Sky Talk April 2011: Binoculars for Stargazing

Many books and articles have been written over the years extolling the pleasures of stargazing using binoculars, all based on the premise that “two eyes are better than one.” And indeed they are! Every major class of celestial wonder from the Moon and planets to star clusters, nebulae and galaxies lie within reach of even the humblest glasses. But some are better suited than others for the purpose, and knowing something about their specifications will help in selecting a pair.

Two primary parameters of a binocular are its aperture and its magnification. Those most often used for sky viewing have objective lenses 35mm or 50mm in diameter. Ones much larger in size are difficult to hold steady and require tripod mounting, while mini-binocs really don’t pull in enough light for celestial viewing. The magnification most typically found on stargazing glasses is either 7x or 10x; thus a 7×50 binocular has objectives about 2-inches in diameter and provides 7 power. This famed “night glass” (as it’s often called) is the one typically recommended for stargazing. But while certainly very serviceable, this is not the optimum combination due to an important factor known as the exit pupil.

The exit pupil relates to that of both a binocular and the opening of the human eye. In the former case, the diameter of the bundle of light emerging from a binocular’s eyepieces is found by dividing the aperture by the magnification. Thus a 7×50 glass has exit pupils 7mm in size. In the case of the eye, the exit pupil is often given as 7mm in size if fully dark-adapted. But in reality that’s rarely the case. As people age, the pupil does not open as wide as when they were younger. And any ambient light from various sources of light-pollution (so prevalent in today’s world) around the observer also contracts the pupil. In practice, 5mm diameter exit pupils are more typically found when viewing the night sky. So some of the light of a 7×50 binocular is not able to enter the eye and wasted, while the 5mm pupils provided by a 7×35 or 10×50 glass allow full use of their objectives’ light-gathering power — making either ideal for stargazing.

In actual practice, 10×50 glasses have a decided advantage. Not only do they pick up more light and offer somewhat better resolution due to their larger objectives, but that extra 3x of power is enough to tease more detail from their images. As an example, the satellites of Jupiter (depending on where they are in their orbital dances about the planet) can be glimpsed in a 7×50 binocular but stand out much better in a 10×50.

The subject of binoculars is a vast one and space prohibits covering such matters as wide-angle and extra-wide-angle glasses, porro-prism verses roof-prism designs, and image-stabilized and zoom binoculars. But there is one other feature of binocular stargazing that simply must be mentioned. And that is using both eyes gives an amazing sense of depth perception or 3-dimentionality to what’s seen! Just look at how the Moon appears to be suspended in space — especially during a lunar eclipse, or when it glides across (or occults) a glittering star cluster like the Pleiades. While simply a physiological illusion, it’s nonetheless quite spectacular!

(See Edmund’s binocular category to browse their selection.)

–James Mullaney
Former assistant editor at Sky & Telescope magazine and author of eight books on stargazing.