This image is a composite from black and white images taken with the Palomar Observatory's 48-inch (1.2 meter) Samuel Oschin Telescope as a part of the second National Geographic Palomar Observatory Sky Survey (POSS II). The images were recorded on two glass photographic plates - one sensitive to red light and the other to blue and later they were digitized. In order to produce the colour image seen here a total of 62 different frames were processed - 31 frames for each colour band, coming from 4 different plates taken from 1990 to 1993. The original file is 14,264 x 15,429 pixels with a resolution of about 1 arc-second per pixel. The image covers an area of sky larger than 4› x 4.3› (for comparison, the Full Moon is about 0.5› in diameter).
Explanation from: https://www.spacetelescope.org/images/heic0510a/