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ACS Data Handbook > Chapter 4: ACS Data Processing Considerations > 4.3 Dark Current, Hot Pixels, and Cosmic Rays

4.3
4.3.1 Dark Current
The procedure for creating ACS dark reference files and applying dark subtraction to ACS science data is described in detail in Section 3.4.4. Because it takes a few weeks to collect enough frames to create the dark reference file, the “best” dark reference file is typically not available in the pipeline until 2 to 3 weeks after the date of observation. Users may verify whether the dark reference file most appropriate to their observations has been delivered for pipeline use by checking the ACS reference file Web pages:
http://www.stsci.edu/hst/observatory/cdbs/SIfileInfo/ACS/
reftablequeryindex
Using an old dark reference file will produce a poor dark correction, either leaving too many hot pixels uncorrected and unflagged, or creating many negative “holes” caused by the correction of hot pixels which were not actually hot in the science data (i.e., if the detectors were annealed in the interim).
WFC and HRC
The dark current is not constant across the CCDs. Figure 4.6 shows dark current features in the WFC1 (above) and WFC2 (below) chips. These features were observed in pre-flight tests, and have generally remained stable in orbit.
There is a gradient, most noticeable on the WFC1 chip, going from a dark edge in the amplifier A quadrant (upper left) to a bright corner in the amplifier B quadrant (upper right). There are two horizontal bright bands of elevated dark current in the center of the WFC2 chip. Many faint rings are also visible, all concentric with the center of both chips. These features are likely intrinsic to the chips themselves, artifacts embedded in (or on) the silicon during various stages of the CCD manufacturing process.
Figure 4.6: High S/N Combination of WFC Dark Frames Illustrating Dark Current Structure
The WFC1 and HRC histograms in Figure 4.7 and Figure 4.8 show the growth of hot pixels (for more information, please refer to Section 4.3.2). A less obvious result is that the peak of the normal pixel (Poisson) distribution (i.e., the mean dark current, excluding the hot pixels) has also increased.
The increase in mean dark current for WFC has gone from 6.8 e/pixel/hour at launch in March 2002 to 11.1 e/pixel/hour (an average of 11.4 for WFC1 and 10.8 for WFC2) in April 2004. Following SM4, a dark current of 20 to 25 e/pixel/hour was measured.
For HRC the change in dark current has been from 9.3 e/pixel/hour at launch to 13.4 e/pixel/hour in April 2004. The dark calibration tracked the mean dark current very closely at 2 week intervals (see Section 3.4.4).
Figure 4.7: WFC Dark Current Histogram for WFC1
Statistics for WFC2 are nearly identical. The figures illustrate the growth of hot pixels over time. Decrease in dark current, and in the number of hot pixels, were seen when the WFC temperature was changed from -77C to -81C on July 4, 2006 (note differences shown for June and July 2006). For post-SM4, dark current in January 2010 (-81C) was almost identical to June 2006 (-77C), but the former still had fewer hot pixels.
Figure 4.8: HRC Dark Current Histogram
Data from the first (March 2002) and the last (January 2007) superdarks are shown. Dark current and hot pixels increased over time, as expected. Unlike Figure 4.7, HRC was not affected by the temperature change in WFC.
SBC
The SBC, or ACS MAMA detector, intrinsically has no read noise and very low detector noise levels which normally will be negligible compared to statistical fluctuations. Dark frames for SBC are typically taken 1 to 2 times per year to monitor their levels. The dark rate for these is typically 2.0x10-5 counts/pixel/second.
Summed SBC dark images are delivered to CDBS from time to time. These are only valid for a period within 2 hours of the SBC turn-on because the temperature will increase with the time that the SBC is turned on, which causes the dark current to rise (see Figure 4.9). However, the dark current remains very low, even if the SBC has been switched on for as long as 6 hours. The principal reason for taking these images is to provide a measure of the health of the SBC. No major fluctuation has been noted. Therefore, the dark reference files available in CDBS are not used in the pipeline because the correction is negligible.
Figure 4.9: SBC Dark Rate and Operating Temperature
Top panel: SBC dark rate (counts/sec/pixel) as a function of operating temperature (C).
Bottom panel: the operating temperatures vs. time elapsed after SBC is switched on.
Red crosses and green circles are data from May 2002 and March 2010, respectively. Dark rates have remained stable since ACS activation in 2002 and are unaffected by SM4. Dark rates for both epochs are stable for operating temperatures below ~28 C (for about ~4 hours after being switched on).
4.3.2 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 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 non-uniformity.
Field-enhanced pixels have been classified into two categories: warm and hot pixels. The definition of “warm” and “hot” pixel is somewhat arbitrary. A limit of 0.08 e/pixel/seconds was chosen as a threshold above which a pixel is considered as “hot.” “Warm” pixels are identified as pixel values that exceed, by about 5σ, the normal distribution of the pixels in a dark frame, up to the threshold of the hot pixels (see Figure 4.7) for a typical dark rate pixel distribution—this used to be 0.02 e/pixel/second but after SM4 it is 0.04 e/pixel/second).
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 rate increases every day by few to several hundreds depending on the threshold. The reduction of the operating temperature of the WFC CCDs has dramatically reduced the dark current of the hot pixels and therefore many pixels previously classified as hot are now warm or normal pixels.
Table 4.4: Creation Rate of New Hot Pixels (Pixel/Day)
(e/pixel/second)
(-77C)
815  56
125  12
616  22
427  34
96  2
480  13
292  8
66  1
390  9
188  5
48  1
328  8
143  12
35  1
16  1
10  1
1  0.5
Table 4.5: Annual Permanent Hot Pixel Growth (%)
(e/pixel/second)
> 0.02
> 0.04
> 0.06
> 0.08
> 0.10
> 1.00
