An astronomical survey is a general map or image of a region of the sky (or of the whole sky) that lacks a specific observational target. Alternatively, an astronomical survey may comprise a set of images, spectra, or other observations of objects that share a common type or feature. Surveys are often restricted to one band of the electromagnetic spectrum due to instrumental limitations, although multiwavelength surveys can be made by using multiple detectors, each sensitive to a different bandwidth. Surveys have generally been performed as part of the production of an astronomical catalog. They may also search for transient astronomical events. They often use wide-field astrographs. Scientific value Sky surveys, unlike targeted observation of a specific object, allow astronomers to catalog celestial objects and perform statistical analyses on them without complex corrections for selection effects. In some cases, an astronomer interested in a particular object will find that survey images are sufficient to make new telescope time entirely unnecessary. Surveys also help astronomers choose targets for closer study using larger, more powerful telescopes. If previous observations support a hypothesis, a telescope scheduling committee is more likely to approve new, more detailed observations to test it. The wide scope of surveys makes them ideal for finding foreground objects that move, such as asteroids and comets. An astronomer can compare existing survey images to current observations to identify changes; this task can even be performed automatically using image analysis software. Besides science, these surveys also detect potentially hazardous objects. Similarly, images of the same object taken by different surveys can be compared to detect transient astronomical events such as variable stars. List of sky surveys Optical Hipparchus - created the first known star catalogue with more than 850 stars. The data was incorporated into the Almagest along with the first list of stellar magnitudes and was the primary astronomical reference until modern times, 190-120 BC. Bonner Durchmusterung - whole-sky astrometric star catalog, 1859-1903 Astrographic Catalogue - international astronomical survey of the entire sky. The survey was performed by 18 observatories using over 22,000 photographic plates. The results have been the basis of comparison for all subsequent surveys, 1887-1975. Henry Draper Catalogue - spectral classifications based on photographic plates, 1918-1924, extension 1925-1936 Catalina Sky Survey - an astronomical survey to discover comets and asteroids. Pan-Andromeda Archaeological Survey National Geographic Society – Palomar Observatory Sky Survey (NGS–POSS) – survey of the northern sky