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Observational astronomy infers astronomical properties from measured light, particle arrivals, positions and time variations. A detector first measures counts or signals; calibration, the point spread function, background and selection effects connect those data to an observed radiative flux or an inferred source parameter.

Different observables constrain different quantities. Spectroscopy supplies line shifts and composition-sensitive features, astrometry supplies positions and apparent motions, and a light curve supplies temporal structure. Their combination can break ambiguities that remain in a single observation. An astronomical selection function is then needed to infer a population from the objects that were detected.