This is a charge coupled device (CCD), SITE Model SI 424, representative of the 30 CCDs used in the Sloan Digital Sky Survey camera focal plane array. CCDs consist of two dimensional arrays of large numbers of tiny silicon diodes that convert light photons directly to electrical signals that can be processed digitally to produce images. The surface of this front illuminated CCD contains a square array of 2048 by 2048 pixels for a total of over 4 million elements. This example was manufactured by Scientific Imaging Technologies Inc. and donated by them to NASM in 2000. It is now on display in the Explore the Universe gallery.