photometry

Photometry is the measurement of the flux or intensity of electromagnetic radiation from astronomical sources. It is a fundamental tool in astronomy, used to quantify how bright an object appears and how its brightness changes over time. Photometry can be performed over broad wavelength ranges or in narrow spectral bands, and it serves as the basis for deriving physical properties such as luminosity, color, and temperature when distances are known.

Measurements are typically expressed in magnitudes, a logarithmic scale with standard zero points defined by reference stars. Photometric systems use filters that define passbands (for example UBVRI, ugriz, or near-infrared sets) and enable comparison across observations. Calibration involves correcting for instrumental response, atmospheric extinction, and other biases, often using standard stars observed under similar conditions. Techniques include aperture photometry, which sums light within a radius around the source, and PSF fitting for crowded fields. Differential photometry compares a target with stable comparison stars to track relative changes.

Detectors such as charge-coupled devices and historically photomultipliers convert incoming photons to measurable signals. Data processing includes bias or dark subtraction, flat-fielding, and sky subtraction, followed by aperture corrections or PSF modeling. Photometry supports time-domain studies of variable stars, transiting exoplanets, and active galactic nuclei; it also provides color indices and luminosities for population studies and distance estimates via standard candles. Limitations include atmospheric variability, instrumental stability, and the need for accurate calibration to achieve precise measurements.