Most of these new hot pixels are transient. Like other CCDs on HST, the ACS devices undergo a monthly annealing process. (The CCDs and the thermal electric coolers are turned off and the heaters are turned on to warm the CCDs to ~19C.) 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 4.10). The anneal rate depends on the dark current rate; very hot pixels are annealed more easily 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 was ~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 was also seen with STIS and WFC3 CCDs during ground radiation testing. Since the anneal cycles do not repair 100% of the hot pixels, there is a growing population of permanent hot pixels (see Figure 4.7).
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 array. The hot pixels can be discarded during image combination if multiple exposures have been dithered.
The standard “CR-SPLIT” approach allows rejection of cosmic rays, but hot pixels cannot be eliminated in post-observation processing without dithering between exposures.
Given the transient nature of hot pixels, users are reminded that a 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 4.10: Hot Pixel Growth Rate for HRC and WFC
These figures show hot pixel growth rates (DQ flag 16) in the WFC and HRC. The sawtooth patterns correspond to anneal cycles. For HRC, the growth rate increased slightly when the anneal duration was reduced from 12 hours to 6 hoursa slight drop coincided with the switch to Side 2 electronics. In the lower figure for WFC, sawtooth patterns also correspond to anneal cycles. The growth rate slightly increased when the anneal duration was switched from 12 hours to 6 hours. Post-SM4 anneals are back to12 hours long. The temperature change from -77C to -81C significant reduced hot pixels.
4.3.3 Cosmic Rays
Like all HST cameras before it, the ACS HRC and WFC images are heavily peppered by cosmic rays in even the shortest of exposures. For full orbit integrations, approximately 5% of the pixels receive significant charge from cosmic rays via direct deposition or from diffusion from nearby pixels. Great care must be taken in planning and analyzing HST ACS observations to minimize the impact of cosmic rays on science images.
Many science observations require a careful consideration of individual cosmic ray events. To either remove cosmic rays or distinguish them from astrophysical sources, users might consider the distributions of observed cosmic ray fluxes, sizes, anisotropies, and the number of attached pixels per event.
Fractional Coverage
For most users of the HRC and WFC, the most important characteristic of cosmic rays is simply the fraction of pixels they impact. This number provides the basis for assessing the risk that the target(s) in any set of exposures will be compromised. For ACS the observed rate of cosmic ray impacts on an individual frame varies by a factor of two depending on the proximity of the spacecraft to the confluence of the Earth’s magnetic field lines (e.g., the South Atlantic Anomaly). For a 1000 second exposure, the fraction of pixels affected by cosmic rays (in non-SAA passages) varies between 1.5% and 3%. This fraction is the same for the WFC and HRC despite their factor of two difference in pixel areas because the census of affected pixels is dominated by charge diffusion, not direct impacts. This fraction is also consistent with what was observed for WFPC2.
For most science observations, a single “CR-SPLIT” (i.e., two exposures) is sufficient to insure that measurements of the targets are not compromised by cosmic rays. More consideration is required for survey-type observations with WFC, a bonafide survey instrument. Observers seeking rare or serendipitous objects as well as transients may require that every single WFC pixel in at least one exposure among a set of exposures is free from cosmic ray impacts. For the cosmic ray fractions of 1.5% to 3% in 1000 seconds, a single ~2400 second orbit must be broken into 4 exposures (4 “CR-SPLIT”s of 500 to 600 seconds each) to reduce the number of uncleanable pixels to 1 or less. (It is also recommended that users dither these exposures to remove hot pixels.)
Electron Deposition
The flux deposited on the CCD from an individual cosmic ray does not depend on the energy of the cosmic ray but rather the length it travels in the silicon substrate. As a result, the deposition distribution has a well-defined minimum with few events of less than 500 electrons (where such low-electron events correspond to cosmic rays which pass through the CCD at a normal angle of incidence). As seen in Figure 4.11, the median charge deposited for WFC and HRC is about 1000 electrons, the same as for WFPC2.
A useful characteristic of the deposition distribution is its well-defined minimum; e.g., multi-pixel events which have an apparent magnitude of 25th or fainter, in a 500 second broad-band exposure, are unlikely to be caused by cosmic rays. Such information can help with the removal of false positives from searches for faint transients (e.g., high-redshift SNe).
Figure 4.11: Electron Deposition by Cosmic Rays on HRC (top) and WFC (bottom)
A minimum deposition of ~500 e is seen corresponding to cosmic rays with normal incidence. The median deposition is ~1000 e.
Attached Pixels
As seen in Figure 4.12, for HRC and WFC, the salient features of electron deposition are a strong peak in the distribution function at 4 to 5 pixels. On the smaller side there is a sharp decline in events. Although a few events are seen which encompass only one pixel, examination of these events indicate that at least some and maybe all of these sources are actually transient hot pixels or unstable pixels which can appear hot in one exposure (with no charge diffusion) and normal in the next. Such pixels are very rare but do exist. There is a long tail in the direction towards increasing numbers of attached pixels. Some of these are likely due to two individual events associated by their chance superposition, but more are from oblique incidence cosmic rays which skim the surface of the CCD leaving a long trail (which is wider near the surface). Unfortunately the number of attached pixels is not a very useful characteristic to distinguish cosmic rays from unresolved astrophysical sources.
Figure 4.12: Distribution of the Number of Pixels Associated with a Single Cosmic Ray Event for the HRC (top) and WFC (bottom)
Some bias exists for events > 6 pixels which may be composed of two events with chance superposition.

ACS Data Handbook > Chapter 4: ACS Data Processing Considerations > 4.3 Dark Current, Hot Pixels, and Cosmic Rays

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