Limiting magnitude

In astronomy, limiting magnitude is the faintest apparent magnitude of a celestial body that is detectable or detected by a given instrument.^[1]

In some cases, limiting magnitude refers to the upper 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.

The limiting magnitude of an instrument is often cited for ideal conditions, but environmental conditions impose further practical limits. These include weather, moonlight, skyglow, and light pollution. The International Dark-Sky Association has been vocal in championing the cause of reducing skyglow and light pollution.