In the late 1980s and early 1990s, the Palomar Observatory (pictured below) conducted a complete survey of the northern sky using the 48-inch Samuel Oschin Telescope. The survey used 900 overlapping fields that were each 6.5 degrees on a side. To give you a sense of scale, the Moon as seen from Earth has a diameter of about half a degrees. For each field, plates were taken in three colors: blue-green, red, and far-red (Kodak J, F, and N emulsions). All of those plates were subsequently digitized by the Space Telescope Science Institute (STScI) and Caltech. STScI then used these digitized images as a source for guide stars for the Hubble Space Telescope (HST).
The complete Digital Palomar Observatory Sky Survey (DPOSS) contains about 3 Terabytes of images (3,000,000,000,000 bytes!). A single byte corresponds to a single letter of text. If the average book contains about half a million bytes, the complete digital sky survey is equivalent to approximately 6 million books (DPOSS). If you’re curious, you can search and retrieve images from the Digitized Sky Survey here: