Binoculars greatly expand your view of the night sky

Saturday, Nov 14, 2020 4:03 AM

Greetings, stargazers.

Every couple of years, I write a column about binocular observing because there are lots of things in the sky that benefit from the extra light gathering with minimal magnification. But it has been eight years since I have written much about binoculars themselves.

Not much has changed with the binocular marketplace in the last decade, including the price. Binoculars are great in their versatility, portability and affordability, and for the price of a truly entry-level telescope that you will either outgrow or get bored with in a couple of years, you can get a nice pair of binoculars that should last a lifetime.

It is easy to think about a pair of binoculars as being two tiny telescopes that are attached side-by-side that you can hold in your hands. While this is true, the optical path through binoculars is much more complicated than the one through a telescope. For binoculars to have an image that is both right side up, and not inverted left to right, there must be four reflections along that light path. Telescopes have at most one reflection, and those attached to cameras have no reflections. In binoculars, the reflections are made using prisms in one of two configurations – either a Porro prism or a roof prism. Roof prism binoculars are more compact, but also more expensive, and because there is no compelling optical advantage, most astronomical binoculars are the Porro prism design.

Binoculars are identified with a pair of numbers that represent the magnification and diameter of the front lens. For example, 10x50 binoculars will magnify by a factor of 10, and the diameter of the front lens is 50 mm (about 2 inches). Ultraportable bird-watching binoculars might be 7x20, and you might find giant 20x100 binoculars mounted on a permanent pier at a national park scenic overlook. The pupils of young, healthy eyes are about 7 mm in diameter, so astronomers jokingly refer to naked eyes as 1x7 binoculars.

For stargazing, 10x50 is the size I most often recommend. It falls in that range of not too small and not too big. Because light-gathering ability is the only thing that matters for stargazing (that goes for telescopes, too), the bigger the lens, the better the view. Binoculars that are 10x50 gather about 50 times as much light, so you can see things 50 times dimmer. To get the same factor increase in light gathering over 50 mm binoculars, you would need to get a pair of 14-inch diameter telescopes.

Tripods really do improve the view, but consequently reduce the portability. Binoculars bigger than 50 mm are too heavy for most people to hold steady for any length of time. Similarly, magnifications greater than 10 will give you a shaky view that is much more noticeable when stargazing.

Shopping for binoculars can be like many other things – there are bargains and lemons. But for a wide price range you get what you pay for. My opinion is that almost any pair of binoculars will give you an improved view of the sky, but there are some things that you might find worthwhile to pay extra for.

The most useful extra for night viewing is anti-reflective coatings. All binoculars have numerous air-to-glass transitions, where reflections will happen, and your image gets worse at each surface. If you wear eyeglasses, you may know anti-reflective coatings make more of a difference in cutting down glare at night, and this is especially true when stargazing. More expensive binoculars tend to have more surfaces coated with better coatings.

Another thing to consider is what is called eye relief. That is how far from the lens your eye needs to be to see the full image. Lower cost binoculars generally have short eye relief. If you wear glasses, or even just sunglasses during daytime use, you might consider binoculars made with extra-long eye relief so you don’t need to remove your other eyewear to use them.

Image stabilization and weatherproofing are more options you could consider.

This month

Jupiter and Saturn are in the southwestern sky at dusk. Most binoculars will let you see the four Galilean moons of Jupiter, and those will be in a different configuration in a line along the ecliptic every night. If you can only see three of the moons, then likely the other one is in front of or behind Jupiter, or maybe just too close to resolve. Some binoculars can give you a hint that Saturn isn’t round, but looks elongated, or more like a football. Mars is still very bright in the southeastern sky, but binoculars won’t show you more than a bright orange spot.

The Pleiades, or seven sisters, are a nice naked-eye cluster, but are an outstanding sight in binoculars. They are rising in the east at dusk. A little after 9 p.m., look for the three belt stars of Orion rising due east, right along the celestial equator. The Orion nebula, a bit to the right of the belt, is the brightest star-forming region that we can see and is one of the most rewarding targets in binoculars.

Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at hakes_c@fortlewis.edu.