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
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).
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
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
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 3.8 and 3.6 e−
/pixel/hour/year for WFC1 and WFC2 respectively, and 2.1 e−
/pixel/hour/year for the HRC CCD.
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
). As of September 2014, we measure a dark current of 48 to 53 e-/pixel/hour among the four amplifiers.
We have reverted to 12-hour anneals.
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.
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.
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.
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:
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.
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.
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 CTE loss as of 2015 is ~10% for a fiducial 100 e- star at the center of the CCD with a typical background of 30e-. This loss can rise to >50% in worst cases (faint sources, low backgrounds, farthest from the CCD serial register), rendering faint sources undetectable and thus unrecoverable with current pixel-based CTE mitigation algorithms.
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
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).
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
In the optical, each photon generates a single electron. However, in the near-UV, shortward of ~3200 Å there is a finite probability of creating more than one electron per UV photon (see Christensen, O., J. App. Phys
, 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 Å)), 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.