The Binocular Site

Finding the Right Astronomy Binoculars

Viewing the Moon Viewing the night sky offers some interesting challenges for binoculars.

While telescopes are a popular choice for astronomy, binoculars can provide views that are just as powerful while remaining more portable, lighter, and easier to use.

The trick is to carefully consider the best binocular features for this purpose. A dark environment and strong contrast between sky and stars requires optimum use of light, magnification, and focus.

The Importance of Light

Light can be maximized through choosing the best prism type, objective lens, and exit pupil.

  • Porro prisms are the best choice here – they transmit the most light and offer the sharpest image. The glass type used for the prism is paramount, and Bak-4 is the preferred option. Bak-4 is available in almost all astronomy binoculars.
  • The objective lens must be as wide as possible, ensuring again that the most light will be picked up by the binoculars. A higher diameter, which is given as the second number in specifications such as “7x50”, will maximize your viewing experience.
  • The exit pupil is an opening inside the lens through which light can pass on its journey through the binoculars. Since your pupils will be dilated in the dark, you need your binoculars to have a larger exit pupil as well. To calculate the exit pupil, divide the objective lens diameter by the magnification. For example, if your binoculars are “7x50”, divide 50 by 7, to get an exit pupil of approximately 7mm. For the best results look for an exit pupil of 7mm or less, as this is the average size of the human pupil at night. By contrast, binoculars meant for day use would need to be even smaller.

What Kind of Magnification Do You Need?

Magnification must be considered against both focus and light. Very high magnifications in binoculars are as effective as those in telescopes, yet require as large an objective lens as possible to keep that viewing area wide.

The magnification, which is given as the first number in specifications such as “7x50”, can be anywhere from 7x up for effective astronomy use. Yet a magnification of 25x or 30x combined with “giant binocular” objective lenses of 60mm or more will reveal an amazing view of the stars that can rival a telescope.

On the other hand, obtaining this level of magnification and lens size requires a heavy set of binoculars that must be supported by a tripod to ensure they remain steady and usable.

Clearly Focused

Focus is perhaps the least imperative for a good view, yet still can make the difference for your astronomy experience. Both center- focus and individual eyepiece focus can be used. However, while center focus is more flexible and easier when you use your binoculars by day as well, individual eyepiece focus is more robust and practical otherwise.

You will also want to consider eye relief (ie. how far you can hold the binoculars from your eyes) for greatest comfort, especially if you use glasses.

Binoculars are an excellent choice for gazing into the night sky, wondering what mysteries exist in the universe around us. They offer a portable, easy to use option for night time viewing, when the right features are available.

Take the time to consider optimum light, magnification, and focus, so you can contemplate the infinite sky.

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