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Advanced Camera for Surveys Instrument Handbook for Cycle 22 > Chapter 4: Detector Performance > 4.3 CCD Operations and Limitations

4.3
WFC
4.3.1 CCD Saturation: The CCD Full Well
 
The average full well depths for the ACS CCDs are given in Table 3.1 as 84,700 e for the WFC and 155,000 e for the HRC, but the pixel-to-pixel values vary by ~10% and ~18% across the fields of view of the WFC and the HRC, respectively. When the CCD is over-exposed, pixels will saturate and excess charge will flow into adjacent pixels along the column. This condition is known as “blooming” or “bleeding.” Extreme overexposure does not cause any long-term damage to the CCDs, so there are no bright object limits for the ACS CCDs. When using ADC gains of 2 for the WFC and 4 for the HRC, the linearity of the detectors deviates by less than 1% up to the full well depths. On-orbit tests using aperture photometry show that this linearity is maintained when summing over pixels that surround an area affected by charge bleeding, even when the central pixel is saturated by 10 times its full well depth (see ACS ISR 2004-01 for details).
4.3.2 CCD Shutter Effects on Exposure Times
The ACS camera has a very fast shutter; even the shortest exposure times are not significantly affected by the finite travel time of the shutter blades. On-orbit testing reported in ACS ISR 2003-03 verified that shutter shading corrections are not necessary to support 1% relative photometry for either the HRC or WFC. We note that the commanding overheads of WFC introduce an additional ~3.0 seconds of dark current integration time beyond the commanded exposure time. This additional dark-time is included in the dark-current subtraction stage of the CALACS pipeline (see the ACS Data Handbook for further details).
Four exposure times are known to have errors of up to 4.1%; e.g., the nominal 0.1 seconds HRC exposure was really 0.1041 seconds. These errant exposure times are accommodated by updates to the reference files used in image pipeline processing. No significant differences exist between exposure times controlled by the two shutters (A and B), with the possible exception of non-repeatability up to ~1% for WFC exposures in the 0.7 to 2.0 second range. The HRC provided excellent shutter time repeatability.
4.3.3 Read Noise
 
WFC
The read noise levels in the physical overscan and imaging regions of the four WFC amplifiers were measured for all gain settings during the orbital verification period following the installation of the CEB-R in SM4. The average WFC read noise is about 25% lower than before SM4. This reduction is due to the use in the CEB-R of a dual-slope integrator (DSI) in the pixel signal processing chain instead of the clamp-and-sample method of pixel sampling used in the original CEB. The DSI method offers lower signal processing noise at the expense of a spatially variable bias level.
In Figure 4.4, the read noise of the four WFC amplifiers are plotted versus date. The read noise values are steady to 1% or better, except for four discrete events during the ACS lifetime. Two of these events were associated with ACS electronics changes: 1) a jump in the amplifier C noise after the 2006 failure of the side-1 electronics; 2) the overall drop in noise once the ACS was repaired during SM4 in 2009. The two other events, 20-30% noise increases in amplifier A (June 2003) and amplifier D (January/February 2013), have been ascribed to radiation damage on the detector. Fluctuation read noise levels stabilized after subsequent anneals (see Section 4.3.5).
Although amplifier D now has significantly elevated read noise compared with the other amplifiers, most WFC broad-band observations remain sky-limited rather than read-noise limited. Note also that the WFC pre-defined subarray apertures all use the lowest-noise amplifier B.
HRC
Prior to January 2007, the read noise of the HRC was monitored using only the default amplifier C and the default gain of 2 e/DN. No variations were observed with time. The read noise measured in the physical overscan and image areas were consistent with the pre-flight values of 4.74 e (see Table 4.2 and Table 4.3).
Figure 4.4: History of ACS/WF read noise
Read noise history of the four read-out amplifiers of the ACS Wide Field Channel. Amplifiers A and B read out WFC1; amplifiers C and D read out WFC2 (see Figure 7.3 for schematic). These values are measured near the serial register using calibration bias frames taken regularly throughout the ACS lifetime. See text for discussion.
4.3.4 Dark Current
 
