Aluminium spectroscopic plate from the Sloan Digital Sky Survey (SDSS)

Mapping the cosmos

The Sloan Digital Sky Survey is the largest astronomical survey ever completed. Between 2000 and 2008 it created a 3D map of over a million stars and galaxies, covering a quarter of the night sky. This plate is one of about 4000 custom-built to fit the telescope. Each plate carries a unique pattern of 640 holes, used to select hundreds of stars and galaxies for each exposure. The circled guide stars marked on the plate help astronomers align it to the sky.