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

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


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


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


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:


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


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


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


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.


Number of hot pixels vs Temp


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


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:


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


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


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.


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


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.


WFC cool down profile

WFC housing temp.

WFC TEC current

WFC CCD temp.


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

Documents

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