Binocular Stargazing

Quietly gazing skyward on a clear night, far from the hustle and bustle of city life and the glow of city lights, is one of life's most tranquil pleasures. As the tapestry of stars passes silently overhead, we seem somehow removed from normal day-to-day concerns, instead becoming part of a bigger picture, part of the universe.

With our eyes alone, we can marvel at all of those distant stars, each similar to our own Sun, but lying at incredible distances away. Scattered among those stars are riches that go unsuspected with the eye alone. Huge clouds of gas and dust called nebulae, large swarms of stars clustered densely together, and giant island universes called galaxies all lie in wait for an astronomer's gaze.

Many people are under the false impression that these celestial wonders can only be seen through large, expensive telescopes. In reality, while telescopes will reveal a treasure trove of stellar jewels, our voyage into the universe can begin with just a pair of binoculars. The same pair that you might already own and use for birding, or while hiking, or when watching a sporting event will start you on your flight through the cosmos.

Understanding Binoculars
Let's begin with some binocular jargon. Light enters each side of the binoculars through a large lens called the objective and passes through a set of prisms so that the resulting image is right-side-up when it exits through the eyepiece and into the observer's eye. There are two types of prism assemblies used in today's binoculars: porro prisms and roof prisms.

All binoculars are rated by two numbers separated by an x, such as 7x35 (pronounced "7 by 35"), 10x50, and so on. The first specifies the magnification, while the second refers to the aperture, or the diameter, of the binoculars' front objective lenses, in millimeters. Magnification, a measure of how much larger an object will appear, is one of the most important considerations when selecting a pair of binoculars. Low-power binoculars (e.g., 7x to 10x) are best to begin with, since they are light enough to hold by hand easily. Larger, heavier binoculars usually require a tripod or other support.

Selecting the proper aperture is just as important as choosing the right magnification, since that governs how much starlight the binoculars will gather. The objective diameters should be selected to match your observing conditions. We know that the amount of light entering the eye is controlled by the central opening in the iris called the pupil. For most people, their pupils contract to about 2.5 millimeters in bright daylight and expand, or dilate, to about 7 millimeters in total darkness. From more light-polluted environs, the eye's pupil may only open to 4mm or 5mm. Age will also lessen the pupil's dilation.

Ideally, the diameter of the beams of light exiting the binoculars' eyepieces, known as their exit pupils, will be the same as the diameters of the observer's pupils. To calculate the exit pupil, simply divide the objective's diameter by the power to find out. Therefore, a 7x50 pair has a 7.1-millimeter exit pupil and is great for dark skies, while 10x50 glasses have a 5-millimeter exit pupil and are better suited for suburban and urban stargazers.

The best astronomical binoculars also feature fully coated optics (better still, fully multi-coated) and prisms made of a type of glass called BaK-4. These guarantee the brightest, sharpest views while also minimizing glare.