Binoculars are a valuable tool for observing the night sky. While there are many merits to their use, you may find yourself disappointed with a binocular view of the sky if you don’t know what kind of binoculars are best suited for this use, and how to properly use them. To get the most out of a binocular observing session, you’ll want to select an appropriate pair of binoculars, and know how to focus them correctly.
If your first question to assess how desirable a pair of binoculars is is about magnification, you’re asking the wrong question. The most important function of these optical aids is to make faint things brighter. Used in conjunction, magnification does help us see some things in the sky better, but there are many beautiful sights in the sky that don’t need much or any magnification. They are big enough to see with the unaided eye, but they are not bright enough. The size of the main lens determines how much light is collected by a telescope. Binoculars are a pair of telescopes—one for each eye. Binoculars intended for daytime, brightly lit use, do not need big lenses. For sky gazing, 50mm lenses are a good size. You can go bigger to get more light, but as the lenses get bigger, they get heavier and more difficult to hold. Some observers choose to get a stand for their binoculars, so they can use ones with really big lenses.
When using handheld binoculars, you won’t want too much magnification, especially if they are new, and you are getting adjusted to using them. Your hands will not hold binoculars perfectly steady, and the more magnification, the more you notice the constant movement of your field of view. You don’t need much magnification anyway. Leave the objects that need high magnification for the telescope. Binoculars are best used for objects that don’t need much magnification, but are too faint to be seen with the naked eye. Binoculars with around 7x magnification are ideal. Only try higher magnification after you are used to something below 10x first.
Prisms do a number of things for binoculars, namely shortening the barrels, and flipping the image right side up. There are two main kinds of prisms you will find in binoculars: roof prisms, and porro prisms. Both kinds of prisms serve the same purpose; they are merely different designs. Porro prism binoculars are a little better for low light use, because they allow a little more light to reach your eye. Roof prisms allow for more compact binocular design, so they are often chosen over porro prim binoculars for daytime use. Roof prism binoculars also tend to cost a bit more, because they are a bit harder to manufacture. Since porro prism binoculars get a little more light through, and are more inexpensive, they are usually a better choice for nighttime use. If a desire for compact design is more important or useful to you than getting the absolute highest amount of light through, roof prism binoculars are a perfectly acceptable option.
Glass is transparent, but you’ve surely seen light reflecting off of glass. Only a small percentage of light is lost through reflection, but it’s better to not lose any light at all when observing extremely faint objects. Optics can be coated in order to reduce reflection, and increase the amount of light that passes through the lenses and prisms within binoculars. Binoculars advertised as having coatings let more light through—and every bit helps. They will, however be a little more expensive for it—it’s worth it for nighttime use.
How to Focus
Focusing binoculars is not quite as simple as just twisting a knob. Each side of your binoculars will focus separately. This feature is beneficial for when your eyes have different focal needs, but can cause problems, if the two sides are not focused with the right offset for your eyes, and you are unaware how to correct it. In the video below, Kitt Peak Visitor Center guide Brian Robinson shows you how to focus your binoculars, and identifies a few more key features of binoculars.