JohnsonCousins

JohnsonCousins refers to the widely used optical broadband photometric system in astronomy that defines a standard set of five passbands: U, B, V, R, and I. The system is based on the original Johnson UBV photometric system and was extended by Patrick B. Cousins in the 1970s with the addition of the redder R and I filters, producing the UBVRI set that remains in common use for ground-based observations. The passbands are chosen to enable consistent measurements of stellar brightness and color across different telescopes and detectors.

The central wavelengths for the Johnson-Cousins filters are approximately 365 nanometers for U, 445 nanometers for B, 551 nanometers for V, 658 nanometers for R, and 806 nanometers for I. Zero points are typically tied to Vega or to modern calibrations, and many observers rely on standard star networks such as the Landolt catalogs to calibrate measurements. The system is designed to work with CCD detectors and atmospheric transmission, providing a practical framework for comparing observations.

In use, the Johnson-Cousins system supports color indices such as U−B and B−V, which correlate with stellar temperature, composition, and reddening. It remains a foundational reference in stellar photometry and in the calibration of many historical and contemporary datasets. While newer survey systems (for example, SDSS) have introduced alternative filter sets, Johnson-Cousins magnitudes are still widely transformed and reported for compatibility with a vast body of published work.