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Dark Current Calibration Summary

Date: 2017-Jan-10

Data Quality: Telemetry 100% Complete of 5 Analyzed replicas

Summary Statistics: Previous and Current Calibrations
Date (Year:DOY)CCD Temp (C)Mean (e-/s)Peak (e-/s)Mean near Peak (e-/s)N>100 e-/sN>200 e-/sN>2000 e-/sN>3000 e-/s
2016:188 -13.79 62.1 13.0 14.1201338 75370 93 23
2016:278 -13.00 67.3 13.7 15.0219023 87868 107 30
2017:010 -13.55 71.0 19.6 20.9224430 85396 95 25

Complete Calibration Report: ACA Dark Current 2017-Jan-10

Contents

Calibration Procedure

CCD Temperature Changes

Statistics

Warm Fraction

Histograms

Background Brightness and Intrinsic Dark Current



Calibration Procedure

On 2017-Jan-10, an ACA dark current calibration was performed. This consisted of 5 pointings at slightly offset (2 arcmin) attitudes. At each pointing, two full frame readouts were performed, one with a 5 second integration and one with 10 seconds, giving a total of 10 full frame readouts. For each integration time, the 5 images were median filtered on a pixel-by-pixel basis to remove star images. Then the 5-second median-filtered image was subtracted from the 10-second median-filtered image to remove dark current accumulation during the readout period. This is important as it takes approximately 8 seconds to read out a CCD quadrant. Finally, the dark current image was converted from counts/integration to electrons per second (e-/s).

Standard processing was used to reduce the data, the same as used for previous calibrations.

Comparison to previous calibrations

The first in-flight ACA dark current calibration was performed on 1999-Aug-11 (1999:223) during Orbital Activation and Checkout (OAC) while Chandra's Sunshade Door was closed. We tabulate and plot results from each flight calibration in this document.

CCD Temperature Changes

The commanded temperature of the ACA has been changed several times during the mission. Each temperature change has had a significant effect on the CCD dark current. The approximate dates of these temperature changes are included in the table below.
DateTemp (C)Event
Launch-10
July 2003-15Completion of Cooling to -15
2006:360-20Completion of Cooling to -20
2007:016-19Reversal of last cooldown step

Basic statistics

The table below gives a summary of the statistical properties of the pixel dark current values. The 'Peak' is calculated by fitting a 5th order polynomial to the histogram values around the mode of the histogram (see Figure 4) and then quoting the peak of the fitted polynomial. The histogram data for this fit is first reduced with sigma-clipping with a sigma of 2 to only include the points around the mode. The 'Mean near peak' is the mean for pixels within the sigma-clipped range (which is also the range of the fit line on Figure 4).

