Space Telescope Science Institute   4.3.4 Dark Current  4.3.6 Cosmic Rays

4.3.5 Warm and Hot Pixels


In the presence of a high electric field, the dark current of a single pixel can be greatly enhanced. Such pixels are called dark spikes or hot pixels. Although the increase in the mean dark current with proton irradiation is important, of greater consequence is the large increase in dark current nonuniformity.

We have chosen to classify the field-enhanced dark spikes into two categories: warm and hot pixels. The definition of "warm" and "hot" pixel is somewhat arbitrary. We have chosen a limit of 0.08 e-/pixel/seconds as a threshold above which we consider a pixel to be "hot". We identify "warm" pixels as those which exceed by about 5  (~0.02 e-/pixel/second) the normal distribution of the pixels in a dark frame up to the threshold of the hot pixels (See Figure 4.8) for a typical dark rate pixel distribution)

Warm and hot pixels accumulate as a function of time on orbit. Defects responsible for elevated dark rate are created continuously as a result of the ongoing displacement damage on orbit. The number of new pixels with a dark current higher than the mean dark date increases every day by few to several hundreds depending on the threshold.


Table 4.3: Creation rate of new hot pixels (pixel/day).
Threshold (e-/pixel/second)
WFC
HRC
0.02
815 ± 56
125 ± 12
0.04
616 ± 22
96 ± 2
0.06
480 ± 13
66 ± 1
0.08
390 ± 9
48 ± 1
0.10
328 ± 8
35 ± 1
1.00
16 ± 1
1 ± 0.5

Most of these new hot pixels are transient. Like others CCDs on HST, the ACS devices undergo a monthly annealing process. The CCDs and the thermal electric coolers (TECs) are turned off and the heaters are turned on to warm the chips to ~19 ºC. Although the annealing mechanism at such low temperatures is not yet understood, after this "thermal cycle" the population of hot pixels is greatly reduced (see Figure 4.6). The anneal rate depends on the dark current rate with very hot pixels being annealed easier than warmer pixels. For pixels classified as "hot" (those with dark rate > 0.08 e-/pix/sec) the anneal rate is ~82% for WFC and ~86% for HRC.

Annealing has no effect on the normal pixels that are responsible for the increase in the mean dark current rate. Such behavior is similar to what is seen with STIS SITe CCD and WFC3 CCDs during ground radiation testing.

Figure 4.6: Hot pixel growth in the WFC CCDs.

 
The vertical dashed lines indicate the annealing dates.
 

During early annealing episodes the ACS CCDs were warmed up for 24 hours. But the annealing time was reduced to 6 hours in spring of 2004 to allow better scheduling of HST time. Even with a shorter cycle the effectiveness of the anneal in ACS CCDs has remained the same. It is interesting to note that since ACS launch, four HST safing events have occurred; after each event the population of hot pixels was reduced as if a normal anneal cycle has occurred. During the safing events the CCDs and the TECs were turned off. Since the heaters were not turned on, the CCDs warmed-up to only about -10 ºC. After a period of time ranging from 24 to 48 hours, HST resumed normal operation. The dark frames taken after the safing events showed a reduction in hot pixel population similar to those observed during normal annealing cycles.

Since the anneals cycle do not repair 100% of the hot pixels, there is a growing population of permanent hot pixels (see Figure 4.7 and Figure 4.8).

Figure 4.7: A subsection of WFC1 dark frames taken at different epochs showing the increasing population of hot pixels.

 
From left to right: before launch, and after 1, 2 and 3 years on orbit.
 
Figure 4.8: Histogram of WFC dark frames taken at different epochs.

 
Epochs pre-launch, and after 1,2, and 3 years on orbit. Both the mean dark rate increase and the growing population of permanent hot pixels are visible.
 

According to the current trend, about 0.3% of WFC (0.22% of HRC) pixels became permanently hot every year (see Table 4.4). In a typical 1000 second exposure the percentage of pixels contaminated by cosmic rays ranges between 1.5% and 3% of the total. By spring of 2007 the contamination from hot pixels will impact about 2% of the pixels (See Figure 4.9.).


Table 4.4: Permanent warm and hot pixel growth (%/yr).
Threshold (e-/pix/sec)
WFC
HRC
0.02
174
1.52
0.04
0.86
0.50
0.06
0.50
0.31
0.08
0.30
0.22
0.10
0.25
0.18
1.00
0.04
0.03

In principle, warm and hot pixels could be eliminated by the superdark subtraction. However, some pixels show a dark current that is not stable with time but switches between well defined levels. These fluctuations may have timescales of a few minutes and have the characteristics of random telegraph signal (RTS) noise. The dark current in field-enhanced hot pixels can be dependent on the signal level, so the noise is much higher than the normal shot noise. As a consequence, since the locations of warm and hot pixels are known from dark frames, they are flagged in the data quality. The hot pixels can be discarded during image combination if multiple exposures have been dithered.

While the standard CR-SPLIT approach allows for cosmic ray subtraction, without additional dithering, it will not eliminate hot pixels in post-observation processing.

We recommend that observers who would have otherwise used a simple CR-SPLIT now use some form of dithering instead. Any form of dithering providing a displacement of at least a few pixels can be used to simultaneously remove the effects of cosmic ray hits and hot pixels in post-observation processing.

For example, a simple ACS-WFC-DITHER-LINE pattern has been developed, based on integer pixel offsets, which shifts the image by 2 pixels in X and 2 pixels in Y along the direction that minimizes the effects of scale variation across the detector. The specific parameter values for this pattern are given in the Phase II Proposal Instructions.

Given the transient nature of hot pixels, users are reminded that few hot pixels may not be properly flagged in the data quality array (because they spontaneously "healed" or because their status changed in the period spanning the reference file and science frame acquisition), and therefore could create false positive detections in some science programs.

Figure 4.9: Growth of permanent warm and hot pixel population as a function of time (WFC top, HRC bottom).

 

The contamination level due to cosmic rays accumulated in a 1000 second exposure is shown with a dashed line.
 

 4.3.4 Dark Current  4.3.6 Cosmic Rays
Space Telescope Science Institute
http://www.stsci.edu
Voice: (410) 338-1082
help@stsci.edu