ACA Dark Current Calibration 2004-Dec-24

On 2004-December-24, 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. This calibration was the sixth ACA dark current calibration since the ACA CCD operating temperature was reduced to -15C in July 2003.

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.

Date 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-Aug-11 11.9 9.7 10.2 3495 1778 4 3

2000-Nov-20 22.2 11.5 11.9 34097 18442 28 10

2001-Mar-01 23.9 12.5 12.9 37381 19962 27 13

2002-Feb-26 29.2 12.5 13.6 56671 29657 46 19

2002-May-17 29.8 12.2 13.6 59754 31238 52 21

2002-Aug-25 31.1 11.9 13.6 64023 33701 62 21

2002-Dec-04 31.7 11.9 13.6 66850 34954 64 21

2003-Apr-26 33.6 12.8 14.5 71418 36831 76 21

2003-Jul-14 19.6 7.6 7.8 44179 14708 21 5

2004-Jan-02 21.6 7.8 8.4 51338 17339 23 6

2004-Mar-24 23.2 8.5 9.4 54534 18700 29 8

2004-Jun-19 22.7 7.7 8.4 55605 18941 26 6

2004-Sep-09 23.1 7.6 8.4 57613 19715 29 8

2004-Dec-24 25.7 9.0 10.3 61944 21270 30 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, and also since then. 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 calibrations since mid-2003 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.

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Last modified: 12/27/13