Introduction and Background
The limiting visual magnitude is technically defined as the faintest magnitude that can be seen by the unaided eye at the zenith. This paramater is influenced by the transparancy or clarity of the atmosphere. This parameter is also influenced by the degree of light pollution present at an observing sight. The limiting visual magnitude is often used as an indicator of atmospheric transparancy and the general quality of the observing conditions at an observing site. For example, meteor observers always record the limiting magnitude during an observation session, and this parameter is used to facilitate the comparison of observations made by different observers under different observing conditions.
The limiting magnitude will certainly be related to the total number of stars (or meteros) that can be seen. The fainter the limiting magnitude, the more stars can be seen during a particular observing session.
Of course, many factors in addition to atmospheric clarity affect your ability to see faint stars. These factors include:
In general, optical instrumentation improves your ability to see faint stars. The limiting magnitude using binoculars or a telescope will be fainter than the limit for the unaided eye.
Dark AdaptionIn order to see faint stars, you must allow your eyes to adapt to the dark. If you go outside at night (or sit in a dark room during the day) your eyes will slowly become more sensitive to light. The process goes quickly at first. This is why you can walk into a dimly lit room from the outdoors on a sunny day and not walk into furniture or walls. The increase in sensitivity to light proceeds far more slowly after the first few seconds.
Two processes determine the sensitivity of your eyes to light. First, your eyes dilate their pupils under low light level conditions. Under dark conditions, your pupils may open as wide as 7 mm in diameter. This is much larger than the 1-2 mm which is the normal opening. Second, chemical processes take place in the cells of your retina which result in increased sensitivity to light.
The eyes can dilate rather quickly, but the chemical changes proceed much more slowly. It takes 10 to 20 minutes before the eyes reach their maximum sensitivity to light. This maximum sensitivity is termed dark adaption. Any bright light that enters the eyes when they are more sensitive to light immediately reverses the dark adaption process, and it will again take 10 to 20 minutes to reach full dark adaption. Light sources such as a flashlight, a match or cigarette lighter, car headlights, or a computer monitor will destroy dark adaption. If you want to achieve full dark adaption, make sure your eyes are shielded from any nearby direct lighting.
Red light is less effective than white light at reversing the chemical processes that make the eye more sensitive. Thus, dark adaption is not as adversley affected by red light. This is why observatories often have red lights and astronomers often carry red flashlights.
Light Pollution
Any artificial light that is directed upward toward the sky or reflected upward will make the sky appear brighter. This will decrease the contrast for objects in the sky and make it more difficult to see faint stars and meteors, as well as nebulous objects such as the milky way, comets, and sky phenomena such as the zodiacal light.
Distant lights from parking lots, airports, sports facilities, and commercial buildings send large numbers of photons upwards into the sky. Some of these photons are scattered from dust particles and air molecules, and the head back toward the ground. You can detect such distant sources of light as bright areas near the horizon. All such areas have light domes surrounding them that can be seen from great distances. Cities themselves have light domes. The closer and brighter the source, the farther up from the horizon the light pollution will spread..
If there are any clouds in the sky, they will appear as slightly brighter areas due to the light from below which reflects off the clouds. At a very dark observing sight clouds at night can be very difficult to detect. Scattered clouds will appear as dark areas where no stars or fewer stars ar visible.
A Limiting Magnitude Model
A theoretical model has been developed by a grad student in astronomy at Columbia University to predict the limiting visual magnitude. The model considers such factors as the longitude and latitude of the observer, elevation above sea level, air temperature, age of the observer, altitude and azimuth being viewed on the sky, date, time, position and phase of the moon, etc. The model is available on the web and is very easy to use.
Limiting Magnitude Model Calculations This model does not consider any artificial sources of sky illumination. Thus, any amount of light pollution will result in differences from the model predictions. A model such as this can be used to document the degree of light pollution at specific locations.
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The classical determination of limiting magnitude involves comparing the sky with charts which shows stars with various magnitudes. Limiting magnitude is determined by determining the faintest star on the chart that can be seen. This magnitude is then considered the limiting magnitude.
Of course, technically, the actual limiting magnitude could be somewaht fainter than the faintest star actually seen. Also, since the limiting magnitude is defined as the faintest star that can be seen at the zenith, the actual limiting magnitude could be somewhat fainter than that observed unless the sky region used was actually at the zenith for the observer. These technicalities are generally ignored.
For northern hemisphere observers, it is convenient to use the vicinity of the sky surrounding the north celestial pole. Thus, the constellation of UMi is often used for limiting magnitude determinations. As an example, the following illustration shows some magnitudes for stars in the vicinity of the little dipper.
Limiting magnitude charts are available for other regions as well.
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A system has been developed to determine limiting magnitude using star counts. A number of standard regions have been established around the sky. The regions are triangular in shape and formed by relatively prominent bright stars. Limiting magnitude can be determinee by counting the number of stars that can be observed in these standard regions. A table for each region provides a calibration of limiting magnitude versus number of stars counted. (The count includes the three stars that define each triangle.) This system was originally developed by amateur astronomers in Finland.
An example of one of these standard regions and the calibration table is indicated in the following illustration.
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A web site devoted to this method is available.
A special web site is also available which automates the process of using the star count method. From this web page an observer enters latitude plus date and time. The web page then selects three optimized standard regions that are observable. Finding charts are presented for these three regions. The observer then enters the star counts for each region and the web page determines the limiting magnitude using the entered data.
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