Astronomical binoculars are instruments used to view astronomical features in the night sky, such as the moon, the Milky Way, and the planets. Astronomical binoculars are less expensive and often easier to use than many telescopes, since people can look through the binoculars with both eyes. As a result, they are usually good introductory instruments for people interested in astronomy. Although most binoculars look the same, there are distinct differences between brands and models. When choosing between types, the magnification of the astronomical binoculars is quite important, as are the objective power and the exit pupil size.
All astronomical binoculars have specific numbers dedicated to them indicating the magnification and the objective power. These numbers for astronomical binoculars are usually 7x50 or 10x50. The first number is the magnification number. For example, for a 10x50 pair of binoculars, the objects in the sky are ten times larger than they would be if they were viewed without binoculars.
The second number indicated on most astronomical binoculars is the objective diameter in millimeters. This is the diameter of the big lenses that are seen in the front of the binoculars. For example, in a 7x50 pair of binoculars, the objective diameter would be 50 mm. Although the objective diameter of some binoculars is quite large, many astronomers find that the larger sizes make the binoculars cumbersome and heavy. Generally, stargazers like to use binoculars because they are lightweight and not strapped to a tripod. As the size increases, so does the need to use a tripod.
Another feature of astronomical binoculars is the exit pupil number. Basically, the exit pupil should be no larger than the pupils of the stargazer’s eyes. If it is larger than the stargazer’s pupils, the extra light is simply not used by the eye. Generally, people in their thirties have a pupil size at night of about 7 mm. When the stargazers reach 40 years old, their pupil sizes drop to 6 mm. The pupil size then continues to drop by about a millimeter every ten years.
To determine the exit pupil size in millimeters, divide the objective diameter by the magnification. So, the exit pupil size for a 10x50 pair of binoculars would be 5 mm. Typically, the larger the magnification number, the smaller the exit pupil will be. The smaller the exit pupil is, the less light is wasted. So, there is no sense paying for a larger exit pupil size, when it will not be used by the stargazer.