Limiting magnitude
Encyclopedia
In astronomy
, limiting magnitude is the faintest apparent magnitude
of a celestial body
that is 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 reducing skyglow
and light pollution
.
, limiting magnitude frequently 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.
There is even variation within metropolitan areas. For those who lives 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.
As a first approximation, the gain in magnitudes of a telescope is 5 * Log10(D1/D0), 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 6-7mm (~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 is 5, the telescope will allow one to see stars as faint as about magnitude 13.
In reality a telescope allows to see much fainter stars because at higher powers the background is darkened and contrast increased. A typical 10-inch scope at high power (250X or more) will easily reach magnitude 15. See the Telescope Limiting Magnitude Calculator. Derived from this site the formula is
mv = m_nakedeye -2 + 2.5*log10(D*P*t)
where
D = objective or main mirror diameter in mm
P = power or magnification
t = transmission factor, usually 0.85-0.9.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, limiting magnitude is the faintest apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
of a celestial body
Celestial Body
Celestial Body is a Croatian film directed by Lukas Nola. It was released in 2000....
that is 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
Standard deviation
Standard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" there is from the average...
"). Sometimes limiting magnitude is qualified by the purpose of the instrument (e.g., "10th magnitude for photometry
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
") This statement recognizes that a photometric detector can detect light far fainter than it can reliably measure it.
The International Dark-Sky Association
International Dark-Sky Association
The International Dark-Sky Association is a U.S.-based non-profit organization incorporated in 1988 by founders Dr. David Crawford, a professional astronomer, and Dr. Timothy Hunter, a medical doctor/amateur astronomer...
has been vocal in championing the cause of reducing skyglow
Skyglow
Skyglow is the illumination of the night sky or parts of it. The most common cause of skyglow is artificial light that emits light pollution, which accumulates into a vast glow that can be seen from miles away and from high in the sky...
and light pollution
Light pollution
Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...
.
In amateur astronomy
In amateur astronomyAmateur astronomy
Amateur astronomy, also called backyard astronomy and stargazing, is a hobby whose participants enjoy watching the night sky , and the plethora of objects found in it, mainly with portable telescopes and binoculars...
, limiting magnitude frequently refers to the faintest stars that can be seen with the unaided eye near the zenith
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...
on clear moonless nights. The quantity is most often used as an overall indicator of sky brightness
Sky brightness
The fact that the sky is not completely dark at night can be easily observed. Were the sky absolutely dark, one would not be able to see the silhouette of an object against the sky....
, in that light polluted and humid areas generally have brighter limiting magnitudes than remote desert or high altitude areas.
There is even variation within metropolitan areas. For those who lives in the immediate suburbs of New York City
New York City
New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...
, 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
Brooklyn
Brooklyn is the most populous of New York City's five boroughs, with nearly 2.6 million residents, and the second-largest in area. Since 1896, Brooklyn has had the same boundaries as Kings County, which is now the most populous county in New York State and the second-most densely populated...
, Queens
Queens
Queens is the easternmost of the five boroughs of New York City. The largest borough in area and the second-largest in population, it is coextensive with Queens County, an administrative division of New York state, in the United States....
, Staten Island
Staten Island
Staten Island is a borough of New York City, New York, United States, located in the southwest part of the city. Staten Island is separated from New Jersey by the Arthur Kill and the Kill Van Kull, and from the rest of New York by New York Bay...
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
Manhattan
Manhattan is the oldest and the most densely populated of the five boroughs of New York City. Located primarily on the island of Manhattan at the mouth of the Hudson River, the boundaries of the borough are identical to those of New York County, an original county of the state of New York...
, 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.
Calculating limiting magnitude
Limiting magnitude can be calculated by using a telescope.As a first approximation, the gain in magnitudes of a telescope is 5 * Log10(D1/D0), 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 6-7mm (~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 is 5, the telescope will allow one to see stars as faint as about magnitude 13.
In reality a telescope allows to see much fainter stars because at higher powers the background is darkened and contrast increased. A typical 10-inch scope at high power (250X or more) will easily reach magnitude 15. See the Telescope Limiting Magnitude Calculator. Derived from this site the formula is
mv = m_nakedeye -2 + 2.5*log10(D*P*t)
where
D = objective or main mirror diameter in mm
P = power or magnification
t = transmission factor, usually 0.85-0.9.
See also
- Night skyNight skyThe term night sky refers to the sky as seen at night. The term is usually associated with astronomy, with reference to views of celestial bodies such as stars, the Moon, and planets that become visible on a clear night after the Sun has set. Natural light sources in a night sky include moonlight,...
- Sky brightnessSky brightnessThe fact that the sky is not completely dark at night can be easily observed. Were the sky absolutely dark, one would not be able to see the silhouette of an object against the sky....
- Light pollutionLight pollutionLight pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...
- SkyglowSkyglowSkyglow is the illumination of the night sky or parts of it. The most common cause of skyglow is artificial light that emits light pollution, which accumulates into a vast glow that can be seen from miles away and from high in the sky...
- Dark-sky movementDark-sky movementThe dark-sky movement is a campaign by people who want to reduce light pollution so people can see the stars, to reduce the effects of unnatural lighting on the environment, and to cut down on energy usage....
- International Dark-Sky AssociationInternational Dark-Sky AssociationThe International Dark-Sky Association is a U.S.-based non-profit organization incorporated in 1988 by founders Dr. David Crawford, a professional astronomer, and Dr. Timothy Hunter, a medical doctor/amateur astronomer...
(IDA)