Binoculars for Astronomy

Binoculars offer a number of advantages for stargazing. They are portable, easy to use, and provide wide-field views. There are objects which can be seen in binoculars that are difficult to view with a telescope. The Pleiades star cluster or the Andromeda Galaxy, for example, are too large to fit in the field of view of a typical telescope but are easily seen in a pair of binoculars. Binoculars are often recommended for beginning amateur astronomers, but there are also a number of misconceptions about binoculars for stargazing. Read on to learn all about binocular astronomy!

Advantages of Binoculars for Stargazing

Binoculars can be the easiest stargazing tool (aside from the unaided eye). You simply pick them up and look around. They are highly portable, making them well suited to traveling astronomers and anyone who wants to just run outside and view the stars for a few minutes.

Aside from their portability and ease of use, the primary advantage of binoculars is the wide field of view they provide. This makes sweeping the sky very simple. There are many objects which look better in binoculars than in a telescope, due to the large size of these objects. A typical telescope field of view is about 1 degree, or twice the size of the full moon. Many deep sky objects are actually larger than this. This seems counterintuitive at first; how can an object so far away look bigger than the moon? Despite its relative proximity, the moon is vastly smaller than any deep sky object, so even though they are far away, many nebulae, galaxies, and star clusters appear quite large.

Above: The Andromeda Galaxy, an ideal target for binoculars

The Andromeda Galaxy is a classic example. It appears six times as large as the full moon, three degrees across! Despite being 70 trillion times farther away than the moon, Andromeda appears this large because it is also 420 trillion times bigger than the moon! Andromeda's three-degree diameter makes it three times larger than a typical low-power telescope field. Binoculars, on the other hand, typically have fields of view in the range of 5-7 degrees, easily allowing the full extent of the Andromeda Galaxy to be seen.

Another primary advantage of binoculars is that they can be well-suited to multiple tasks. A good pair of stargazing binoculars will often be excellent for birdwatching, hunting, hiking, and watching sporting events.

Some people find viewing with two eyes to be much more comfortable than the usual one-eyed view through a telescope. This makes binoculars more comfortable to use. However, most experienced observers would say they find viewing with one eye to be fine, especially after having done it for so long. And for observers who find two-eyed viewing to be much more preferable, a binocular viewer may be added to a telescope to allow viewing with both eyes.

Disadvantages of Binoculars for Stargazing

The primary drawback to binocular stargazing is the limited magnification of binoculars. Most binoculars have magnifications in the range of 7x to 10x. While this is ideal for wide-field viewing, it is not nearly enough to resolve detail on the planets or in small star clusters. Higher power binoculars are available, but there is a significant trade-off. First of all, the highest power binoculars only provide about 25x magnification. Even a small telescope is capable of 50x to 100x or more. Also, increasing the magnification of a binocular decreases the brightness of the image, unless the objective lenses are increased in size to compensate (see below). This means that high-power binoculars require large lenses and therefore are quite large and heavy. For this reason alone, high-power binoculars require the use of a tripod.

Additionally, high-power binoculars (usually anything over 10x) are not easily hand-held, even if they are light enough and small enough. Few observers can hold even a 10x binocular steady, let alone a 25x pair. Again, a tripod is required. A large, heavy, expensive pair of binoculars that requires the use of a tripod tends to defeat the purpose of binoculars for stargazing: having a portable, easy-to-use setup. At some point, having a telescope becomes more practical. That said, many people like large, high-power binoculars for wide-field viewing to compliment their telescope. However, these observers often have a smaller, portable pair of binoculars as well, and the large binoculars are treated more like a type of telescope.

Types of Binoculars

The simplest way to think of binoculars is as a set of two very small refracting telescopes. There is a lens at the front and an eyepiece at the back. The difference is primarily that binoculars use prisms to fold the light path, making them more compact. There are two basic types of binoculars based on the type of prisms used.

Above: Porro prism (left) and roof prism binoculars

Porro prism binoculars, like those pictured above on the left, have offset prisms resulting in the familiar "bent" shape of many binoculars. Roof prism binoculars still fold the light through prisms, but do so without offsetting the light path. This results in a more compact design. Below are optical diagrams of the internal workings of each binocular type.

Above: Optical diagram of one half of a porro prism binocular. Light is focused by the objective lens (left side), through two prisms which offset the light path, and to the eyepiece (lower right).

Above: Optical diagram of one half of a roof prism binocular. Light is focused by the objective lens (left), through two prisms which keep the light path straight, and to the eyepiece (right). Note the extra reflection within the prisms versus the porro prism design above.

All thing being equal (which they rarely are), porro prisms can give a slightly better image than a roof prism. This is because of the extra reflection within the roof prisms which causes a slight loss of light. In actuality, roof prism binoculars offer many advantages for other applications (such as birding and hunting), including compactness and close-focus ability. For these reasons, roof prism binoculars are often the top-of-the-line models in many manufacturers' product lines. Often roof prism binoculars will outperform porro prisms because better glass and coatings go into the roof type because of their popularity.

However, for stargazing, a good pair of porro prism binoculars are often the best choice. The advantages of a roof prism are less important for astronomy. But one of the advantages of binoculars is that they are excellent dual-purpose instruments. Many astronomers use their binoculars for birdwatching and other activities, so a purchase is made with multiple uses in mind.

Choosing Binoculars for Stargazing

Every binocular has two numbers in its description, such as 7x50 or 10x25 or 25x100. The first number is the magnification. 7x binoculars, for example, magnify everything 7 times. The second number indicates the diameter (or aperture) of the objective lens in millimeters. So, a 7x50 binocular has 50mm (2") objective lenses, while a 25x100 has 100mm (4") objective lenses.

