Instrument Science Report ACS 2003-004

Elevated Temperaturemeasurements of Hot Pixels

Colin Cox, Max Mutchler and Doug van Orsow.June 23, 2003

ABSTRACT

The number of hot pixels on the Wide Field Channel of the Advanced Camera for Surveyswill increase over the next few years as radiation damage accrues. A considerable reduc-tion in dark rate and hence hot pixel counts should result from lowering the detector tem-perature. When the Aft Shroud Cooling System is installed in Servicing Mission 4,temperatures as low as -85C or -90C might be available. At present we cannot signifi-cantly lower the detector temperature. We have therefore investigated the temperatureresponse of dark rates and hot pixel counts by raising the temperature, which we did intwo steps. We find that the predominant effect of a change in temperature is a simple scal-ing of the dark rate in each pixel. There is a 20% increase in dark rate for every degree oftemperature elevation. The number of hot pixels (with more than 0.08 electrons per sec-ond) more than triples between -77C and -71.5C.

Introduction

The hot pixels on the Wide Field Channel (WFC) of the Advanced Camera for Surveys(ACS) are more resistant to annealing than those on the High Resolution Channel (HRC)or those on other CCDs on HST. As described in Instrument Science Report ACS 2002-09by Adam Riess, the number of hot pixels on WFC after several years of operation isexpected to be comparable to the number affected by cosmic rays in a 1000 secondexposure.The count rate, both in normal and hot pixels, is very sensitive to the temperature at whichthe CCD operates. Ground measurements of dark counts had shown a doubling of countrate for every 5 degree increase in temperature. Operating at the lowest available tempera-

Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.

Instrument Science Report ACS 2003-004

ture clearly has advantages. Servicing Mission 4 is intended to include the installation ofan Aft Shroud Cooling System (ASCS) which includes extra radiators and CapillaryPumped Loop coolers which will enhance the cooling of the instruments. There may bethe possibility of running the WFC at -85°C or even -90°C. This is subject to how well thecooling system performs and the competing heat loads of WFC3 and COS.The ACS team was therefore interested in seeing how well the WFC would perform attemperatures lower than its current operating value of -77°C. Unfortunately the currentsystem is not capable of reducing this temperature by more than a degree or so. To investi-gate the temperature sensitivity of the count rate, we therefore decided to takemeasurements at slightly higher temperatures and then extrapolate backwards to estimatethe low temperature conditions.

Method

Proposal 9707 was written with special commanding to change the set point temperaturesof the thermo-electric coolers in contact with the WFC CCDs. The three temperature set-tings were the normal -77°C and the elevated values -71.5°C and -66.7°C. At each of thesetemperatures two bias images straddling three 1000 second dark frames were taken.(Images to characterize charge transfer efficiency were also taken and will be the subjectof a separate report.) To analyze the dark images, the usual bias subtractions and cosmicray filtering operations were performed. Conclusions were derived from the image histo-grams and extracted count rates.

Results

Figures 1 and 2 show the temperature history and the timing of the observations. A tenminute settling time was allowed following each temperature change before beginning anyobservations. Histograms of the count rates per pixel are shown in Figure 3. With increas-ing temperature there is an increase in the hot pixel tail as well as an increase in the modeof the dark count distribution. Table 1 lists the main results illustrated by these plots.The mean rate is calculated iteratively and excludes pixels differing from the mean value

by more than three standard deviations. In this way we predominantly sample the normal

pixels. The hot pixel count rate values all use the hot pixels defined by the -77°C thresh-

old; the rate is measured for the same set of pixels in each case.

The hot pixel number for the -66.7°C case had to be modified to allow for the fact that 5%

of pixels were dropped by the on-board compression. A small irregularity may be seen in

the peak of the histogram for the hottest case. This reflects the fact that the missing pixels

are detected and replaced by a values equal to the local median. The factor of two com-

pression works with no loss for the normal case and showed a negligible loss in the -

71.5°C case. Any future measurements at elevated temperatures should be done without

image compression, or at least should not attempt to compress by as much as a factor of

two.

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Instrument Science Report ACS 2003-004

Analysis

The temperature dependence of dark rate in CCD detectors follow the form given in Jane-

sick, 2001, , where T is the absolute temperature in

degrees Kelvin, k is Boltzmann’s constant, and Eg is the silicon band-gap energy given by

. The three data points have been scaled to

match this function. Although this is a very small sample, the fit looks very reasonable,(see Figure 4), and gives a good indication of the behavior we can expect at lower temper-atures. Also shown are the results of ground measurements made on detectors belongingto earlier batches of the same design as the flown CCDs. All sets of data match the theoret-ical shape fairly well although the -70° and −75°C points in the ground data are suspectedof being contaminated by a light leak.The total number of hot pixels is probably a more complex function of temperature since itwould depend on the intensity profile and the chosen threshold, although to the extent thatall pixels have the same temperature dependence we can expect similar behavior. We haveanalyzed the hot pixel count in the same way and shown them on the same diagram. Eventhough these results are approximate, it is clear that lowering the detector temperatureeven by a few degrees has a large impact on the dark count rate and hot pixel count. In theplausible operating temperature range, the theoretical function falls by more than 20% forevery degree of temperature reduction.As a further check that we understand the temperature dependence of hot pixels we tookthe standard -77°C dark image and multiplied the intensity of every point by the ratio ofthe average intensities in the -71.5°C image to that in the -77°C image. Overlaying the his-togram of this scaled image with that from the -71.5°C image, as we have done in Figure 5shows that the hot pixel distribution is the same for each.

