May 1, 2012

About This Article

1. Release of CALACS 2012.2 and new CTE-corrected data products

A new version of the data calibration pipeline, CALACS has been released. It includes corrections for charge transfer efficiency (CTE) degradation and the electronic artifacts introduced by the repair of the Wide Field Channel (WFC) during Servicing Mission 4 (SM4). Users will see new data products when they retrieve ACS data from the Mikulski Archive for Space Telescopes (MAST).

Please note that to retrieve the new CTE-corrected data products, users should request "calibrated" data. The new extensions (see below) will not be present in the "Files Extensions Requested" menu until after the MAST database has been reprocessed, which will occur in July-August.

We anticipate a high demand from users wishing to re-process their ACS data in the next few weeks. The new CALACS takes ~ 40% longer to process an image and retrieval times may therefore be longer than normal. 

We ask that users who wish to submit large requests (more than 200 datasets) to please notify the Help Desk and we will ensure that these requests are handled properly by our processing and archive teams.

The new CALACS is also available under IRAFX. Users wishing to reprocess data themselves should request the raw and reference files from MAST, and then run CALACS under HSTCAL (not STSDAS) in IRAFX.

Pixel-based CTE correction

The pixel-based CTE correction is based on the original work of Anderson & Bedin (2010, PASP, 122, 1035) but has been modified to include the time and temperature dependence of CTE losses (Ubeda & Anderson, ACS ISR 2012-03). An improved correction at low signal and background levels has also been incorporated as well as a correction for column-to-column variations.

Bias shift, bias stripe and crosstalk corrections

The new CALACS contains corrections for three artifacts introduced by the new electronics installed during SM4. The “bias shift” (Golimowski et al. 2012, ACS ISR 2012-02) is a signal-dependent phenomenon associated with the CCDs’ external pre-amplifiers and the dual-slope integrators within the CCD Electronics Box Replacement. The pixel-to-pixel bias level of each pixel is offset by 0.02–0.3% of the pixel signal, and this offset decays slowly in the serial (horizontal) clocking direction. Such small bias shifts, while unimportant for most ACS imaging, can be a significant hindrance for studies of faint, extended sources in the proximity of very bright or highly exposed sources. All full frame post-SM4 ACS data are now corrected for this effect.

“Bias striping” is the second artifact introduced by the SM4 repair. It is a low-amplitude, horizontal striping caused by electronic “1/f” noise in a reference voltage inside the replacement electronics. Grogin et al. 2011 (ACS ISR 2011-05) recently described this effect—and how to correct for it using the image area. 

The standard deviation of the bias striping is 0.9 e–, compared with the WFC read noise of ~4 e–. The new CALACS corrects for this effect using the physical prescan region of all four quadrants rather than the image area discussed in Grogin et al. (2011). This new method is feasible because the bias-striping pattern is uniform across the quadrants of a given WFC CCD and is mirror-reflected across the CCD gap. Thus, all four prescan regions can be co-added and fitted together to provide an accurate correction for the bias striping. This task is done after the bias-shift correction, because this latter effect can affect the counts of the prescan regions. 

The third artifact that is corrected in the new CALACS is crosstalk between the amplifiers as the image is read out. Crosstalk has been present at a low level in WFC since the camera was first installed (Giavalisco 2004, ACS ISR 2004-13). This effect depends on the gain setting and a correction is now provided for post-SM4 observations taken with the default gain setting of 2 (Suchkov et al. 2010, ACS ISR 2010-02).

The new CALACS

The new CALACS includes additional steps for full-frame images only.  First, all images are corrected for the signal-dependent bias shift, cross talk, and bias striping. The new CALACS then has two branches.

The first branch contains traditional CALACS processing using standard darks (_DRK reference files) to produce standard data products: _CRJ, _FLT and _DRZ.fits files. The new second branch will correct for CTE degradation, perform dark correction using new CTE- corrected darks (_DKC files), and then standard processing to produce new data products, called _CRC, _FLC and _DRC.fits files. The user will be able to choose whether to use the CTE-corrected or standard data products.