Table 1
Date (Year:DOY)Mean (e-/s)Peak (e-/s)Mean near Peak (e-/s)N>100 e-/sN>200 e-/sN>2000 e-/sN>3000 e-/s
1999:223 11.9 8.9 10.1 3495 1778 4 3
2000:326 22.2 11.3 11.6 34097 18442 28 10
2001:060 23.9 12.4 12.8 37381 19962 27 13
2002:057 29.2 12.5 12.9 56671 29657 46 19
2002:137 29.8 12.2 12.6 59754 31238 52 21
2002:237 31.1 12.0 12.5 64023 33701 62 21
2002:338 31.7 12.0 12.5 66850 34954 64 21
2003:116 33.6 13.0 13.4 71418 36831 76 21
2003:195 19.6 7.4 7.8 44179 14708 21 5
2004:002 21.6 7.6 7.9 51388 17339 23 6
2004:084 23.2 8.4 8.8 54534 18700 29 8
2004:171 22.7 7.6 7.9 55605 18941 26 6
2004:253 23.1 7.5 7.9 57613 19715 29 8
2004:359 25.7 9.1 9.5 61944 21270 30 6
2005:070 26.7 8.9 9.3 66981 23434 35 8
2005:191 31.1 12.8 13.1 72427 25014 35 7
2005:247 27.6 8.4 8.9 72060 25309 36 5
2005:316 27.8 8.5 8.9 72420 25098 36 8
2006:064 29.0 8.4 8.9 77247 27288 44 8
2006:220 30.1 8.2 8.6 82171 29121 50 11
2006:329 31.7 9.2 9.8 85404 30076 45 11
2007:006 18.6 5.9 6.3 38089 6956 9 2
2007:069 20.7 6.3 6.5 47039 9792 12 4
2007:251 21.9 6.2 6.5 51683 10915 9 3
2007:345 23.6 7.3 7.6 55093 11707 13 3
2008:101 23.5 6.5 6.9 56901 12191 12 5
2008:188 25.4 7.7 8.0 60648 13100 16 6
2008:273 25.3 7.2 7.5 61492 13305 15 4
2009:050 26.1 6.9 7.4 65217 14302 19 4
2009:113 25.7 6.3 6.8 65623 14335 16 4
2009:190 27.7 7.5 7.9 70139 15533 20 5
2009:279 27.7 6.7 7.1 72454 16316 19 6
2010:015 30.1 7.3 7.9 81618 19106 20 7
2010:108 29.3 6.9 7.5 77984 17715 21 6
2010:189 30.1 7.1 7.6 80774 18440 22 6
2010:288 30.3 7.0 7.5 81702 18687 21 7
2010:301 31.5 7.9 8.5 84113 19374 20 7
2011:018 33.4 8.0 8.5 92889 22850 24 8
2011:079 31.7 7.6 8.3 85278 19391 20 7
2011:211 29.6 7.7 8.4 73545 14515 18 7
2011:234 32.1 8.1 8.6 85821 18995 26 9
2011:346 31.8 7.8 8.4 83805 18030 25 7
2012:033 29.2 7.1 7.6 72713 14044 18 6
2012:115 30.9 8.0 8.6 77344 15251 23 7
2012:198 30.6 7.6 8.1 76760 14820 19 7
2012:287 38.8 9.3 10.2112327 28948 41 11
2013:011 40.4 8.9 9.6120674 33475 42 12
2013:096 42.6 9.7 10.7127936 36529 47 13
2013:191 46.7 9.9 10.9145276 46183 56 16
2013:281 44.8 9.6 10.6137892 41276 53 14
2014:041 52.2 11.1 12.1165093 57337 70 22
2014:104 46.6 10.3 11.3143852 43511 55 14
2014:220 56.5 14.8 15.8175264 60420 64 19
2014:289 58.8 12.3 13.4188168 71764 83 26
2015:014 52.8 11.4 12.4167276 56474 60 19
2015:090 60.0 11.9 13.0193595 74950 85 22
2015:183 54.4 11.7 12.8173089 59287 68 19
2015:273 58.1 12.0 13.3187331 68343 71 20
2015:355 53.0 12.1 13.0167317 53830 57 17
2016:048 55.6 11.6 12.8178830 61398 62 18
2016:125 60.7 12.3 13.4196945 73517 89 22
2016:188 62.1 13.0 14.1201338 75370 93 23
2016:278 67.3 13.7 15.0219023 87868 107 30
2017:010 71.0 19.6 20.9224430 85396 95 25

The statistics above show the significant effect of the CCD cooldown to -15C in July 2003. There was little change in the peak dark current between OAC until then. There then appeared to be a dramatic shift in the peak with the data from the 2005-Jul-10 dark current measurement. This has since been explained as an increase due to the use of an acquisition attitude with a much brighter background than previous attitudes due to zodiacal light. (See the analysis here: Background Brightness and Dark Current Calibration). This dark current calibration report has now been modified to include estimates of "intrinsic dark current" with removal of the zodiacal contributions to the dark current. Another expected shift of the peak of the dark current occurred due to the 2006 CCD cooldown.

Warm Pixels

With regard to the other elements of Table 1, the number of "warm" (> 100 e-/s) and "hot" pixels (> 3000 e-/s) appears to be increasing fairly linearly with time at each CCD temperature. Hot pixels are flagged as bad in the star selection process. A plot of the fraction of pixels which are warm versus time is shown below, where the changes in behavior are clearly seen in 2003 and 2006 due to CCD cooldowns. There is a single point at -20C before the CCD setpoint was raised to -19C. Figure 1b shows the warm pixel fraction for the -19C calibrations with corrections for temperature and zodiacal brightness. See Warm Pixel Count Correction for more details on the implementation of the correction.
Figure 1a
Figure 1b (at -19C) [Click image for large plot]
Figure 1c (at -14C) [Click image for large plot]

Histograms

Differential histogram
The plot below shows the differential distribution of dark current values, in the number of pixels per 1.0 e-/s bin. The factor of 10 increase in warm pixels since OAC is evident in the tail above ~30 e-/s. For clarity, only the OAC calibration and the most recent four calibrations are shown in this and following plots.
Figure 2a
Figure 2b

The graph below shows the same data, plotted on a linear scale near zero. There is a substantial non-gaussian tail of negative dark current values. It is not clear if this is real or a processing artifact. The OAC dark calibration shows only a gaussian tail consistent with electronic read-out noise.

