photometric filter

A detector like a CCD just counts photons; on its own it cannot tell red light from blue. To recover colour — and far more — astronomers place a coloured glass or coating, a photometric filter, in the light path. Each filter passes only a chosen band of wavelengths, so an exposure through it measures the object's brightness in just that slice of the spectrum.

By taking images through several standard filters and comparing the brightnesses, astronomers measure an object's colour precisely. A widely used set is the UBVRI system — ultraviolet, blue, visual, red, and near-infrared bands — and modern surveys use systems like the ugriz filters of the Sloan Digital Sky Survey. The difference between two filter magnitudes is the colour index; for a star it reveals surface temperature, since hot stars are bluer and cool stars redder, while for a galaxy colour hints at the ages of its stars and how much dust reddens its light.

Filters turn a brightness measurement into crude spectroscopy on the cheap. Spreading light into a full spectrum is powerful but spreads the photons thin, so it needs bright targets or long exposures; filter photometry instead bins the light into a few broad channels, keeping enough signal to measure billions of faint galaxies a full spectrum could never reach. Filters also isolate specific features — a narrow filter tuned to the red glow of hydrogen lets astronomers map glowing nebulae and star-forming regions across a galaxy.