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by Shelly McRae
Telescopes have long been regarded as the primary tool for amateur and professional astronomers. But for backyard stargazers, the right tool could be a high-quality pair of binoculars. Less expensive than telescopes, binoculars can provide the stargazer with views of the moon, planets and even the occasional galaxy. Just as if you were buying a telescope, though, you want to consider several factors and features of the different kinds of binoculars before making your purchase.
Aperture: The aperture for binoculars refers to the diameter of the front lenses. The diameter influences the brightness of the optics image. The larger the diameter, the brighter the optics image appears. The size of the aperture can be found on the binoculars themselves. Manufacturers place this information, along with the magnification, in varying locations. On the binoculars, you'll see a pair of numbers, such as 7x50. The second number, 50 in this example, is the aperture size in millimeters, so the diameter of the lenses is 50 mm. For stargazing, your binoculars should have an aperture of at least 40 mm.
Magnification: As mentioned in the explanation of aperture, the binoculars have a pair of numbers, such as 7x50, listed on them. The first number represents the magnification power of the binoculars. The times sign between the two numbers is linked to magnification. In the example of 7x50, the binoculars have a magnification of seven times the naked eye. The object viewed through the lens is magnified by a factor of seven. The magnification should ideally be between seven and ten. Coupled with an aperture of no more than 50, you can hold your binoculars to stargaze. With larger binoculars, you should use a tripod. Otherwise, your image may appear shaky.
Field of view: Another set of numbers may be stamped on binoculars, something similar to 372 feet at 1,000 yards. This is the field of view. Because binoculars can be used for viewing earthbound vistas and objects, the field of view is a necessary feature. In this example, it means the binoculars offer a 372-foot "side-to-side" view of a vista viewed from 1,000 yards away. In stargazing, the distances are so great that the field of view becomes irrelevant. However, this feature is important for using binoculars for other purposes, such as watching a ball game or spotting wildlife.
Exit pupil: As you stargaze using binoculars, the light that reaches your eye as it leaves the lenses has a specific width. This is known as the exit pupil. The exit pupil is calculated by dividing the aperture by the magnification. The exit pupil of a 7x50 pair of binoculars is 7 mm. Your eyes dilate in the dark. If you are under the age of 30, your eyes may dilate to 7 mm. Between the ages of 30 and 40, your eyes may dilate to 6 mm. Over the age of 40, your eyes may dilate to no more than 5 mm. If you are a mature stargazer, you may not need an exit pupil of 7 mm, because your eyes are only going to take in 5 mm of light. The image may not be as crisp as it would be if you used binoculars with a smaller exit pupil.
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