Photometry (astronomy)

Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation. Usually, photometry refers to measurement over large wavelength bands of radiation; but, when not only the amount of radiation but its spectral distribution are measured the term spectrophotometry is used. When the distance of the measured object can be estimated, photometry can give information about the total energy emitted by the object, its size, its temperature and other physical properties. Accurate photometry is more difficult when the apparent brightness of the object is fainter.

Historically photometry was done with a photoelectric photometer, an instrument that measured the light intensity of a single object by directing its light on to a photosensitive cell. They have largely been replaced with CCD cameras, though photoelectric photometers are still used in special situations, such as where high time resolution is required.

At its most basic, photometry is conducted by gathering light in an optical telescope, passing the light through colored filters, and then recording the wavelengths on a CCD. Generally at least three different photometric images are taken, as well as images of photometric standard stars, each using different filters; and the data is then used to calculate physical and chemical parameters of the object.

Photometry is generally used to generate light curves of objects such as variable stars and supernova where the quantity of interest is the variation of total light energy output over time. Photometry can be used as a technique to discover exoplanetss. By measuring the intensity of a star's light over a period of time, astronomers can examine deviations in its spectral output and determine possible causes.