In summary, there will be three pairs of data products from the new CALACS:

_CRJ = Cosmic-ray corrected FITS image
_CRC = Cosmic-ray and CTE-corrected FITS image

_FLT = Flat-field corrected FITS image
_FLC = Flat-field and CTE-corrected FITS image

_DRZ = Drizzled FITS image
_DRC = Drizzled CTE-corrected FITS image

Note: any data-file type ending in “C” is the CTE-corrected equivalent of a standard calacs file.

Note: we have not yet tested the efficacy of the CTE-correction algorithm for data taken with the grisms. We therefore ask users to compare old and new data products carefully for any grism data.

2. Updated photometric zeropoints for WFC and HRC

Revised detector quantum efficiency and filter transmission curves were delivered to SYNPHOT on March 22, 2012. The Sirianni et al. (2005, PASP, 117, 1049) published zeropoints should no longer be used. They have been replaced with time-dependent values which account for the loss of sensitivity with time.

Bohlin (2012, ACS ISR 2012-01) revised the ACS flux calibration for the standard filters in the HRC and WFC cameras. This new calibration accounts for the loss of sensitivity over time through 2007.1, which is as much as 0.6% per year for the HRC F220W but is less than 0.3%/yr for F330W and longer wavelength filters in both CCD cameras (Bohlin, Mack, & Ubeda 2011, ACS ISR 2011-03). Retrieving old data from the archive will provide the new PHOTFLAM keyword value that is appropriate for the epoch of the observation. The smooth trend toward lower sensitivities has a discontinuity for WFC at 2006.5, when the operating temperature was lowered from -77C to -81C. Following SM4, there are insufficient data to define trends, so the sensitivities are constant after 2009.4.

All archival ACS/CCD data retrieved prior to March 22, 2012 are populated with out-of-date photometric flux calibration PHOTFLAM keywords. Changes are due to the implementation of corrections to the changing sensitivity with time in Table 1 of Bohlin, Mack, & Ubeda (2011) and to the updated QE and filter transmissions in Table 5 of Bohlin (2012). After SM4 on 2009.4 when the ACS/WFC was revived, the gain was set to approximately reproduce the sensitivity at launch. Figure 3 of Bohlin, Mack, & Ubeda (2011) shows that the redefined sensitivities were successfully reset to within +/- 1% of the initial values. Currently, there are not enough data to define trends with time, so that the post-SM4 sensitivities remain constant at the 2009.4 values. A QE update is required only for HRC, where data processing improvements mimic a time-independent sensitivity increase by as much as 1.9% for F220W. The filter transmission changes for the wide filters are typically less than 0.5% but reach as much as 3% for WFC F550M and 4% for HRC F344N.

More information as well as a calculator to calculate zeropoints for a given date for any filter are available on the ACS Zeropoints page.

In addition, bandwidths, aperture corrections and plots of the filter throughputs are now also available on the ACS Aperture Corrections page and the ACS System Throughputs page.

3. New Documentation

  • Study of the evolution of the ACS/WFC charge transfer efficiency (ACS ISR 2012-03) Ubeda & Anderson
  • Pixel-based correction of the ACS/WFC signal-dependent bias shift (ACS ISR 2012-02) Golimowski, Suchkov, Loose, Anderson & Grogin 
  • Flux Calibration of the ACS CCD Cameras IV. Absolute Fluxes (ACS ISR 2012-01) Bohlin
  • Post-SM4 ACS/WFC Bias Striping: Characterization and Mitigation (ACS ISR 2011-05) Grogin, Lim, Maybhate, Hook & Loose 
  • ACS after Servicing Mission 4: The WFC Optimization Campaign (ACS ISR 2011-04) Golimowski et al.
  • Flux Calibration of the ACS CCD Cameras III. Sensitivity Changes over Time (ACS ISR 2011-03) Bohlin, Mack & Ubeda
  • Flux Calibration of the ACS CCD Cameras II. Encircled Energy Correction (ACS ISR 2011-02) Bohlin
  • Flux Calibration of the ACS CCD Cameras I. CTE Correction (ACS ISR 2011-01) Bohlin & Anderson


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