The Faint Object Camera (FOC) was a camera installed on the Hubble Space Telescope from launch in 1990 until 2002. It was replaced by the Advanced Camera for Surveys. In December 1993, Hubble's vision was corrected on STS-61 by installing COSTARS, which corrected the problem with Hubble's mirror before it reached an instrument like FOC. Later instruments had this correction built in, which is why it was possible to later remove COSTARS itself and replace it with a new science instrument. Rather than CCDs the FOC used photon-counting digicons as its detector.