2MASS

2MASS, the Two Micron All Sky Survey, was an astronomical survey that mapped the entire sky in the near-infrared. It produced uniform photometric data in three infrared bands and became a foundational resource for studies of the Milky Way, nearby galaxies, and extragalactic objects. The survey operated from the late 1990s into the early 2000s, with data processing and public release following shortly after.

The project used two identical 1.3-meter telescopes, located at Mount Hopkins, Arizona, and Cerro Tololo in Chile. Between 1997 and 2001, the instruments captured millions of images in the J (1.25 μm), H (1.65 μm), and Ks (2.16 μm) bands, enabling automated source detection and photometry across the entire sky. The measurements were calibrated to provide a uniform near-infrared catalog suitable for cross-wavelength studies.

The primary data products are the All-Sky Catalogs of Point Sources (PSC) and Extended Sources (XSC). The PSC contains hundreds of millions of objects, while the XSC catalogs millions of extended sources such as galaxies and resolved nebulae. Public releases occurred in the early 2000s, with continued usage and cross-matching in subsequent decades. The 2MASS data have been widely incorporated into a broad range of astronomical research, including stellar population studies, the structure of the Milky Way, discovery of cool stars and brown dwarfs, and cross-surveys with optical and mid-infrared data.

Legacy and significance: 2MASS established a benchmark for all-sky near-infrared surveys, providing a comprehensive reference framework that has influenced later missions and multi-wavelength studies. It remains a widely cited resource for identifying infrared-bright objects and for calibrating broader astronomical analyses.