SDSSugriz

SDSSugriz refers to the five-band optical photometric system used by the Sloan Digital Sky Survey. The system comprises five broad-band filters, labeled u, g, r, i, and z, designed to span from near-ultraviolet to near-infrared and to provide color information that aids in object classification and redshift estimation.

The approximate effective wavelengths are u ≈ 355 nm, g ≈ 468 nm, r ≈ 616 nm, i ≈ 748 nm, and z ≈ 893 nm. The filters and their passbands were defined for SDSS imaging, and the resulting photometry is calibrated to the AB magnitude system. In practice, SDSS reports magnitudes using the asinh (luptitude) formalism to improve performance at low signal-to-noise, while many analyses use the corresponding AB magnitudes.

SDSS ugriz photometry has been central to studies of stars, galaxies, and quasars, enabling color-based selection and robust photometric redshifts. The system is widely used beyond SDSS, with various surveys and studies adopting the ugriz framework or providing transformations to other systems (for example, Johnson-Cousins). The ugriz filters remain a reference standard in optical astronomy, illustrating how broad-band photometry can support large-scale surveys, population studies, and cross-survey comparisons.