All ACS CCDs are buried channel devices which have a shallow n-type layer implanted below the surface to store and transfer the collected signal charge away from the traps associated with the Si-SiO2 interface. Moreover, ACS CCDs are operated in Multi-pinned Phases (MPP) mode so that the silicon surface is inverted and the surface dark current is suppressed. ACS CCDs therefore have very low dark current. The WFC CCDs are operated in MPP mode only during integration, so the total dark current figure for WFC includes a small component of surface dark current accumulated during the readout time.
Like all CCDs operated in a Low Earth Orbit (LEO) radiation environment, the ACS CCDs are subject to radiation damage by energetic particles trapped in the radiation belts. Ionization damage and displacement damage are two types of damage caused by protons in silicon. The MPP mode is very effective in mitigating the damage due to ionization such as the generation of surface dark current due to the creation of trapping states in the Si-SiO2 interface. Although only a minor fraction of the total energy is lost by a proton via non-ionizing energy loss, the displacement damage can cause significant performance degradation in CCDs by decreasing the charge transfer efficiency (CTE), increasing the average dark current, and introducing pixels with very high dark current (hot pixels). Displacement damage to the silicon lattice occurs mostly due to the interaction between low energy protons and silicon atoms. The generation of phosphorous-vacancy centers introduces an extra level of energy between the conduction band and the valence band of the silicon. New energetic levels in the silicon bandgap have the direct effect of increasing the dark current as a result of carrier generation in the bulk depletion region of the pixel. As a consequence, the dark current of CCDs operated in a radiative environment is predicted to increase with time.
Ground testing of the WFC CCDs, radiated with a cumulative fluence equivalent to 2.5 and 5 years of on-orbit exposure, predicted a linear growth of ~1.5 e/pixel/hour/year. The dark current in ACS CCDs is monitored three days per week with the acquisition of four 1000 seconds dark frames (totaling 12 images per week). Dark frames are used to create reference files for the calibration of scientific images, and to track and catalog hot pixels as they evolve. The four daily frames are combined together to remove cosmic rays and to extract hot pixel information for any specific day. The dark reference files are generated by combining two weeks of daily darks in order to reduce the statistical noise. The hot pixel information for a specific day is then added to the combined bi-weekly dark. In order to study the evolution of the dark current with time, the modal dark current value in the cosmic-ray free daily darks is calculated. As expected, the dark current increases with time. The observed linear growth rates of dark current are 2.1 and 1.6 e/pixel/hour/year for WFC1 and WFC2 respectively, and 2.1 e/pixel/hour/year for the HRC CCD. These rates are in general agreement with the ground test predictions.
At the beginning of the side-2 operation in July 2006 the temperature set point of the WFC was lowered from -77 C to -81 C (documented in ACS TIR 2006-02). Following SM4, we measure a dark current of 22 - 25 e/pixel/hour among the four amplifiers. We have reverted to 12-hour anneals.
The dark current growth with time post-SM4 is measured to be 2.2 and 1.3  e-/pixel/hour/year for the periods 2009-10 and 2010-11 for both CCDs. Dark current and hot pixels depend strongly on the operating temperature. The reduction of the operating temperature of the WFC CCDs reduced the number of hot pixels by almost 50% (See Figure 4.5). The dark rate shows a clear drop on July 4, 2006, when the temperature was changed. Figure 4.6 illustrates the before-and-after effect directly. The new operating temperature brought the dark current of the WFC CCDs back to the level eighteen months after the launch.
Figure 4.5: Hot Pixel Growth Rate for HRC and WFC
The figures above show hot pixel (DQ flag 16) growth rates in the WFC, and in the HRC during most of its entire mission. The sawtooth 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.
