During the third EVA of SM4 on 16 May 2009, astronauts John Grunsfeld and Drew Feustel replaced the ACS CCD Electronics Box and Low Voltage Power Supply that incrementally failed in June 2006 and January 2007, causing the loss of the ACS Wide Field Channel and High Resolution Channel. The replacement components (CEB-R and LVPS-R) immediately 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 in this cycle and beyond.
The CEB replacement (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 (1-2 DN) 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 of the pixel previously sampled at the output node. These artifacts are removable in post-image processing and should have little to no effect on most WFC science programs. Because the bias gradient depends on the timing pattern of the CCD read out, fewer WFC subarray options are supported in this cycle than were supported in previously observing cycles.
Table 2.1 lists the basic characteristics of the WFC CCDs before the failure of ACS in January 2007 and 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.
