ACA Dark Current Calibration 2002-Dec-4

On 2002-Dec-4, 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. This gave a total of 10 full frame readouts. For a given 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).

Comparison to previous calibrations

Previous calibrations have been done on 1999-Aug-11 (during Orbital Activation and Checkout (OAC) with the SSD closed), 2000-Nov-21 (2000:326), 2001-Mar-01 (2001:060), 2002-Feb-26 (2002:057), 2002-May-17 (2002:137), and 2002-Aug-25 (2002:237). In the following plots we compare the 2002-Dec-4 (2002:338) data to the previous datasets.
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-/sec of the median, and then quoting the peak of the fitted polynomial. The 'Mean near peak' is represents the mean for pixels with a dark current within 5 e-/sec of the median.

   1999-Aug-11 2000-Nov-20 2001-Mar-01 2002-Feb-26 2002-May-17 2002-Aug-25 2002-Dec-04
Mean (e-/s) 11.9 22.2 23.9 29.2 29.8 31.0 31.7
Peak (e-/s) 10.3 11.5 12.5 12.5 12.2 11.9 11.9
Mean near peak (e-/s) 10.2 11.8 12.8 13.6 13.6 13.6 13.6
N > 100 e-/s 3495 34346 37381 56671 59754 64023 66850
N > 3000 e-/s 3 10 13 19 21 21 21

The statistics above show that there has been little change in the peak dark current between OAC and the present time. The incidence of "warm" ( > 100 e-/sec) and "hot" pixels (> 3000 e-/sec) appears to be increasing fairly linearly since launch. 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.

Differential histogram
The plot below shows the differential distribution of dark current values, in number of pixels per 1.0 e-/sec bin.   The factor of 10 increase in warm pixels since OAC is evident in the tail above ~30 e-/sec.

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 rate of increase of warm pixels is about 20000 per year, or 2.0% of the focal plane per year. This is probably an upper limit to the rate, since we have been going through the Solar max. At this rate, there should no problems with ground aspect determination nor on-board star acquisition in the next 2-3 years. However, in the 5-10 year time frame, warm pixels could make up 10-20% of the CCD focal plane. With sufficient operational effort, dark current maps will allow subtraction of warm pixels and hence high-quality ground aspect solutions. A bigger concern is on-board acquisition of faint stars in the presence of many warm pixels. The magnitude of this potential problem is difficult to determine, since it depends on details of the ACA search/acquisition algorithm. A simulation of this algorithm is likely the best way to address this issue.
Brett Unks

Last modified: 12/27/13