Figure 4.6: WFC Dark Current Histogram for Chip 1
The lines illustrate both the global rise in WFC dark current over time, and also the increased incidence of hot pixels. Decrease in dark current and in the number of hot pixels, were seen when the WFC temperature was changed from -77 C to -81 C on July 4, 2006 (note differences shown for June and July 2006). By June 2013 (cyan histogram), the mode of the dark current had surpassed the June 2006 values, and there were nearly as many hot pixels.
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 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 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 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 σ (was 0.02 e/pixel/second but after SM4 it is 0.04 e/pixel/second) the normal distribution of the pixels in a dark frame up to the threshold of the hot pixels (See Figure 4.6) 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 one-time reduction of the WFC operating temperature in 2006 dramatically reduced the overall dark current and hot pixel fraction but, by 2013, these values have degraded past their previous levels.
Table 4.4: Creation rate of new hot pixels (pixel/day).
Threshold (e/pixel/second)
815 56
125 12
616 22
427 34
96 2
480 13
66 1
48 1
35 1
16 1
1 0.5
Table 4.5: Annual permanent hot pixel growth (%).
Threshold (e/pixel/second)
0.52
0.21
Most hot pixels are transient. Like others CCDs on HST, the ACS WFC undergoes a monthly annealing process. The WFC CCDs and the thermal electric coolers are powered off, and heaters are powered 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 4.5). 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 and Figure 4.6).
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 1, 2, 3, and 4 years on orbit.
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.
For example, a simple ACS-WFC-DITHER-LINE pattern has been developed that 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 on the ACS dithering webpage at:
http://www.stsci.edu/hst/acs/proposing/dither.
Additional information can be found in the Phase II Proposal Instructions. 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.
4.3.6 Cosmic Rays
Studies have been made of the characteristics of cosmic ray impacts on the HRC and WFC. The fraction of pixels affected by cosmic rays varies from 1.5% to 3% during a 1000 second exposure for both cameras, similar to what was seen on WFPC2 and STIS. This number provides the basis for assessing the risk that the target(s) in any set of exposures will be compromised. The affected 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. 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 of 500 to 600 seconds each to reduce the number of uncleanable pixels to 1 or less. Users seeking higher S/N (lower read noise) may prefer the trade-off of doing 3 exposures of ~800 seconds, where CR-rejection should still be very good for most purposes. But we do NOT recommend 2 long exposures (i.e. 1200 seconds), where residual CR contamination would be unacceptably high in most cases. We recommend that users dither these exposures to remove hot pixels as well as cosmic rays (see Section 7.4).
The flux deposited on the CCD from an individual cosmic ray does not depend on the energy of the cosmic ray but rather the distance it travels in the silicon substrate. The electron deposition due to individual cosmic rays has a well defined cut-off below 500 e and a median of ~1000 e (see Figure 4.8 and Figure 4.9).
Figure 4.8: Electron deposition by cosmic rays on WFC.
Figure 4.9: Electron deposition of cosmic rays on HRC.
The distribution of the number of pixels affected by a single cosmic ray is strongly peaked at 4 to 5 pixels. 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.
Distributions of sizes and anisotropies can be useful for distinguishing cosmic rays from astrophysical sources in a single image. The size distribution for both chips peaks near 0.4 pixels as a standard deviation (or 0.9 pixels as a FWHM). This is much narrower than for a PSF and is thus a useful discriminant between unresolved sources and cosmic rays.
4.3.7 Charge Transfer Efficiency
 
