Zero point (photometry)

In astronomy, the zero point in a photometric system is defined as the magnitude of an object that produces 1 count per second on the detector.

[2] Traditionally, Vega is used as the calibration star for the zero point magnitude in specific pass bands (U, B, and V), although often, an average of multiple stars is used for higher accuracy.

where M is the magnitude of an object, F is the flux at a specific wavelength, and S is the sensitivity function of a given instrument.

[4] Under most circumstances, Vega is used as the zero point, but in reality, an elaborate "bootstrap" system is used to calibrate a detector.

[3][5] The calibration typically takes place through extensive observational photometry as well as the use of theoretical atmospheric models.