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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.
In naked-eye visibility
The limiting magnitude for naked eye visibility refers to the faintest stars that can be seen with the unaided eye near the zenith on clear moonless nights. The quantity is most often used as an overall indicator of sky brightness, in that light polluted and humid areas generally have brighter limiting magnitudes than remote desert or high altitude areas. The limiting magnitude will depend on the observer, and will increase with the eye's dark adaptation. On a relatively clear sky, the limiting visibility will be about 6th magnitude.
There is even variation within metropolitan areas. For those who live in the immediate suburbs of New York City, the limiting magnitude might be 4.0. This corresponds to roughly 250 visible stars, or one-tenth the number that can be perceived under perfectly dark skies. From the New York City boroughs outside Manhattan (Brooklyn, Queens, Staten Island and the Bronx), the limiting magnitude might be 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 possibly 2.0, meaning that from the heart of New York City only approximately 15 stars will be visible at any given time.
From relatively dark suburban areas, the limiting magnitude is frequently closer to 5 or somewhat fainter, but from very remote and clear sites, some amateur astronomers can see nearly as faint as 8th magnitude. Many basic observing references quote a limiting magnitude of 6, as this is the approximate limit of star maps which date from before the invention of the telescope. Ability in this area, which requires the use of averted vision, varies substantially from observer to observer, with both youth and experience being beneficial.
Limiting magnitude is traditionally estimated by searching for faint stars of known magnitude. In 2013 an app was developed based on Google's Sky Map that allows non-specialists to estimate the limiting magnitude in polluted areas using their phone.
In amateur astronomy, limiting magnitude refers to the faintest objects that can be viewed with a telescope. A two-inch telescope, for example, will gather about 16 times more light than a typical eye, and will allow stars to be seen to about 10th magnitude; a ten-inch (25 cm) telescope will gather about 400 times as much light as the typical eye, and will see stars down to roughly 14th magnitude, although these magnitudes are very dependent on the observer and the seeing conditions.
Calculating limiting magnitude
Limiting magnitude can be calculated by using a telescope.
As a first approximation, the gain in magnitudes of a telescope is , where D1 is the diameter of the telescope's primary light gathering component, and D0 is the diameter of the eye's dark adapted pupil. Both quantities must be measured in the same units. D0 varies from person to person but is typically 6–7 mm (~1/4").
A 10-inch (D1 = 254mm) telescope therefore would provide a gain of about 8 magnitudes beyond what could be observed without it. Thus, if one is at a site where the naked eye limiting magnitude (NELM) is 5, the telescope will allow one to see stars as faint as about magnitude 13.
In reality a telescope allows one to see much fainter stars because at higher powers the background is darkened and contrast increased. A typical 10-inch scope at high power (250× or more) will easily reach magnitude 15. A formula including a correction for improved contrast is
D = objective or main mirror diameter in mm
P = power or magnification
t = transmission factor, usually 0.85–0.9.
Telescopes at large observatories are typically located at sites selected for dark skies. They also increase the limiting magnitude by using long integration times on the detector, and by using image-processing techniques to increase the signal to noise ratio. The Keck Telescope, for example, 10 meters in diameter, can detect stars at 24 to 26th magnitude using a one-hour integration and adaptive optics techniques.
Even higher limiting magnitudes can be achieved for telescopes above the Earth's atmosphere, such as the Hubble Space Telescope, where the sky brightness due to the atmosphere is not relevant. For orbital telescopes, the background sky brightness is set by the zodiacal light. The Hubble telescope can detect objects as faint as 31st magnitude, and the James Webb Space Telescope (operating in the infrared spectrum) is expected to have a magnitude limit of 34th magnitude.
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