ACS / WFC Crosstalk after Servicing Mission 4

The ACS/WFC detector consists of two CCDs, each of which is read out through two amplifiers. While reading each quadrant of the detector, the electronic crosstalk between the amplifiers induces faint, typically negative, mirror-symmetric ghost images on the other three quadrants. The effect is strongest for high-signal offending (source) pixels. Analysis of pre-SM4 crosstalk showed that its impact on ACS/WFC science is not significant and can be ignored in most science applications. In this report, we analyze crosstalk after SM4. Crosstalk due to low-signal offenders is much weaker than before SM4 and does not produce ghosts similar to those seen in pre-SM4 images. For high-signal offending pixels, we find substantial differences between the gain=1 eˉ/DN and gain=2 eˉ/DN cases. For the default gain setting of 2, the crosstalk is similar to what it was before the SM4, up to 5–8 eˉ per pixel on the same CCD. For gain=1, the crosstalk is ~100 eˉ per pixel for saturated offending pixels on the same CCD, which is more than an order of magnitude above the pre-SM4 level. The crosstalk from saturated pixels is ~20–30 eˉ per pixel on the other CCD, which is also much higher than it was before SM4.