Complete Calibration Report: ACA Dark Current 2009-Oct-06
Contents
Background Brightness and Intrinsic Dark Current
Calibration Procedure
On 2009-Oct-06, 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.
| Date | Temp (C) | Event |
|---|---|---|
| Launch | -10 | |
| July 2003 | -15 | Completion of Cooling to -15 |
| 2006:360 | -20 | Completion of Cooling to -20 |
| 2007:016 | -19 | Reversal 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-/s | N>200 e-/s | N>2000 e-/s | N>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 |
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 occured 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 1
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 Launch | RA | DEC | EL | EB | SUN EL | EL- SUNEL | ZodiB (see below) |
|---|---|---|---|---|---|---|---|---|
| 1999:223 | 0.05 | 0 | ||||||
| 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 |
| 2009:279 | 10.20 | 348.17 | 16.46 | 355.88 | 19.76 | 192.88 | 163.00 | 150.88 |
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 intrisic dark current.
Table 3
| Date (Year:DOY) | Years Since Launch | ZodiB | Peak (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 | 88.27 | 11.27 | 0.95 | 10.32 | |
| 2001:060 | 1.60 | 205.59 | 12.37 | 2.22 | 10.15 | |
| 2002:057 | 2.59 | 193.85 | 12.47 | 2.09 | 10.38 | |
| 2002:137 | 2.81 | 162.67 | 12.21 | 1.76 | 10.45 | |
| 2002:237 | 3.08 | 91.43 | 12.01 | 0.99 | 11.02 | |
| 2002:338 | 3.36 | 89.92 | 12.02 | 0.97 | 11.05 | |
| 2003:116 | 3.75 | 219.88 | 12.98 | 2.37 | 10.61 | |
| 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 | 432.00 | 12.83 | 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 | |
| 2009:279 | 10.20 | 150.88 | 6.74 | 1.63 | 5.11 |
Int_peak = Intrinsic peak dark current, Peak-ZB_part, not adjusted for degredation
In the following figure, we've plotted the intrisic 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 degredation over time.
Figure 6
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Last modified: 12/27/13