Figure 3

The following plot shows the most recent dark current histogram near its peak and includes the polynomial fit to the data.

Figure 4

Cumulative histogram
Below, the cumulative histogram is shown, indicating the fraction of pixels with dark current greater than a given value.

Figure 5

Implications

The CCD cooldown to -15C, which reduced the overall dark current by about a factor of 2, effectively removed almost 2 years of CCD dark current degradation, as measured by the fraction of warm pixels on the CCD. The CCD cooldown in 2006 seems to have had a similar impact. However, the latest dark current histograms continue to be clearly different to the OAC calibration: pixel-to-pixel dark-current variations are now larger because of the high dark current tail of the distribution.

Background Brightness and Intrinsic Dark Current

The background brightness of the attitude used for the calibration can be estimated using tables of the zodiacal light. Here we've found the ecliptic coordinates of the calibration attitudes and the ecliptic coordinates of the sun during the calibration acquisition. The resulting estimates of Zodiacal brightness are also given.

Table 2
Date (Year:DOY)Years Since LaunchRADECELEBSUN ELEL- SUNELZodiB (see below)
1999:2230.050
2000:326 1.33 30.00 -40.00 8.21 -47.91 238.98 129.24 88.27
2001:060 1.60 30.00 -40.00 8.21 -47.91 340.46 27.75 205.59
2002:057 2.59 30.86 -38.25 10.36 -46.74 337.20 33.16 193.85
2002:137 2.81 30.86 -38.25 10.36 -46.74 55.93 45.56 162.67
2002:237 3.08 30.86 -38.25 10.36 -46.74 151.63 141.27 91.43
2002:338 3.36 30.86 -38.25 10.36 -46.74 251.65 118.72 89.92
2003:116 3.75 30.86 -38.25 10.36 -46.74 35.36 25.00 219.88
2003:195 3.97 30.86 -38.25 10.36 -46.74 111.17 100.81 97.64
2004:002 4.44 30.86 -38.25 10.36 -46.74 280.91 89.45 103.58
2004:084 4.66 30.86 -38.25 10.36 -46.74 3.69 6.67 261.60
2004:171 4.90 30.86 -38.25 10.36 -46.74 88.05 77.69 112.91
2004:253 5.13 30.86 -38.25 10.36 -46.74 166.65 156.29 97.93
2004:359 5.42 111.03 23.98 109.15 1.93 272.52 163.37 193.01
2005:070 5.63 182.52 7.49 179.31 7.87 350.51 171.20 194.16
2005:191 5.96 162.37 24.46 154.29 15.68 107.85 46.44 432.00
2005:247 6.11 14.97 -13.50 8.34 -18.31 161.56 153.23 144.44
2005:316 6.30 10.64 18.98 17.31 13.24 229.70 147.61 154.76
2006:064 6.61 166.78 33.93 153.88 25.89 344.27 169.61 142.87
2006:220 7.04 87.62 -58.27 81.34 -81.64 135.33 53.99 87.87
2006:329 7.34 33.34 10.49 34.67 -2.74 242.57 152.10 180.71
2007:006 7.45 155.00 36.90 142.86 24.60 285.27 142.41 126.55
2007:069 7.62 166.78 33.93 153.88 25.89 349.03 164.85 140.07
2007:251 8.12 224.40 37.42 205.30 51.13 164.98 40.32 155.40
2007:345 8.38 111.04 23.95 109.17 1.90 258.55 149.38 178.80
2008:101 8.71 229.51 11.99 223.36 29.11 20.50 157.14 125.11
2008:188 8.95 42.54 66.75 64.52 47.46 104.31 39.78 173.69
