Photometry

Photometry is the measurement of the brightness, or flux, of light coming from an object, typically a star, galaxy, or other astronomical source, usually expressed on the magnitude scale, where a smaller or more negative number means brighter.

A detector, historically a photomultiplier, almost always a digital CCD or CMOS camera today, records the light collected through a telescope over a set exposure, and the raw counts are calibrated against reference stars of known brightness to produce a physically meaningful measurement, a step closely tied to broader sensor calibration practice. Repeated photometric measurements of the same object over time produce a light curve.

Photometry underlies most of observational astronomy, from measuring a star's distance and temperature to detecting exoplanets and supernovae, and it increasingly feeds automated pipelines that process astronomical survey data at scale, well beyond what any human observer could review manually. Pitfalls include atmospheric conditions and instrument calibration drift, which can shift measurements over a single night, and crowded fields, where light from nearby sources blends together, both of which can introduce systematic errors a single uncalibrated measurement will not reveal.