The Two Micron All-Sky Survey, or 2MASS, was an astronomical survey of the whole sky in infrared light. It happened between 1997 and 2001, in two different places, Fred Lawrence Whipple Observatory in Arizona, and at the Cerro Tololo Inter-American Observatory in Chile, each using a 1.3-meter telescope for the Northern and Southern Hemisphere, in order. It was organized in the short-wavelength infrared at three different frequency bands (J, H, and K) near 2 micrometres, from which the photometric survey with its Mercury cadmium telluride (HgCdTe) detectors takes its name.
2MASS made an astronomical catalog with over 300 million observed objects, including minor planets of the Solar System, brown dwarfs, low-mass stars, nebulae, star clusters and galaxies. Also, 1 million objects were cataloged in the 2MASS Extended Source Catalog (2MASX). The cataloged objects are named with a "2MASS" and "2MASX"-prefix, in order.