astronomical survey

An astronomical survey is a project to gather astronomical data over some portion of the sky, in some cases, all. It may constitute a single image or set of images. The term observing program means roughly the same thing. Currently, some are carried out automatically. The targets may be based upon appearance (e.g., portion of the spectrum at which the object is evident), based upon object type (e.g., galaxies), or may be some aspect of cosmic background radiation (CBR). It can result in an associated astronomical catalog and survey-based designator.

Surveys are naturally magnitude-limited, and it is now common to preselect the magnitude limit and adhere to it as much as possible, to allow for accommodating the Malmquist bias and similar issues, so the resulting catalog can serve as a reasonable sample set by some useful criteria. Magnitudes for specific bands are often collected, cited as the band's designating letter, e.g., citing k = 19.0 to indicate an apparent magnitude of +19 as seen through in the K band.

By this definition, thousands of surveys have been carried out, but most are so small the term would seldom be used. Some large, well-known surveys are the Palomar Observatory Sky Survey (POSS) in the 1950s and more recently, the Two Micron All-sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS). Some space-observatories have been designed to carry out surveys, to cover electromagnetic radiation not viewable from the ground, and/or for other advantages such as sidestepping seeing issues (e.g., Gaia). See list of surveys and list of survey/catalog indicators.

The term astronomical survey is also sometimes used for efforts gathering ("surveying") existing observational data and constructing a catalog regarding some particular topic. The term observing program used for some surveys effectively excludes this meaning.