Limiting magnitude

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Limiting magnitude is the faintest apparent magnitude detectable or detected by a given instrument.

In some cases, limiting magnitude refers to the utter threshold of detection. In more formal uses, limiting magnitude is specified along with the strength of the signal, e.g., "10th magnitude at 20 sigma." Sometimes limiting magnitude is qualified by the purpose of the instrument, e.g., "10th magnitude for photometry;" this statement recognizes that a photometric detector can detect light far fainter than it can reliably measure it.

The International Dark-Sky Association has been vocal in championing the cause of limiting skyglow and light pollution / photopollution.

[edit] Limiting Magnitude in Amateur Astronomy

In amateur astronomy, limiting magnitude frequently refers to the faintest stars that can be seen with the unaided eye. The quantity is most often used as an overall indicator of sky quality, in that light polluted and humid areas generally have brighter limiting magnitudes than remote desert areas.

There is even variation within metropolitan areas. For those who live in the immediate suburbs of New York City, the limiting magnitude is approximately 4.0. This corresponds to roughly 250 visible stars, or one-tenth the number that can be perceived under perfectly dark skies. From the outer boroughs of New York City (Brooklyn, Queens, Staten Island and the Bronx), the limiting magnitude is approximately 3.0, suggesting that at best, only about 50 stars might be seen at any one time. From brightly-lit Midtown Manhattan, the limiting magnitude is approximately 2.0, meaning that from the heart of New York City only approximately 15 stars will be visible at any given time.

From suburban areas, the limiting magnitude is frequently closer to 6.0, but from very remote and clear sites, amateur astronomers can see nearly as faint as 8.0 magnitude.