Charge transfer efficiency (CTE) is a measure of how effectively the CCD moves charge between adjacent pixels during read out. A perfect CCD would be able to transfer 100% of the charge as the charge is shunted across the CCD and out through the serial register and its CTE would be unity. In practice, small traps in the silicon lattice compromise this process by holding on to electrons, releasing them at a later time. Depending on the trap type, the release time ranges from a few microseconds to several seconds. For large charge packets (several thousands of electrons), losing a few electrons along the way is not a serious problem, but for smaller (~100e or less) signals, it can have a substantial effect.
The CTE numbers for the ACS CCDs at the time of installation are given in Table 4.6. While the numbers look impressive, remember that reading out the WFC CCD requires 2048 parallel and 2048 serial transfers, so that almost 2% of the charge from a pixel in the corner opposite the readout amplifier was lost.
Table 4.6: Charge transfer efficiency measurements for the ACS CCDs after installation in March 2002. (Based on an experiment performed with Fe55)
Like other CCD cameras aboard HST, ACS has suffered degraded CTE due to radiation damage since its installation in 2002. Since 2003, specific calibration programs aimed at characterizing the effects of the CTE on stellar photometry have been performed. Results from the internal CTE calibration programs showed that CTE appears to decline linearly with time, and that CTE losses are stronger for lower signal levels (Mutchler & Sirianni, ACS ISR 2005-03). However, in order to derive an accurate correction formula for photometry, a number of observations at different epochs and for different levels of sky background and stellar fluxes have to be accumulated. The results of the observations taken through November 2011 are described in several ISRs (ACS ISR 2009-01, ACS ISR 2011-01, ACS ISR 2012-05) and are summarized here. For WFC, significant photometric losses are apparent for stars undergoing numerous parallel transfers (y-direction). The losses as of 2011 are between 5-10% for typical observing parameters, rising to ~50% in worst cases (faint stars, low background). The size of the photometric loss appears to have a strong power-law dependence on the stellar flux, as seen in other CCDs flown on HST.
The dependence on background is significant, and it is possible to increase the background by using the post-flash capability. This is unlikely to be useful for most observations, but may make a significant difference in observations with extremely low background.
It may also be useful, in some specific cases, to choose the filter in order to obtain a higher background (e.g. F606W). CTE degradation also has an impact on astrometry (see ACS ISR 2007-04). Therefore, for astrometric programs of relatively bright objects, the use of post-flash may be considered. No losses are apparent for WFC due to serial transfers (x-direction). Correction formulae are presented in ACS ISR 2012-05 to correct photometric losses as a function of a source’s position, flux, background, and time. Users are encouraged to check the ACS webpage for updates. Further details on CTE corrections of very bright stars on subarrays can be found in the ACS ISR 2011-01 (Bohlin & Anderson, 2011). The study of the CTE evolution is presented in ACS ISR 2012-03 (Ubeda & Anderson, 2012).
The predicted photometric losses (due to imperfect CTE) in observations carried out with the ACS/WFC may be calculated with the formulae in the ACS ISR 2012-05. Here we consider three “typical” science applications:
1.
2.
3.
A point source with a flux of 1 e/second observed during 600 seconds with a filter that gives a sky background level of 30 electrons has a magnitude change of 0.055 mag.
An online CTE correction calculator is available for users at the ACS website.
Anderson & Bedin (2010 PASP 122, 1035) have developed an empirical approach based on the profiles of warm pixels to characterize the effects of CTE losses for the ACS Wide Field Camera. Their algorithm first develops a model that reproduces the observed trails and then inverts the model to convert the observed pixel values in an image into an estimate of the original pixel values.
In 2012 a new and improved version of CALACS was released. This software version contains corrections for CTE degradation and the electronic artifacts introduced by the repair of the WFC during SM4, namely the bias shift effect (ACS ISR 2012-02), the bias striping effect (ACS ISR 2011-05), and the crosstalk effect (ACS ISR 2010-02). In addition to the standard data products (CRJ, FLT and DRZ files) there are three new files (CRC, FLC and DRC) which contain the CTE-corrected data products. Users are now able to choose whether to use the standard or CTE-corrected products. This new version of CALACS applies the CTE correction to the raw images and requires CTE-corrected dark reference files. The ACS Team has produced the dark reference frames for the entire ACS archive (ACS TIR 2012-02).
The latest version of the algorithm works very well for intermediate to high flux levels (> 200 electrons) and it has been greatly improved in order to become more effective at low flux levels (< 100 electrons). It also employs a more accurate time and temperature dependence for CTE over the ACS lifetime (ACS ISR 2012-03).
The results of the Anderson & Bedin correction on stellar fields are found to be in agreement with the photometric correction formula of Chiaberge et al. (ACS ISR 2012-05). Please refer to the ACS website for the latest information.
4.3.8 UV Light and the HRC CCD
In the optical, each photon generates a single electron. However, in the near-UV, shortward of ~3200  there is a finite probability of creating more than one electron per UV photon (see Christensen, O., J. App. Phys. 47, 689, 1976). At room temperature the theoretical quantum yield (i.e., the number of electrons generated for a photon of energy E > 3.5eV (λ~3500 )), is Ne = E(eV)/3.65. The HRC CCDs quantum efficiency curve has not been corrected for this effect. The interested reader may wish to see the STIS Instrument Handbook for details on the signal-to-noise treatment for the STIS CCDs.

Advanced Camera for Surveys Instrument Handbook for Cycle 22 > Chapter 4: Detector Performance > 4.3 CCD Operations and Limitations

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