How to Buy Binoculars for Stargazing

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How to Buy Binoculars for Stargazing

Binoculars are often used by stargazing enthusiasts to view the night sky. While not giving the high detail possible with fixed telescopes offering high magnifications, binoculars have several advantages that make them popular tools amongst even experienced astronomers.

About Binoculars for Stargazing

Some advantages of binoculars over telescopes when it comes to stargazing include size, weight, and portability, making it easier to transport them and use them in different locations. Binoculars also have a wider field of view than telescopes, which typically require a finder to locate objects. Both eyes can be used. Their arrangement of prisms means that objects are orientated the correct way up, whereas the view offered by astronomical telescopes is inverted. All this makes them easier and more natural to use. Binoculars are also typically cheaper than telescopes and are versatile, having various other applications besides stargazing.

Choosing Binoculars for Stargazing

While binoculars have various applications, when it comes to stargazing, there are some specific considerations. Stargazing is done at night, so it is essential to choose binoculars that are suitable for dark conditions. Objective lens diameter is one of the most important factors here. A high enough magnification is required, but it is also a good idea to have binoculars with a wide enough field of view to appreciate a decent section of sky. All these factors need to be weighed up against matters of budget, size and portability.

Binoculars for Night Use

As well as objective lens diameter, exit pupil and light transmission are other important considerations.

Objective Lens Diameter

The objective lens diameter is simply the measurement across the objective lenses, which is measured in millimetres (mm). It is written on the binoculars after the magnification. When viewing objects at night or even in lower light conditions, such as dawn and dusk, a larger objective lens is necessary to obtain a clear image. This is because the primary role of objective lenses is to allow the entry and transmission of light. Larger lenses allow more light to pass through, and more light means that the image appears brighter. Of course, image brightness is also dependent upon several factors, so this assumes that all other factors are equal. Objective lens diameter has a direct bearing on how large the binoculars are, so binoculars with larger objective lenses will be larger than those with smaller objective lenses. The objective lens diameter is actually used to classify binoculars into size, with standard sized binoculars over 30 mm and compact binoculars under 30 mm. Generally speaking, compact binoculars are not suitable for stargazing as they do not allow enough light to enter.

Exit Pupil

Exit pupil is the diameter of the light beam that is transmitted from the ocular lenses in the eyepiece to the eye. It refers to how bright the image is. Exit pupil can be calculated by dividing the objective lens diameter by the magnification. So a 10x25 pair of binoculars, with a 10x magnification and a 25 mm objective lens diameter, has an exit pupil diameter of 2.5 mm. An 8x42 pair of binoculars, with an 8x magnification and a 42 mm objective lens diameter, has an exit pupil diameter of 5.25 mm. The light that can then pass into the eye is also affected by iris diameter. This varies according to light conditions. In ordinary daylight, it is about 2.5 mm. Taking the exit pupil diameter of the 10x25 binoculars, this means that the diameter of light that can enter the eye is the same as iris’ diameter in daylight. A larger objective lens diameter is not necessary, and most binoculars, including compacts, are fine for daylight viewing. However, at night, iris diameter can widen to about 6 mm or more. So the 8x42 binoculars would provide around twice as much light as the 10x25 binoculars, and just slightly less than can enter the eye. They would be suitable for night time viewing, though even larger objective lenses could give even brighter images. Generally speaking, binoculars with an exit pupil of 4 mm or more are fine for stargazing. Another benefit of a large exit pupil is that the image remains clearer if the binoculars move.

Twilight Factor

Another measurement sometimes given is the twilight factor, which is the square root of the value obtained by multiplying magnification by objective lens diameter. Higher twilight factors indicate more detailed images under low light conditions. It is not as frequently used as exit pupil diameter.

Light Transmission and Optical Coatings

Other factors also affect the amount of light that can enter the eye. These include the coatings used on lenses and prisms to aid the transmission of light. Which coatings are used should be stated in the specification or product description. Coatings include anti-reflective coatings, phase correction coatings, aluminium or silver mirror coatings and dielectric coatings. Fully multi-coated binoculars are best as they have been layered throughout with multiple coatings. Fully coated binoculars have been layered throughout with a single coating.
The coatings help light transmission by minimising the amount lost through reflection at air to glass interfaces. This enables more light to reach the eye and results in a brighter, clearer image. Binoculars have a light transmission measurement, given as a percentage, which indicates how much light reaches the eye from its entry at the objective lenses.

Other Considerations

Other considerations when purchasing a pair of binoculars for stargazing include magnification, field of view, and image stabilisation.

Magnification

The magnification is the amount by which the binoculars magnify the image, or appear to bring it closer. It is written on the binoculars before objective lens size. Higher magnifications allow more detail to be revealed, but they also give a narrower field of view and make the object more difficult to locate and hold steady. Standard magnifications are 8x and 10x, and anything greater really requires use of a tripod unless it has in-built image stabilisation.

Field of View

Field of view is expressed either as an angle or as the width at a specific distance, for example 5 degrees or 100 m at 1000 m. Field of view is affected by various factors, including binocular design and size, but especially by magnification and the eyepiece. Porro prism binoculars, with a traditional M shaped design, generally offer a wider field of view, as do larger binoculars and those with a smaller magnification. However, the eyepiece also plays a part, with more complex eyepieces affording wider fields of view for a given magnification. It can be worth looking for wide view binoculars if this is an important feature. However, these binoculars are also more expensive. Depending on budget, they can certainly be recommended as binoculars with a wider field of view allow a larger section of sky to be observed.

Image Stabilisation

Image stabilising microscopes have in-built mechanisms to hold the image steady, reducing blurring and any tendency to lose the image from the field of view. They can be activated at the press of a button, and the steady image obtained means that more detail can be observed, with all else equal. Disadvantages of image stabilising microscopes include their size and weight, as well as the additional expense, but they are highly recommended for stargazing.

Find Binoculars for Stargazing on eBay

There are many listings of binoculars suitable for stargazing on eBay. Simply go to the eBay home page, follow the Electronics tab on the left to the Cameras & Photography page, and select the option for Telescopes & Binoculars followed by Binoculars & Monoculars and Binoculars. Browse the listings for those binoculars with the features suggested above.
Binoculars can also be searched for using the search tab at the top of the screen. A search for “astronomy binoculars” will bring up binoculars that are specifically designed for stargazing.

Conclusion

Binoculars have several advantages over telescopes when it comes to stargazing, including size and portability, width of view, ability to use both eyes, and correct orientation. This makes them easier to use, especially for the beginner, but also for more experienced astronomers. Certain considerations need to be kept in mind when purchasing binoculars for night time use. The objective lens diameter should be large to allow a decent amount of light to enter the lens and pass through to the eye. Exit pupil diameter is the diameter of the light beam transmitted to the eye, and exit pupils of 4 mm or more, which allow brighter and clearer images, are recommended for stargazing. Optical coatings also help light transmission. A wide field of view enables a good section of sky to be observed. Lastly, image stabilising microscopes have in-built mechanisms to hold images steady. This helps the image to be observed in more detail.

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