2008:273 9.18 37.43 13.99 39.59 -0.73 186.22 146.63 179.00
2009:050 9.57 180.89 14.98 174.73 14.07 330.47 155.74 156.87
2009:113 9.75 195.88 25.46 183.67 29.52 32.98 150.69 118.20
2009:190 9.96 164.91 49.40 143.94 38.99 106.94 37.01 242.19
2009:27910.20 348.17 16.46 355.88 19.76 192.88 163.00 150.88
2010:01510.48 145.06 19.00 141.06 4.77 294.72 153.66 181.12
2010:10810.73 180.40 -13.47 185.81 -12.18 27.86 157.95 165.29
2010:18910.95 318.74 -37.49 309.55 -20.51 105.75 156.20 142.60
2010:28811.22 356.78 38.96 15.14 36.45 201.53 173.61 121.37
2010:30111.26 7.83 1.00 7.58 -2.19 214.46 153.12 181.76
2011:01811.48 356.70 38.95 15.07 36.48 297.53 77.54 134.84
2011:07911.65 150.70 0.00 152.75 -11.23 359.03 153.72 164.52
2011:21112.01 337.55 -20.48 331.64 -10.29 126.52 154.89 168.73
2011:23412.08 318.74 -37.49 309.55 -20.51 148.58 160.97 146.33
2011:34612.38 59.11 -0.00 56.89 -19.96 259.59 157.30 146.01
2012:03312.52 152.38 46.89 136.18 32.94 312.53 176.35 129.40
2012:11512.75 318.74 -37.49 309.55 -20.51 34.22 84.67 178.00
2012:19812.98 219.12 15.98 210.94 29.59 113.87 97.06 128.84
2012:28713.22 42.69 10.48 43.42 -5.64 200.06 156.64 183.60
2013:01113.47 136.09 56.97 119.99 38.23 290.92 170.93 117.24
2013:09613.70 225.88 -3.62 224.49 13.12 16.35 151.86 155.85
2013:19113.96 244.61 4.50 241.74 25.48 107.92 133.82 122.40
2013:28114.21 110.56 -85.74 265.51 -70.50 194.88 70.64 87.78
2014:04114.55 173.13 21.48 164.99 16.96 321.16 156.17 152.36
2014:10414.72 182.52 7.49 179.31 7.87 23.97 155.34 177.47
2014:22015.04 282.74 -2.00 283.64 20.84 135.40 148.24 136.60
2014:28915.23 352.68 -8.00 350.12 -4.44 202.55 147.57 178.20
2015:01415.47 145.06 19.00 141.06 4.77 293.47 152.42 180.15
2015:09015.68 195.88 25.46 183.67 29.52 9.96 173.71 135.03
2015:18315.94 164.91 49.44 143.92 39.02 99.83 44.09 203.72
2015:27316.18 29.35 -19.48 19.52 -29.33 186.52 167.00 133.25
2015:35516.41 227.14 16.98 219.01 33.14 268.78 49.77 217.20
2016:04816.57 150.40 36.70 139.23 23.00 327.75 171.49 148.60
2016:12516.78 219.12 15.98 210.94 29.59 43.96 166.97 133.25
2016:18816.95 164.91 49.44 143.92 39.02 104.36 39.56 224.92
2016:27817.20 37.43 13.99 39.59 -0.73 191.20 151.61 180.13
2017:01017.46 340.02 -1.50 340.99 6.42 289.93 51.06 586.60
Date = Year and Day of Data Acquisition
Time = Years since launch
RA,Dec = Target Coordinates (Equatorial)
El,Eb = Target Coordinates (Ecliptic)
Sun El = Sun position (Ecliptic, Sun Eb defined as 0)
EL - SUN EL = Target EL - Sun EL
ZodiB = Zodiacal Brightness from Table 17 of
Leinert et al. (1998), A&AS, 127, 1 . Values are given in 10-8 W m-2 sr-1 µm-1, for a wavelength of 0.50 µm.

In the next table, we've estimated the fraction of the Peak that is actually due to the background brightness and we've subtracted this to estimate the intrinsic dark current.

