Introduction

As the decline in charge transfer efficiency (CTE) became a very serious issue for ACS/WFC, there was increased interest in developing a method to correct its effects pixel-by-pixel. Anderson & Bedin (2010) present a model for how charge is trapped and released during readout of the ACS/WFC CCDs over the full range of background intensity and source flux. This model was developed by studying the charge trails of warm and hot pixels in a set of calibration dark images, which determined the distribution of charge traps in the detector and the release profile of trapped charge. In the spring of 2012, pixel-based CTE correction software using the Anderson & Bedin model was included in the data processing pipeline CALACS, which produces one standard (CRJ or FLT) and one CTE-corrected (CRC or FLC) data product for each WFC observation. An in-depth description of the pixel-based CTE correction software is provided in the ACS Data Handbook.

Recent Updates

In the summer of 2017, a new version of the pixel-based CTE correction software was implemented in CALACS. As described in ACS ISR 2018-04, the updated model includes a technique for mitigating amplification of readnoise during the CTE correction process, updated charge trap distribution and trapped charge release profile, and the ability to run the software over multiple CPUs. The new correction provides a photometric accuracy of better than 5% and an astrometric accuracy of better than 0.05 pixels for moderately faint sources (>200e-) on moderate backgrounds (>20e-). 

In fall of 2018, the capability to simulate the effects of CTE losses during readout on input images, i.e., adding the charge trails to images rather than correcting them, was added to CALACS. Called the ACS CTE forward model, this software is intended for users who wish to more fully understand the effects of the pixel-based CTE correction on their measurements by forward-modeling synthetic data (or other data unaffected by CTE losses) and then CTE-correcting it. A Jupyter notebook is available to guide users in the proper usage of the CTE forward model (See ACS Analysis Tools).

Effects of CTE and a comparison with the forward model
Figure  1: An example of the effects of imperfect CTE and a comparison with the forward modeler.

 

Figure 1 shows a region of 47 Tuc from a March 2016 observation, which was a 339 second exposure in the F775W filter. The left panel shows the FLT image, which has been bias-subtracted, dark-corrected, and flat-fielded, but not CTE-corrected. The middle panel shows the FLC image, which the same as the FLT, except that it has been CTE-corrected. The right panel shows the result of CTE forward-modeling the FLC image according to the guidelines in the Jupyter notebook referenced above. The significant charge trails from stars in the FLT have been removed and added back to the stars which generated them in the FLC. The forward-modeled FLC appears nearly identical to the FLT.

Expected Accuracy

LAST UPDATED: 06/17/2019

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