark Rate does not impact the S/N When the noise due to dark current D [e-/pix/hr] competes(in with read noise? For a given aperture and an exposure time EXPTIME (sec) D =3600*Read_Noise^2/Exptime ~ 90000/EXPTIME 1000 sec => D=90e-/pix/hr

  8. Hot pixel variation • Dark non uniformity is more serious than the increase in the dark current. • Hot pixel threshold: 0.08 e-/pix/sec • The number of hot pixels increases with time due to radiation damage. • The average signal level of the hot pixels shows the same temperature dependence as normal dark pixels.

  9. Number of hot pixels vs Temp

  10. Hot pixel growth • The number of hot pixels changes with time due to radiation damage. In 2008 the number of hot pixels (dark current > 0.08e-/pix/sec) will reach the same level of contamination of cosmic rays in a 1000 sec exposure Hot pixel threshold

  11. Hot pixel growth Percentage of pixels that are hot: • Hot pixels are removed by taking multiple images at offset positions (“dithers”). More hot pixels require more readouts for effective removal.

  12. Hot pixel Mitigation Max number of WFC readouts in 1 orbit CRs in 628 sec -81 C 2008 -77 C 2013 -74 C 2013 -81 C 2013 -77 C 2008 -74 C 2008 For average exposure times, obtaining 3-4 dithered frames is the optimal strategy. The number of readouts needs to be increased only if the temperature changes to -74C. CONCLUSIONS: No impact if increase in temperature can be avoided

  13. QE/Flat Field Variation • We do see small variations (< 0.5%) in the flat field at F435W (WFC) when CCDs are warmer • We need to investigate variations in the near-IR. • Variations in the flat field may require new calibration. • QE variations need to be investigated: some impact is expected in the near-IR where WFC is most used. • After ~ 3.5 years on orbit we do not observe a significant variation of QE. TENTATIVE CONCLUSION: We do not expect QE or Flat Field variations with temperature to have a serious scientific impact. Better on orbit data can be obtained

  14. CTE variations with Temp • difficult to predict without a direct test • Temperature and clocking rate are major player • Broadly speaking, there are two sort of traps responsible for CTE problems: Given the different clocking rate the effect on Parallel/Serial directions and WFC/HRC can be different.

  15. Summary

  16. WFC Cooling Margin • Data from cool down period after anneals indicate that there is additional cooling margin: • TEC current is well away from maximum • TEC hot side temperatures are well below CARD limits (21 C vs 35 C) • Margin should allow: • “cold test” now • some mitigation of aft shroud temperature increase in the future.

  17. WFC cool down profile WFC housing temp. WFC TEC current WFC CCD temp.

  18. Tests on orbit • Previous on-orbit test provided temperature dependence of • Read Noise - Dark Current -Hot pixels • PROGRAM 10771 (Nov-Dec 05) • study temperature dependence of • QE • Flat Field • CTE • HRC and WFC at three different temperatures • WFC [-74,-77,-80] HRC [-77,-80,-84] Mix of internal and external orbits: total 12 internal + 12 external • for CTE and QE : observation of 47 Tuc (or M3) • for Flat Field and CTE : internal EPER tests • for impact of CTE tails on detection threshold: z band HDFN

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