Results of cooling the CCD to -15C

In an attempt to mitigate the effects of an ever increasing CCD dark current, the ACA CCD has been progressively cooled from -10C to -15C. Manufacturer's analysis predicted that this cooling would result in a 50% decrease in the overall dark current, decreasing the amplitude of warm and hot pixels that are impacting both star selection and post-facto aspect processing.

The CCD was cooled in -1C increments using CAP 843 on May 8, May 27, June 12, June26, and July 8, 2003. This progressive cooling allowed for monitoring of both the decrease in dark current and the overall health of the ACA. See previous reports.

Dark current calibrations were performed on April 26, 2003 and July 14, 2003, immediately before and after the cooling process. This allows an analysis of the effects of the reduced temperature over the entire CCD. The August 11, 1999 OAC calibration is included in these analyses as a reference.

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' represents the mean for pixels with a dark current within 5 e-/sec of the median.

   1999-Aug-11 2003-Apr-26 2003-Jul-14
Mean (e-/s) 11.9 33.6 19.6
Peak (e-/s) 9.7 12.8 7.6
Mean near peak (e-/s) 10.2 14.5 7.8
N > 100 e-/s 3495 71418 44179
N > 200 e-/s 1778 36831 14708
N > 2000 e-/s 4 76 21
N > 3000 e-/s 3 21 5

The statistics above show the effect of cooling the CCD. The peak dark current is back down to launch levels. The number of warm pixels (>100 e-/s) has shifted down by 40 percent. The number of very hot pixels (>3000 e-/sec) has been quartered. Below is a plot of the fraction of warm pixels, including all dark current calibrations. Previous calibrations put the increase in warm fraction at 1.9% of the CCD (20000 pixels) per year.

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. Even though the mean has returned to launch levels, it is clear that the fraction of warm pixels is still quite high.

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.

Evaluating Cooling Effectiveness

Below is the histogram of the fractional change in dark current between the April and July calibrations. The peak in the histogram (fit to a polynomial) is at 0.50. The width around the peak of the distribution is consistent with read noise, and the positive tail is likely the result of continuing damage to the CCD.

The above scatter plot shows the starting (2003:116) dark current against the fractional change. This plot matches fairly well with the distribution of fractional changes, indicating that the CCD dark current decreased uniformly. There does, however, appear to be a slight shift in the effectiveness of cooling the hottest pixels (>2000 e-/sec), where the mean decrease is 43%. We see an increased dark current in 3915 pixels to greater than 100 e-/sec, consistent with the damage rate from previous calibrations. The cyclic pattern below around 40 e-/sec is an artifact of log scale plotting of the integer dark current.

ACA Health



The above plot shows several diagnostic plots that are used to gauge the well-being of the ACA. The CCD temp is seen to step in 1C increments, corresponding to steps in the TEC set point. The large jump in the ACA housing is most likely due to the recent activation of IRU-B and is expected to return to previous levels when IRU-A is deactivated. The trend of TEC set point vs ACA - CCD temperature is observed to be fairly linear with the possibility of a slight upturn at higher differential temperatures. The spacecraft bus current has also maintained nominal levels.

Implications

Cooling the CCD and thus lowering the dark current should have a dramatic effect on the performance of the ACA. We will see an improvement in the OBC acquisition of faint stars due to a decrease in the population of warm pixels. The star selection process will have fewer hot pixels to contend with, and thereby increase the area viable for guide stars (especially valuable in sparsely populated fields). Post-facto processing will also benefit from a lower number guide star centroids perturbed by warm pixels.

Future decreases in the CCD temperature may be needed to periodically boost the performance of the ACA, and should certainly be possible if needed. Analysis indicates that the TEC set point could be driven to produce a CCD temperature as low as -30C.


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email:Brett Unks

Last modified: 12/27/13