ACA Dark Current Calibration 2005-Nov-12

On 2005-Nov-12, 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 below.

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 2nd order polynomial to the histogram values within 5 e-/s of the median, and then quoting the peak of the fitted polynomial. The 'Mean near peak' is the mean for pixels with a dark current within 5 e-/s of the median.

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 9.7 10.2 3495 1778 4 3

2000:326 22.2 11.5 11.9 34097 18442 28 10

2001:060 23.9 12.5 12.9 37381 19962 27 13

2002:057 29.2 12.5 13.6 56671 29657 46 19

2002:137 29.8 12.2 13.6 59754 31238 52 21

2002:237 31.1 11.9 13.6 64023 33701 62 21

2002:338 31.7 11.9 13.6 66850 34954 64 21

2003:116 33.6 12.8 14.5 71418 36831 76 21

2003:195 19.6 7.6 7.8 44179 14708 21 5

2004:002 21.6 7.8 8.4 51388 17339 23 6

2004:084 23.2 8.5 9.4 54534 18700 29 8

2004:171 22.7 7.7 8.4 55605 18941 26 6

2004:253 23.1 7.6 8.4 57613 19715 29 8

2004:359 25.7 9.0 10.3 61944 21270 30 6

2005:070 26.7 8.4 10.2 66981 23434 35 8

2005:191 31.1 12.2 14.3 72427 25014 35 7

2005:247 27.6 8.6 9.5 72060 25309 36 5

2005:316 27.8 8.7 9.5 72420 25098 36 8

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.

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. 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 break in behaviour in 2003 is clearly seen.

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.

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.

Cumulative histogram

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

Implications

The CCD cooldown to -15C, which reduced the overall dark current by about a factor of 2, has effectively removed almost 2 years of CCD dark current degradation, as measured by the fraction of warm pixels on the CCD. However, the latest dark current histograms are 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

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 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	9.71	0.00	9.71
2000:326	1.33	88.27	11.47	0.95	10.52
2001:060	1.60	205.59	12.54	2.22	10.32
2002:057	2.59	193.85	12.52	2.09	10.43
2002:137	2.81	162.67	12.16	1.76	10.41
2002:237	3.08	91.43	11.91	0.99	10.92
2002:338	3.36	89.92	11.90	0.97	10.93
2003:116	3.75	219.88	12.78	2.37	10.40
2003:195	3.97	97.64	7.65	1.05	6.59
2004:002	4.44	103.58	7.75	1.12	6.63
2004:084	4.66	261.60	8.53	2.83	5.71
2004:171	4.90	112.91	7.73	1.22	6.51
2004:253	5.13	97.93	7.62	1.06	6.56
2004:359	5.42	193.01	8.99	2.08	6.90
2005:070	5.63	194.16	8.38	2.10	6.28
2005:191	5.96	432.00	12.17	4.67	7.50
2005:247	6.11	144.44	8.62	1.56	7.06
2005:316	6.30	154.76	8.66	1.67	6.99

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 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. Again, see Parameters from Zodiacal Light Inquiry for more information. Future work may involve finding the best model to explain the degredation over time.

Aspect Information main page

Comments or questions: Aspect Help

Last modified: 12/27/13