This is a charge coupled device (CCD), SITE Model SI 424, representative of the CCDs used in the Sloan Digital Sky Survey camera. It is a 2.5-inch square chip containing a grid-like array of 2,048 x 2,048 silicon diodes, each one acting as a tiny picture element, or pixel, for a total of over 4 million pixels (4 megapixels). Each diode collects light and converts it into an electrical charge. This charge is transferred to a computer, retaining information on position and instensity, and the resulting image is viewed on a monitor or stored in a computer. The size and number of the silicon light detectors or pixels on the CCD determine the clarity or resolution of the final image. The surface of this back-illuminated CCD appears deep blue in color as a result of an antireflective coating. 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.