Astronomy Today

Photography and Photometry

What are the standard bandpasses used in modern astronomy?

The difference is that astronomical sources like planets, stars, and galaxies are so far away that they are almost always too faint to study with typical photographic equipment. In astronomical telescopes, special optical components are therefore used to transmit as much light as possible—special CCDs are especially efficient in detecting light—and the entire camera assembly is cryogenically cooled to hundreds of degrees below zero Fahrenheit in a special container called a dewar. These measures help astronomers measure objects millions—even billions—of times fainter than would be possible with ordinary store-bought cameras.

Astronomers often create specific bandpasses with unique filter combinations in order to achieve particular scientific goals. (This is particularly true for photometry obtained in wavelengths that are not in the range of visible or infrared light.) Over the years, though, a number of bandpasses have been established as being generally useful for a wide variety of astronomical analyses. These standard bandpasses constitute photometric “systems” that are broadly used in astronomy today. For obtaining photometry in visible light, the most common bandpasses are called U (“near-ultraviolet,” for light with wavelengths between about 300 to 400 nanometers), B (“blue,” between about 400–500 nanometers), V (“visible,” about 500–600 nanometers), R (“red,” about 600–700 nanometers), and I (“near-infrared,” about 700–900 nanometers). For infrared observations, some of the standard bandpasses are called Z, J, H, and K.


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