Table 3
Date (Year:DOY)Years Since LaunchZodiBPeak (e-/s)ZB_Part (e-/s)Int_Peak (e-/s)
1999:223 0.05 0.00 8.92 0.00 8.92
2000:326 1.33 88.2711.27 0.9510.32
2001:060 1.60 205.5912.37 2.2210.15
2002:057 2.59 193.8512.47 2.0910.38
2002:137 2.81 162.6712.21 1.7610.45
2002:237 3.08 91.4312.01 0.9911.02
2002:338 3.36 89.9212.02 0.9711.05
2003:116 3.75 219.8812.98 2.3710.61
2003:195 3.97 97.64 7.38 1.05 6.33
2004:002 4.44 103.58 7.59 1.12 6.47
2004:084 4.66 261.60 8.42 2.83 5.59
2004:171 4.90 112.91 7.58 1.22 6.36
2004:253 5.13 97.93 7.50 1.06 6.44
2004:359 5.42 193.01 9.15 2.08 7.07
2005:070 5.63 194.16 8.87 2.10 6.77
2005:191 5.96 432.0012.83 4.67 8.16
2005:247 6.11 144.44 8.45 1.56 6.89
2005:316 6.30 154.76 8.48 1.67 6.81
2006:064 6.61 142.87 8.44 1.54 6.90
2006:220 7.04 87.87 8.24 0.95 7.29
2006:329 7.34 180.71 9.25 1.95 7.30
2007:006 7.45 126.55 5.91 1.37 4.54
2007:069 7.62 140.07 6.26 1.51 4.75
2007:251 8.12 155.40 6.17 1.68 4.49
2007:345 8.38 178.80 7.31 1.93 5.38
2008:101 8.71 125.11 6.46 1.35 5.11
2008:188 8.95 173.69 7.69 1.88 5.81
2008:273 9.18 179.00 7.23 1.93 5.30
2009:050 9.57 156.87 6.91 1.69 5.22
2009:113 9.75 118.20 6.30 1.28 5.02
2009:190 9.96 242.19 7.50 2.62 4.88
2009:27910.20 150.88 6.74 1.63 5.11
2010:01510.48 181.12 7.31 1.96 5.35
2010:10810.73 165.29 6.95 1.79 5.16
2010:18910.95 142.60 7.08 1.54 5.54
2010:28811.22 121.37 6.96 1.31 5.65
2010:30111.26 181.76 7.94 1.96 5.98
2011:01811.48 134.84 8.00 1.46 6.54
2011:07911.65 164.52 7.59 1.78 5.81
2011:21112.01 168.73 7.73 1.82 5.91
2011:23412.08 146.33 8.06 1.58 6.48
2011:34612.38 146.01 7.79 1.58 6.21
2012:03312.52 129.40 7.07 1.40 5.67
2012:11512.75 178.00 8.04 1.92 6.12
2012:19812.98 128.84 7.58 1.39 6.19
2012:28713.22 183.60 9.30 1.98 7.32
2013:01113.47 117.24 8.86 1.27 7.59
2013:09613.70 155.85 9.68 1.68 8.00
2013:19113.96 122.40 9.87 1.32 8.55
2013:28114.21 87.78 9.60 0.95 8.65
2014:04114.55 152.3611.13 1.65 9.48
2014:10414.72 177.4710.26 1.92 8.34
2014:22015.04 136.6014.75 1.4813.27
2014:28915.23 178.2012.30 1.9210.38
2015:01415.47 180.1511.42 1.95 9.47
2015:09015.68 135.0311.86 1.4610.40
2015:18315.94 203.7211.67 2.20 9.47
2015:27316.18 133.2512.00 1.4410.56
2015:35516.41 217.2012.07 2.35 9.72
2016:04816.57 148.6011.64 1.6010.04
2016:12516.78 133.2512.28 1.4410.84
2016:18816.95 224.9213.01 2.4310.58
2016:27817.20 180.1313.68 1.9511.73
2017:01017.46 586.6019.59 6.3413.25
ZB_part = Zodiacal brightness contribution to peak, ZodiB*r where r = 0.0108. See Parameters from Zodiacal Light Inquiry for an explanation of this value of r
Int_peak = Intrinsic peak dark current, Peak-ZB_part, not adjusted for degradation


In the following figure, we've plotted the intrinsic peak dark current over time. The first 8 measurements have been scaled by 1/1.69 to adjust for the CCD temperature change before 2003:195. The scale of this figure has not yet been adjusted to reflect the change to -19C while we await more data. Again, see Parameters from Zodiacal Light Inquiry for more information. Future work may involve finding the best model to explain the degradation over time.



Figure 6



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