CTE INFORMATION

Introduction

The charge transfer efficiency (CTE) of the ACS CCDs (WFC and HRC) declines as damage from the space radiation environment accumulates. Here we provide information about our efforts to monitor the CTE, and methods of mitigating and calibrating it. The ACS Team currently provides two tools for correcting signal losses incurred by the imperfect CTE of the ACS/WFC CCD detectors:

ACSCTE

ACSCTE is a standalone python tool included in the python package ACSTOOLS. It is simply a wrapper around the C-code implementation of the pixel-based CTE correction algorithm developed by Jay Anderson and Luigi Bedin described here. The most recent updates to the algorithm are described ACS ISR 2018-04. This is the same algorthim used in the ACS calibration pipeline, CALACS, to correct for CTE losses in ACS/WFC frames.

Photometric CTE calculator

This calculator corrects ACS/WFC photometry for CTE losses using an empirical formula derived from observations of 47 Tucanae described ACS ISR 2012-05. The tool is calibrated for photometry extracted from observations obtained after SM4 in May 2009 only. For information on the expected accuracy, as well as, the observations and model used in the webtool please refer to the Photometric CTE Corrections webpage. To access the webtool, click the link below

ACS/WFC Photometric CTE Webtool

Monitoring

Internal monitoring of ACS CTE:

Internal monitoring of the decay of parallel CTE of the ACS/WFC CCDs is accomplished with the extended pixel edge response (EPER) test. A summary of EPER tests over the lifetime of ACS may be found on the EPER Monitoring Page.

External monitoring of the ACS CTE:

Observations of a single field in globular cluster 47 Tucanae are obtained for the external CTE monitoring program for ACS/WFC. Pairs of images are separated by two large slews about half the size of the WFC field of view (102"). The slews are performed in both the X and in the Y directions to vary the number of transfers for each star. Monitoring of both the serial and parallel CTE is possible by comparing the measured magnitudes of each star at its position in each frame. The figure below shows the magnitude loss for stellar light after 2000 pixel transfers plotted as a function of the stellar flux in electrons. Blue and green points are for sky background levels of ~0.6 and ~14 e-, respectively. The solid lines are weighted linear fits to the data. The empirical photometric correction for stellar data discussed above is determined from this monitoring program, and the analysis is described more fully in ACS ISR 2012-05.