A Guide to Binoculars (Part 1: The Basics)

Why Binoculars?

Newcomers to astronomy often rush to buy the first telescope they see in a department store, and this is perfectly understandable. All the beautiful pictures of celestial objects seen in magazines or in books have this effect on most people.

Unfortunately, after setting the telescope in the backyard and taking the first look at the night sky all the magic is gone. The instrument's narrow field of view and confusing image orientation will soon cause more frustration than fun.

The obvious alternative is to invest in binoculars, which have the advantages of being relatively inexpensive, highly portable and easy to use. The only disadvantage is that the magnification is generally fixed. Zoom models with variable magnification are also available, but for reasons you will find later in this article it is probably better to accept the fixed-power limitation.

Binoculars magnifying 5 to 8 times are ideal for finding the planets, scanning Milky Way star fields, studying star clusters such as the Pleiades and Hyades, and viewing bright comets. The Moon will also prove a constant source of enjoyment; the mountains, valleys and craters are beautifully brought out, and it takes only a few minutes to learn the most prominent features.

How Binoculars Work

To understand the working principle of binoculars, first you need to know a little about telescopes. In fact, this is exactly what binoculars are, two identical telescopes placed next to each other.

At the front of each telescope is a lens, called the objective. Its role is to gather light from whatever it is you're looking at and bring it to a focus in the eyepiece, where the light is formed into a visible image and magnified to take up a large portion of the retina. The magnification depends on the focal length of the eyepiece, and for binoculars it is usually between 5x and 10x.

Binoculars diagram
Binoculars functional diagram. [larger image]

The image produced by this telescope will be upside down and backwards, but for astronomical viewing this is not a major inconvenient. In space there is no up and down or left and right. However, for watching birds or following the action at a baseball game a right- side-up picture is essential. This is why binoculars use corrective elements between the objective and the eyepiece, called prisms.

Prisms used in binoculars are blocks of glass that function as mirrors, but without a mirror's reflective backing. They come in two models and use different types of glass, and we will
discuss about this later in the article. For now let's just mention their role, that is to bring the light beams from the objective closer together by means of internal reflection, and also turn the image right-side up and orient the view properly left to right.

To better understand the working principles take a look at the image above. It shows the path of the light as it enters the objectives, passes through a set of prisms that turn the image right side up, and finally leaves the eyepieces to enter the observer's eyes. This applies to all binoculars, no matter what model or size.

Next » A Guide to Binoculars - Part 2: Understanding the Terms

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