Table 1: Hot pixel temperature dependence

TemperatureMean rate

electrons per sec. per

pixel

Hot Pixels

>0.08 electrons/sec.

Hot pixel count rate

electrons/sec.

-77.0°C WFC1: 2.91 × 10-3

WFC2: 2.80 × 10-3

31062 (0.4%)

31925 (0.4%)

0.69

0.64

-71.5°C WFC1: 6.65 × 10-3

WFC2: 6.33 × 10-3

102063 (1.2%)

103184 (1.2%)

1.29

1.22

-66.7°C WFC1: 1.50 × 10-2

WFC2: 1.41 × 10-2

203400 (2.4%)

203900 (2.4%)

2.14

2.03

D T( ) C T1.5

Eg 2kT( )⁄–( )exp⋅ ⋅=

Eg 1.1557 7.021 104–

T2⋅× 1108 T+( )⁄–=

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Instrument Science Report ACS 2003-004

Another way of studying the relation between the normal and elevated temperature darksis to plot the level of corresponding pixels in two images against each other. Figure 6shows the results of this procedure. The top image compares the -71.5°C and -77°C inten-sities while the lower plot shows -66.7°C against -77°C. In each case the predominanteffect is a simple scaling of dark rate in each pixel with temperature (i.e., there is no sig-nificant population of pixels that make a sharp transition from being relatively normal atcold temperatures to being very hot at higher temperatures; such pixels would have shownup in the top left of the panels in Figure 6).

Conclusions

The temperature dependence of hot pixels is as would have been expected from theory andground based measurements. The dark current and number of hot pixels is a very sensitivefunction of temperature, varying by more than 20% per degree. It will be important to con-sider the influence of temperature on the performance of the ACS following theinstallation of the ASCS. There is a limit to how much heat can be dissipated by the newradiators and it will be necessary to balance the cooling requirements of the new instru-ments against that of the ACS. Both because of cooling and power requirements, acampaign strategy of turning one instrument off to allow the others to operate at full effi-ciency may be worth considering.

Acknowledgments

Thanks are due to Alan Welty for advice and commanding instructions to allow the specialtemperature setting. We also thank George Chapman and Alison Vick for proposal prepa-ration and solving difficult scheduling problems in connection with these observations.

References

The Projected Growth of Hot Pixels on ACS WFC, Instrument Science Report ACS2002-03, Adam Riess, December 2002.

Scientific charge-coupled devices, James R Janesick, SPIE Press Monograph VolPM83, 2001

Status of the WFC build-4 CCDs. Marco Sirianni and Mark Clampin, August 2000,JHU/ACS Internal Report available athttp://adcam.pha.jhu.edu/instrument/detectors/WFC/builds/4/Statuswfc4.pdf

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Instrument Science Report ACS 2003-004

Figure 1: Day 065 detector temperature profile and observation times.

Figure 2: Day 066 detector temperature profile and observation times.

Day 065 Exposures

-80

-78

-76

-74

-72

-70

-68

-66

-64

-62

-60

9:00 10:00 11:00 12:00 13:00 14:00

Time of day

De

tec

tor

tem

pe

ratu

re Bias Dark Bias CTE Bias

Day 065 Exposures

-80

-78

-76

-74

-72

-70

-68

-66

-64

-62

-60

9:00 10:00 11:00 12:00 13:00 14:00

Ti f d

De

tec

tor

tem

pe

ratu

re Bias Dark Bias CTE Bias

Day 065 Exposures

-80

-78

-76

-74

-72

-70

-68

-66

-64

-62

-60

9:00 10:00 11:00 12:00 13:00 14:00

Time of day

De

tec

tor

tem

pe

ratu

re Bias Dark Bias CTE Bias

Day 066 Exposures

-80

-78

-76

-74

-72

-70

-68

-66

-64

-62

-60

12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00

Time of day

De

tec

tor

tem

pe

ratu

re BiasDark Bias CTEBias Dark Bias

Day 066 Exposures

-80

-78

-76

-74

-72

-70

-68

-66

-64

-62

-60

12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00

Time of day

De

tec

tor

tem

pe

ratu

re BiasDark Bias CTEBias Dark Bias

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Instrument Science Report ACS 2003-004

Figure 3: Histograms of pixel count rates at three temperatures. A threshold of 0.08 elec-trons per pixel per second is chosen to define hot pixels.

Figure 4: Dark rates and hot pixel counts compared with theoretical dark rate behavior.

ACS/WFC DARK COUNTS AT THREE TEMPERATURES

1

10

100

1000

10000

100000

1000000

-0.1 0 0.1 0.2 0.3 0.4 0.5

electrons per pixel per second

Co

un

ts

Values > 0.08 designated as hot pixels

-66.7C

-71.5C

-77C

-85 -80 -75 -70 -65 -60 -55 -50

TEMPERATURE Degrees C

1

10

100

1,000

2

3

45

7

2

345

7

2

3

45

79

2

345

79

Coun

ts p

er

pix

el pe

r ho

ur

TEMPERATURE DEPENDENCE OF DARK RATES

In flight

Hot pixels

Ground 1

Ground 2

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Instrument Science Report ACS 2003-004

Figure 5: Histograms of the scaled -77°C image and the observed -71.5°C image

Figure 6: Scatter plot of count rates in corresponding pixels in pairs of images obtained atdifferent temperatures.

COMPARISON OF SCALED PULSE HEIGHTS WITH HIGHER

TEMPERATURE

1

10

100

1000

10000

100000

1000000

-0.1 0 0.1 0.2 0.3 0.4 0.5

electrons per pixel per second

Co

un

ts

Values > 0.08 designated as hot pixels-71.5C

-77C Scaled

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