Astronomical surveys generally involve imaging or mapping of regions of the sky using Telescopes. In the past,
surveys have been usually restricted to one band of wavelengths of electromagnetic radiation (e.g. light or radio) or to measurements of the flux of one type of particle (e.g. cosmic rays), and they were generally performed as part of the
production of an astronomical catalogue for a specific type of astronomical object (like, for example, all the stars
brighter than a certain apparent magnitude). Over the last ten years, taking advantage of technological improvements in
the construction of telescopes, and following a general expansion in our understanding of astrophysics at all levels, it
has become commonplace to conduct surveys that join together many different observations of a given region in the sky,
obtained with different telescopes at different wavelengths. This so called multi-wavelength approach is now the new standard
for surveys, at least in the fields of Extragalactic astronomy and Observational cosmology.
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