tips - oct 13, 2005 m. sirianni temperature change for acs ccds: initial study on scientific...
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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 noise2. Dark Current3. Hot pixel population4. 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 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
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 %
-85
-75
-65
-55
-45
-35
-25
-15
-5
5
15
25
35
45
55
65
75
85
-85 -83 -81 -79 -77 -75 -73
Temperature (C)
variation (%) of dark current
Using -77 C as a reference:
TIPS - Oct 13, 2005 M. Sirianni
On-orbit dark variation due to radiation damage
Dark Rate WFC (-77 C)
0
5
10
15
20
25
30
0 2 4 6 8 10Years from launch
e-/pix/hr
WFC1_trendWFC2_trendWFC1WFC2
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 wewould reach after 18 years on orbit at -77 C.
WFC -74 C -77 C -81 C
2008 31.9 18.6 8.4 (e-/pix/hr)
2013 47.6 27.7 12.5
1
10
100
1000
-95 -90 -85 -80 -75 -70 -65 -60 -55 -50
Temperature (C)
e-/pix/hr
Lot 7b 13-02
10-02_first
10-01_second
10-02_second
fit
10-01_1yr
10-02_yr
fit_1yr
Temperature
Rad
iatio
n D
ama
ge
Flight data at 1 year
Ground Test data
HRC -77 C -80 C -84 C
2008 37.7 21.3 9.8 (e-/pix/hr)
2013 55.8 31.8 14.6
TIPS - Oct 13, 2005 M. Sirianni
Dark variation : scientific impact2008 2013
Temp (C) - 74 - 77 - 81 - 74 - 77 - 81
Signal 303 303 303 303 303 303
Dark 157 91.7 41.3 234 136 61.6
Sky 1775 1775 1775 1775 1775 1775
RN 812 812 812 812 812 812
Noise 55.2 54.6 54.1 55.9 55.0 54.3
S/N 5.49 5.55 5.59 5.42 5.51 5.57
T exp= 628s
CONCLUSION:An increase in Dark Rate does not impact the S/N
€
Noise ≡ Signal+Dark+ Sky+ RN
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/EXPTIME1000 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 pixelthreshold
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.
2008 2013
-74 C 4.8 % 8.7 %
-77C 1.8 % 3.3 %
-81C 1.3 % 2.5 %
Percentage of pixels that are hot:
TIPS - Oct 13, 2005 M. Sirianni
Hot pixel Mitigation
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0 2 4 6 8 10
pixel contamination (%)
number of frames needed
Max number of WFC readouts in 1 orbit
-74
C
2
01
3
-74
C
2
00
8
-77
C
2
01
3
-77
C
2
00
8
-81
C
2
01
3
-81
C
2
00
8
CR
s in
62
8 s
ec
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 changesto -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
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 TrapsShort emission time constant
CTE improves if the emission time is decreased (allowing more time for e- to escape from trap)
CTE if T
Deep TrapsLong 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/Serialdirections 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
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