2.1 SM4 Repair of ACS

The ACS CCD Electronics Box (CEB) and Low Voltage Power Supply (LVPS) incrementally failed in June 2006 and January 2007, causing the loss of the ACS Wide Field Channel (WFC) and High Resolution Channel (HRC). The replacement components (CEB-R and LVPS-R) installed on 16 May 2009 during HST Servicing Mission 4 (SM4) successfully restored the function of the WFC but not that of the HRC. Unfortunately, the damage in 2006 to the circuitry that controlled the HRC occurred upstream of the location repaired by the CEB-R. This situation was not unexpected, as the post-failure analysis of the nature and location of the short in the HRC circuitry was ambiguous. Consequently, only the WFC and the Solar Blind Channel (SBC) are available to observers from 2009 onwards.

The CEB-R features new CCD controller and signal processing electronics based upon a programmable SIDECAR (system image, digitizing, enhancing, controlling, and retrieving) ASIC (Application-Specific Integrated Circuit) manufactured by Teledyne Scientific & Imaging. The dual-slope integrator within the CEB-R delivers nearly 1 e^– lower read noise than was obtained with the old CEB. The CEB-R does, however, induce some low-level (0.9 e^– RMS) correlated noise and a spatially variable bias level in the WFC images. The bias level of each pixel also shifts by 0.02–0.3% of the signal. These artifacts are removed in CALACS pipeline processing and have little to no effect on most WFC science programs. Because the bias gradient depends on the timing pattern of the CCD readout, the original WFC subarray modes were replaced at the end of Cycle 23 with new modes that match the full-frame readout timing. See Section 7.3.1 for details.

Table 2.1 lists the basic characteristics of the WFC CCDs. The second column lists the values before the failure of ACS in January 2007. The third column lists recent values, well after the installation of the CEB-R during SM4 in May 2009. The fourth column in the table lists the section in this Handbook where detailed information about each characteristic can be found. Further details on improvements after SM4 can be found in ACS ISR 2011-04.

Table 2.1: WFC Performance Summary

^a Before application of bias-shift correction algorithm.