The procedure for creating ACS dark reference files and applying dark subtraction
to ACS science data is described in detail in Section 3.4.3
. 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
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:
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).
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.
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
/pixel/hour at launch in March 2002 to 11.1
/pixel/hour (an average of 11.4 for WFC1 and 10.8 for WFC2) in April 2004. Following SM4, we measured a dark current of 20
For HRC the change in dark current has been from 9.3
/pixel/hour at launch to 13.4
/pixel/hour in April 2004. The dark calibration tracked the mean dark current very closely at 2 week intervals (see Section 3.4.3
The SBC, or ACS MAMA detector, intrinsically has no readout 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
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.
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.
We have chosen to classify the field-enhanced pixels 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
/pixel/seconds as a threshold above which we consider a pixel to be “hot.” We identify “warm” pixels as those which 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
/pixel/second but after SM4 it is 0.04
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.
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 are turned off and the heaters are turned on to warm the CCDs to ~19°
C.) 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 >
/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 anneals cycle 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.
The figures below show hot pixel growth rates (DQ flag 16) in the WFC, and in the HRC during its entire mission. The saw
tooth patterns correspond to ACS anneal cycles. In the HRC, the growth rate increased slightly when the anneal duration was switched from 12 hours to 6 hours. The slight drop in hot pixels coincided with the switch to Side 2 electronics. In the lower figure for WFC, the sawtooth patterns also correspond to ACS anneal cycles. The growth rate slightly increased when the anneal duration was switched from 12 hours to 6 hours. Post-SM4 anneals are once again 12 hours long. The temperature change from -77°C to -81°C resulted in a significant drop in hot pixels.
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.
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 is 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.)
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).
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.