Space Telescope Science Institute   7.1.6 Determine Total Orbit Request  7.2.1 How Do You Determine if You Violate a Bright Object Limit for SBC Exposures?

7.1.7 Charge Transfer Efficiency


All CCDs operated in a radiative environment are subject to a significant degradation in charge transfer efficiency (CTE). The degradation is due to radiation damage of the silicon, inducing the creation of traps that impede an efficient clocking of the charge on the CCD. Since reading out the ACS WFC requires 2048 parallel transfers and 2048 serial transfers, it is not surprising that CTE effects have begun to manifest themselves in the first years of ACS operation. For this reason, it is likely that some types of science, particularly those in which the source flux in each image is expected to be low (< 0.1 electrons/second) and compact, will be most effectively performed during the first few years of ACS operation.

Initial expectations for growth of CTE for the Wide Field Camera have proven to be conservative. Multiple iterations of special programs designed to track the growth of CTE over time show that after 1 year of operation, there was a loss of approximately 1 to 2% in the counts from a star with between 50 and 150 total electrons, a nominal sky background of 30 electrons, and a placement at row 1024 (center) in one of the WFC chips. Absolute and relative measurements of the CTE show that the degradation proceeds linearly with time. Thus, such a target would experience a loss of approximately 4% to 8% in early 2006. A target placed at the WFC aperture reference point, near the maximum number of parallel shifts during readout, would have approximately twice the loss. Expected absolute errors after calibration of science data, at these low-loss levels, is expected to be of order 25% the relative loss.

The aperture WFC1-CTE is available to mitigate CTE loss. This aperture has the same area as the WFC1 aperture except that the reference position is 200 pixels from the upper right corner of chip 1, in both the chips x- and y- direction. Therefore, WFC1-CTE is not appropriate for highly extended targets.

As the CTE effects worsen in future cycles, users may want to consider using the post-flash capability (currently an "available-but-unsupported" mode) to add a background level to their images. This causes the Poisson noise from the background level to increase and imposes a non-uniform background, but to-date only marginally improves the CTE performance of the detector. At present, we do not recommend the use of the post-flash capability for any applications, but we will continue to track this carefully.


 7.1.6 Determine Total Orbit Request  7.2.1 How Do You Determine if You Violate a Bright Object Limit for SBC Exposures?
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