Correcting Pixels for CTI in ACS’s WFC

Jay Anderson, jayander@stsci.edu

The harsh radiation environment of space takes a toll on charge-coupled-device (CCD) detectors. When cosmic rays (CRs)—high-energy protons, neutrons, and ions—impact CCDs, they frequently displace photoelectrons, causing the familiar trails in individual exposures.  Particularly energetic events can even displace silicon atoms and create permanent defects in the silicon lattice.  These defects cause two problems for astronomers.  The first is that they can generate short-circuits within pixels, such that the pixels fill with charge even when there are no photons present.  These are called “hot” or “warm” pixels (WPs).  Defects in the silicon lattice can also temporarily trap photoelectrons while the detector is being read out.  The result of this is charge-transfer inefficiency (CTI), which degrades the image quality.  This article reports recent progress that has been made in correcting CTI in the Wide Field Channel (WFC) of the Advanced Camera for Surveys (ACS).

During an exposure, photons impact the detector and generate electrons via the photoelectric effect.  These electrons are collected within the pixel where they were created until it is time to read out the detector.  The detector is read out by shuffling the charge cloud for each pixel down the columns to the serial readout register, then across this register to the readout amplifier, where the charge in each pixel is measured one pixel at a time.  As a cloud of electrons is shuffled down the columns, it may encounter a defect in the silicon lattice, which can trap an electron temporarily.   By the time the trap releases the electron, the original pixel cloud may have already been shuffled several pixels downstream.  The result is that the electron gets released into another pixel, one located upstream from the original pixel.  The recorded image of the source is thus smeared out in the direction away from the serial register (Figure 1).