How to Choose a Good Pair of Astronomy Binoculars
Almost all astronomers, both amateur and professional, own at least one good pair of binoculars and with good reason too. Binoculars offer unparalleled ease of use, while delivering crisp, clear views of the night sky at affordable prices. Unlike telescopes that have to be set up very precisely to be able to track objects, a pair of binoculars is simply pointed in the direction you want to observe, focused with one simple adjustment, and provided the instrument is held steady, it will produce views that are comparable to, and sometimes superior to, a small telescope.
Nevertheless, binoculars suffer from one major drawback. Unless you are using a giant pair of pier-mounted binoculars, their relatively small objective lenses do not gather sufficient light to view dim objects. Note that the distance of an object is not always related to its brightness, so even relatively close objects that have a low surface brightness might be a better target for a telescope, simply because a telescope gathers more light. Fortunately, though, binoculars offer an affordable way to discover the many binocular targets that are visible from the UK. An interactive sky map to find seasonal binocular targets is available online.
However, price alone should never be the sole deciding factor, especially if you are new to the hobby of stargazing. There are many technical aspects to consider to prevent you buying the wrong instrument, so in this guide we will briefly describe some of the most important things to look out for when shopping for a pair of binoculars for stargazing purposes. This guide will also briefly describe three high quality astronomy binoculars that are suitable for astronomical observing.
Get the basics right
Like all binoculars, instruments designed for stargazing are described by two figures, for instance, 10×50, 12×70, or some other combination. In all cases, the first figure denotes the magnification the instrument delivers, while the second denotes the diameter of the objective lenses. Thus, a 10×50 instrument gathers light through two 50mm-diameter lenses, and magnifies the object being viewed ten times.
These are important figures, because two 50mm lenses gather significantly more light than any 50mm aperture telescope, which means that the view through a 10×50 pair of binoculars is often superior to a view of the same object through a similar sized telescope. As with any optical aid, the more light the instrument gathers the better the views get, which in practice, means that a 10×50 instrument is better suited to observing bright objects such as the moon and planets, as opposed to nebulae and other deep sky objects such as galaxies that have low a surface brightness.
Thus, the secret to buying the right instrument is to be sure about what you want to observe; knowing this will make it easier to strike the right balance between magnification and lens diameter to achieve clear images.
Get proper eye relief
“Eye relief” refers to the distance behind the ocular (small) lenses of the instrument at which the image comes into focus. This value, expressed in millimetres, is an important consideration because an observer that wears eyeglasses might not be able to bring his eyes to within the specified distance. This means that such an observer will be unable to see a fully focussed image. The image below shows a typical eye relief problem that wearers of eyeglasses often encounter.
The most common eye relief distance is about 15mm, but most high quality binoculars are designed so that wearers of eyeglasses can comfortably use them. However, there are exceptions to this, so if you wear eyeglasses, be sure to test all binoculars you are considering buying while wearing your eyeglasses to be sure you can bring an image into focus.
Magnification is not everything
While it is tempting for beginner observers to go for the highest magnification available, the fact is that excessive magnification can sometimes make it impossible to view a desired object. There are two main problems with magnification: the first involves the fact that the higher the magnification, the smaller the field of view becomes, meaning that at high magnifications, parts of large objects, such as M45 (Pleiades Cluster), may not fit inside the field of view.
The other, more serious problem involves the fact that as magnification rises, so does image instability. In practice, this means that at say 20 × magnification, every small movement of the instrument also causes a 20-× magnification of that movement, which results in images that are so “shaky” that all detail is lost to the viewer. The obvious solution is to use a sturdy tripod to steady the instrument, but the downside of this is that a descent tripod can be more expensive than the binoculars.
Thus, observers on a tight budget are better served with smaller and lighter binoculars that can be held steady by hand, while still delivering a great view.
Consider the field of view
“Field of view” in this context simply means the total area of the sky that is visible through the instrument. Bear in mind that unlike telescopes, with which the field of view can be changed simply by switching eyepieces, the field of view delivered by a pair of binoculars is fixed. Therefore, if you mostly want to use your binoculars to observe the vast star fields in the Milky Way, or perhaps the surface of the Moon, a powerful, high-magnification instrument might show you more detail, but it will do so over smaller area than a lower magnification instrument will.
While astronomical binoculars are designed to deliver useful magnification as well as reasonable fields of view, the fact is that this ratio is fixed, meaning that above all, the intended use of the instrument should be the deciding factor that determines your choice of binoculars.
Our choice of top quality astronomical binoculars
Although the average observer is spoilt for choice when it comes to the bewildering number of makes, models, and prices of astronomical binoculars, the three instruments described below are widely regarded as being among the best in their respective classes. Note that all three instruments listed here are suitable for use by observers who have to wear eyeglasses.
Olympus 10X50 DPS I Binoculars
While some purists might argue that 10×50 binoculars are too small for serious astronomical observing, the mid-level multi-coated lenses on Olympus 10×50 DPS binoculars are of a high enough quality to deliver crisp images that are essentially free of chromatic aberration (false colour), and distortion around the edges of the images that is a common problem on low quality optics.
Moreover, this instrument delivers a whopping 6.5-degree field of view, which is augmented by a painless focussing adjustment to produce stunning views. Additionally, its lightweight construction means that it can be used without a tripod for extended observing sessions through folding eye caps that makes this the ideal choice for observers who have to wear eyeglasses. Note that the indicated price should be treated as a guide only.
Celestron SkyMaster 25×70 Binoculars
The Celestron SkyMaster 25×70 is and affordable 25-× magnification instrument aimed at serious observers that routinely observe relatively dim objects. The 70mm objective lenses gather sufficient light to bring objects into view that a smaller instrument cannot; however, its 3.3-pound weight make them unsuitable for prolonged use without a tripod.
The instrument comes with a tripod adapter that fits almost all standard camera tripods, and it is strongly recommended that this instrument be mounted on a sturdy tripod for best results. This instrument is also suitable for long distance daytime viewing, when it high magnification can be very handy to spot elusive birds or other game species. Note that the indicated price is a guide only.
Celestron SkyMaster 25×100 Binoculars
The Celestron SkyMaster 25×100 binoculars is widely regarded as one of the best astronomical binoculars ever made. Nonetheless, this instrument is not suitable for use without a tripod, which should be rated as “heavy duty” to carry the 10-pound weight of this instrument without imparting vibrations to the instrument.
The views through this instrument are comparable to a 5-, to 6-inch telescope, but with the added advantage that it takes up only a fraction of the space taken up by a telescope, meaning that the observer gets near-telescopic views from an instrument that fits comfortably inside the average overnight bag. Note that the indicated price is a guide only.