ugriz photometric system

The ugriz photometric system is essentially an expansion of the griz photometric system, ugriz indicating the letters associated with the passbands, u, g, r, i, and z:

The ugriz system was adopted for the Sloan Digital Sky Survey (SDSS) photometric astronomical survey (subsequently termed SDSS-I). SDSS-I, performed with a telescope designed to survey the whole sky efficiently (the Sloan 2.5m Telescope), but used a separate, more-conventional telescope for its calibration observations, with filters of different manufacture and inevitably slightly different passbands. Though the difference was small, the project treated it as a distinct photometric system, and carried out the kind conversion calculations between incompatible photometric systems in order to apply their calibration information (e.g., to take airmass into account). Since this means they dealt with two versions of ugriz, they termed the slightly-different one used for calibration ugriz' ("ugriz prime"), with passbands, u', g', r', i', and z'. Various documents about SDSS have termed ugriz' as the SDSS system without giving this explanation.