TIPS - Oct 13, 2005 M. Sirianni Temperature change for ACS CCDs: initial study on scientific performance M. Sirianni, T. Wheeler, C.Cox, M. Mutchler, A. Riess, K. Sembach, R. Doxsey

TIPS - Oct 13, 2005 M. Sirianni Introduction Variation of the CCD temperature can affect the following aspects: 1.Read noise 2.Dark Current 3.Hot pixel population 4.Quantum Efficiency (and Flat Field) 5.Charge Transfer Efficiency 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 The temperature range investigated is -74 to -80 (WFC) -77 to -84 (HRC) On average ~ 80% of the ACS usage is with WFC

TIPS - Oct 13, 2005 M. Sirianni Dark Current Variation Dark Current changes with Temperature: D(T) = C T 1.5 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 ) 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

TIPS - Oct 13, 2005 M. Sirianni Dark variation with temperature At -74 C the dark rate increases by 71% At -81 C the dark rate decreases by 55 % Using -77 C as a reference:

TIPS - Oct 13, 2005 M. Sirianni On-orbit dark variation due to radiation damage Mean dark current doubles every ~ 4 years

TIPS - Oct 13, 2005 M. Sirianni Dark variation: prediction 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. WFC-74 C-77 C-81 C (e-/pix/hr) Temperature Radiation Damage Flight data at 1 year Ground Test data HRC-77 C-80 C-84 C (e-/pix/hr)

TIPS - Oct 13, 2005 M. Sirianni Dark variation: scientific impact In order to assess the impact of the predicted dark current variation on the science with ACS/WFC, we estimated the noise budget for a typical observation: –Most used filter : F814W –Default gain (2e-/DN) : Read Noise: 5.36 e- –Average Sky: 0.1 e-/pix/sec –Star A0V magnitude F814W = 26 –One single exposure –Aperture 3 pixels (76% of light) –exposure time: 628 secaverage exposure time for WFC observations 339 secmin exposure time for efficient use of WFC Note: potential variations of QE and CTE are not included

TIPS - Oct 13, 2005 M. Sirianni Dark variation : scientific impact Temp (C) Signal 303 Dark Sky 1775 RN 812 Noise S/N 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

TIPS - Oct 13, 2005 M. Sirianni 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.

TIPS - Oct 13, 2005 M. Sirianni Number of hot pixels vs Temp

TIPS - Oct 13, 2005 M. Sirianni 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

TIPS - Oct 13, 2005 M. Sirianni Hot pixel growth Hot pixels are removed by taking multiple images at offset positions (“dithers”). More hot pixels require more readouts for effective removal C4.8 %8.7 % -77C1.8 %3.3 % -81C1.3 %2.5 % Percentage of pixels that are hot:

TIPS - Oct 13, 2005 M. Sirianni Hot pixel Mitigation Max number of WFC readouts in 1 orbit -74 C C C C C C 2008 CRs in 628 sec 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

TIPS - Oct 13, 2005 M. Sirianni 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

TIPS - Oct 13, 2005 M. Sirianni Residuals - WFC2

TIPS - Oct 13, 2005 M. Sirianni 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: Shallow Traps Short emission time constant CTE improves if the emission time is decreased (allowing more time for e- to escape from trap) CTE  if T  Deep Traps Long emission time constant CTE improves if the emission time is increased (keeping the traps filled) CTE  if T  Given the different clocking rate the effect on Parallel/Serial directions and WFC/HRC can be different.

TIPS - Oct 13, 2005 M. Sirianni Summary Lower Temp Higher Temp Scientific impact Notes Read Noise == none Dark Current - + none Hot Pixels - + low Only if temperature increases to -74C QE +/- ? few % Flight test can measure this Flat Field = ? ~ few % CTE +/- ? Unknown

TIPS - Oct 13, 2005 M. Sirianni 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.

TIPS - Oct 13, 2005 M. Sirianni WFC cool down profile WFC housing temp. WFC TEC current WFC CCD temp.

TIPS - Oct 13, 2005 M. Sirianni WFC cool down profile

TIPS - Oct 13, 2005 M. Sirianni Temperature Margin The exponential fit indicates that we could obtain a minimum temperature of -83 C and thus could operate at –80 C with one continuous cool down. This would take approximately three hours or less from the initial TEC turn on A colder detector temperature might be possible because the detector housing temperature is also decreasing. Extended, stepped cool downs, where the temperature stabilizes and is repeatedly reduced, would extend the cool down time.

TIPS - Oct 13, 2005 M. Sirianni Tests on orbit Previous on-orbit test provided temperature dependence of –Read Noise - Dark Current -Hot pixels PROGRAM (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