(The exception to this is zoom binoculars, which we strongly recommend against for stargazing, for reasons given below. Zooms will have three numbers, such as 10-30x50, indicating the magnification range as well as objective diameter.)

Exit Pupil

Just as with a telescope, the larger the objective lens, the more light the binocular will gather. However, there is another important factor to consider when choosing binoculars: exit pupil. The exit pupil of a binocular is easily determined. Simply take the objective diameter and divide by the magnification. 7x50 binoculars would then have a 50/7 = 7.1mm diameter exit pupil. Generally speaking, the exit pupil determines the brightness of the view through the binoculars. A larger exit pupil means a brighter image, with certain exceptions. Therefore, all things being equal, a 7x50 binocular with its 7.1mm exit pupil will be brighter than a 25x100 binocular with its 4mm exit pupil, despite the larger objective size.

So, the obvious extension of the exit pupil idea would be to keep increasing the diameter of the exit pupil to make the image brighter. Unfortunately, there is limit to the size of exit pupil that is effective. The pupil of the human eye, when dark adapted, has a diameter of no more than about 7mm. And this number assumes young, healthy eyes. Older observers tend to have pupils only 5mm in diameter or so. The maximum 7mm diameter of the eye's pupil means that any exit pupil larger than 7mm would be wasted. This is the reason you never see, say, 7x70 binoculars which would have a 10mm exit pupil. In this case the human eye would restrict the exit pupil to 7mm and the effect would be like having a pair of 7x50s, only heavier and more expensive!

An alternate argument is that if many observers have pupils that only open to 5mm, why even use a 7x50? Wouldn't a 10x50, with a perfectly matched 5mm exit pupil, be better? While the 10x50 and 7x50 might have the same brightness for observers with smaller pupils, there are still several advantages to the 7x50s. For one, the 7x50s have a wider field of view than the 10x50s, which is always an advantage. More important, the 7-power binoculars will be much easier to hand hold than the 10-power, because they will appear to shake less due to the lower magnification. The difference between 7x and 10x is very minimal when it comes to stargazing, so the lower power is usually preferable.

Magnification

Brightness is ultimately more important than magnification, just as is the case with telescopes. A lower magnification, such as 7x to 10x, provides a very wide field of view and is easy to hand-hold. As magnification increases, the image becomes dimmer. To compensate, higher power binoculars must have larger objectives, making them heavier, harder to hand-hold, and more expensive. 12x to 25x binoculars typically have 60mm to 100mm objective lenses, making them as large as a pair of small telescopes!

The highest magnification typically available with a binocular is 25x to 30x, though some higher-power pairs exist. For casual stargazing, especially for beginning amateur astronomers, low-power binoculars are ideal because they make finding objects simple due to their wide field, and are easy to use because you can grab them and go. One exception to this generalization is image stabilized binoculars such as Canon's Image Stabilizer series of binoculars. These are available in magnifications up to 18x but still provide a relatively wide field and are easy to hand hold despite the high magnification thanks to the electronic stabilization feature that eliminates image shake.

Optical Coatings

There are three general classes of optical coatings available on binoculars: Fully Coated, Multi-Coated, and Fully Multi-Coated. Standard optical glass reflects about 4% of the light that strikes it, so only 96% is transmitted. While this seems like a lot of light coming through, consider there are usually 7 to 8 pieces of glass in each half of a binocular. This can reduce the light throughput considerably, effectively turning a 50mm diameter objective into a 30-40mm objective. Anti-reflective coatings increase the light transmission of a binocular and reduce internal reflections that also reduce contrast. Excellent coatings can increase overall light throughput to near 90%.

Fully Coated binoculars have coatings on every lens surface to increase light transmission.

Multi-Coated binoculars use multiple coatings on some (but not all) of the lens surfaces. Multiple coatings offer better light transmission characteristics than the single coatings used in a Fully Coated pair of glass.

Fully Multi-Coated binoculars offer the best coatings. They have multiple coatings on every lens surface, providing the highest light transmission and best contrast.

Phase Coatings

One additional type of coating, which differs from the regular optical coatings described above is phase coating. These are used exclusively on roof prism binoculars and are applied to the prisms rather than the lenses themselves. When light passes through the prisms in a roof prism binocular, each wavelength of light follows a slightly different path and the different colors of light exit the prisms slightly out of phase. (In other words, the waves of light no longer line up perfectly.) Phase coatings reduce this effect, providing better contrast, brightness, and clarity.

Binocular Prices

Binoculars cover a wide range in prices. There are numerous cheap binoculars out there aimed at casual hikers and sports fans, etc. The biggest factor to be aware of is that cheap binoculars are typically poorly built. Not only are the optics less sharp and the optical coatings lower quality, the overall build quality can be poor and this may cause the binoculars to lose their alignment, a critical feature for a pair of optical systems such as a binocular.

For stargazing, where a 40-50mm objective diameter is normally recommended, a good quality pair of glass with at least multi-coated optics will start around $150. Some decent beginner's binoculars might be had for around $100, but keep in mind that the optical and mechanical quality will not be that of the better, slightly more expensive glass. Fully multi-coated, high quality (both optically and mechanically) 7x50 or 10x50 binoculars usually cost $200-300. Larger binoculars (80mm+ objectives) typically cost around $400 and up. While some newer large binoculars are available for significantly less money, these cheaper large binoculars have been of very inconsistent quality and would not be highly recommended. The very popular Image Stabilizer binoculars cost from $350 to $1600, depending on size (the $379 10x30s being the most popular).