Binoculars

Binoculars are the ideal instrument for beginners. They are cheap, portable, easy to use, and will help you learn your way around the night sky before you move on to a telescope.

aperture and magnification

Binoculars can be thought of as two small telescopes fixed together. Inside the binocular, two prisms fold up the light path, making them more compact than a normal telescope. Every binocular is marked with two numbers, such as 7 x 40 or 10 x 50. The first figure is the magnification, which is the enlargement relative to the naked-eye view. The second figure is the aperture (that is, the width) of the front lenses measured in millimeters. Wider apertures let in more light, making fainter objects visible. However, larger-aperture binoculars are also inevitably heavier and more expensive than smaller ones. Binoculars with zoom magnification are available but generally produce poorer images.

keeping binoculars steady

For a steadier view when observing through binoculars, support your elbows on something solid, such as the arms of a chair. Alternatively, sit with your elbows resting on your knees.

field of view

The field of view (also simply called the "field") of binoculars is typically 3° to 5° across, equivalent to six to ten Moon diameters. This is a good deal larger than the field of most telescopes, allowing you to scan large areas at a glance. Some objects are ideally suited to observation with binoculars, such as comets with long tails, scattered star clusters, and Milky Way star fields.

focusing binoculars

For best results, binoculars must be adjusted to suit the user's eyes. First, pivot each half of the binocular around the central bar until the distance between the eyepieces matches that of your eyes. For focusing, most binoculars have a knob on the central bar, plus one individually adjustable eyepiece. In some models, however, both eyepieces have to be adjusted individually.

focus LEfT

focus LEfT

1 Close the eye on the side of the adjustable eyepiece. Turn the central knob to focus the image you see with your other eye.

2 Now close the other eye and turn the individually adjustable eyepiece to bring that eye into focus.

3 From then on, you need only turn the central knob to bring both eyes into focus simultaneously.

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