How To Choose Binoculars For Sky Watching (Best UK Guide)

Many people are under the misconception that the only way to watch the sky is with a telescope. Which is a shame because in many cases a decent pair of binoculars will be more effective in viewing some skybound objects than a telescope. Plus binoculars are easier to use, easier to set up and definitely more portable than a telescope.

If you want to have a closer look at the surface of the moon, the andromeda galaxy, or even snatch a quick glimpse of the International Space Station a quality pair of binoculars will be perfect. We have two eyes so it makes sense to use them both when scanning the skies. There are a few features that you’ll need to pay extra attention to, but that’s why we’ve written this article.

What Do You Need In Binoculars For Skywatching?

To get the best from your binoculars there are certain specifications that you’ll have to get right, but once all the dots are in place, you’ll be amazed at just how much you’ll be able to see. Not just at night or in the sky, a decent pair of binoculars will open up a whole new world to you (in more ways than one). Let’s start with the basics and get into specifics later.

Magnification & Objective Lens Diameter

Actually stamped onto the body of the binoculars you will find a sequence of numbers separated by the letter X. This is the magnification and size of the objective lens diameter in millimeters. The first numbers plus the X means a magnification of however many times that number, for instance 12×70 indicates 12 times magnification (the X represents times) from which we can understand that everything observed through that particular pair of binoculars will be 12 times larger than seen with the naked eye.

The numbers after the x indicate the diameter of the objective lens, in this case 70mm. The size of the objective lens indicates just how bright the image you’re viewing will be (in this case something astral). The objective lens is the only means light has of entering the binoculars which in turn, dictates the brightness of the image. From these two numbers we can calculate the exact size of the beam of light that enters the binoculars through the objective lens, that light beam is called the exit pupil.

What Is The Exit Pupil & Why Is It So Important?

Point your binoculars towards a bright light, at arm’s length and look through the objective lens, the circle of light you can see in the centre of the lens, is the exit pupil. The diameter of that circle of light should match, or be as close to our own pupil size as possible because it’s how the images we see through the binocular lenses are illuminated. To calculate the exit pupil of any pair of binoculars, all you need to know is the relevant level of magnification and the diameter of the objective lens. Simply divide the objective lens diameter by the magnification value, the answer will be the exit pupil in millimeters.

Our 12×70 binoculars from earlier have an exit pupil of 5.8 because 70÷12=5.8. This works with every pair of binoculars, as long as you have the values for magnification and the objective lens diameter, you can calculate the exit pupil. 

How Important Is Exit Pupil For Sky Watchers?

As you can gather from what we’ve mentioned so far, there is a direct relationship between the exit pupil and the objective lens diameter & magnification. So when we say a pair of binoculars have a large exit pupil, we’re really saying they have lower magnification. So, what effect does the magnification have on image brightness?

Looking at galaxies or nebulae through binoculars appears to make them larger, this is due to the magnification level, but the galaxy (or nebulae) and the background sky will remain at the same contrast. When looking at stars however, the background sky becomes dimmer but the brightness of the star remains constant (due to the objective lens diameter).

This is because there’s not a magnification level high enough to give the appearance of the star growing larger. The light from the star is governed (through the binoculars) by the size of the objective lens diameter. The increased magnification of the galaxy appears to spread the light it produces over a wider area and increase the overall size of the galaxy. Because of the contrast boost, fainter stars become easier to see as the magnification increases.

What Does It All Mean?

What we can ascertain from this is that a smaller exit pupil is better for stargazing, because a smaller exit pupil increases the darkness of the background sky which gives better views of the galaxies. Stars have an improved contrast due to the darker background, and fainter stars become more visible. You’ll need to make a choice between seeing more with less detail in which case a lower magnification and wider field of view, or seeing far more detail with less of the full image (narrow field of view) and higher magnification.

Clear Images

As we’re going to be using our binoculars for skywatching, we’ll be needing a relatively high magnification, and a large objective lens diameter. This can bring a few problems including the weight and size of the binoculars, and how clear the image appears through the lenses. As the objective lens diameter increases, so does the size and weight of the binoculars, which makes holding them long term, hard work.

Plus the natural tremor that is inherent in all humans becomes far more pronounced under high magnification. Hand held binoculars should have a magnification of no greater than 10x to prevent this tremor from becoming apparent and blurring the image through the lenses. Which is why any binoculars with a magnification of 12x or above should be supported by a tripod.

Of course this is very much dependent on your age and level of physical fitness, there are many people that would find no difficulty in supporting a 12×50 pair of binoculars and experience no shaking or subsequent image blur. Plus as many 12×50 binoculars weigh less than a bag of sugar, weight will not cause any issues either.

Tripod Support For Large Binoculars

The impact of high magnification shake will spoil your view from 12x upwards, the best solution for this is a tripod. Tripods are relatively inexpensive and almost all binoculars have an attachment point for a tripod. The ones that don’t can be attached via an adaptor which is also relatively inexpensive.

There are alternatives to using a tripod when using extremely high magnification binoculars and some of these can be employed for short time periods. They include;

1. Supporting your body against an immovable object (like a wall, fence, tree or car)
2. Employing a reclining chair in the almost horizontal position
3. Lying on the ground
4. Tucking your elbows into your ribs to support your arms
5. Use lower magnification

Of course all of the above are only temporary measures as they’re not practical long term.

Other Issues With High Magnification 

When it comes to high magnification, shaking arms and a blurry image are just part of the problem. If you happen upon a meteor shower one lucky evening, and you study it with too high mag you’re likely to get dazzled and be unable to see very much else for some time. The most important consideration when it comes to binoculars for sky watching is the level of the magnification.

What About Telescopes?

There are probably a few of you thinking, isn’t a telescope better for skywatching? In some ways that’s correct, telescopes do have far higher magnification levels than binoculars, but due to their design that image will take on a duller appearance (unless you have access to a high powered observatory telescope). There’s also the size, weight and immovability of a telescope compared with a pair of high-powered binoculars.

Space Views

Apart from the really large, bright objects in space like the moon (or some of the planets when their orbit brings them close enough), many other objects will appear too dim when viewed through a telescopes’ small lens area. Telescopes also have relatively narrow fields of view, so even if you spot something that looks interesting with your eyes, it’ll take forever to get it into the single lens of a telescope. Binoculars really are a better choice for the amatuer sky watcher that wants to have fast access to any views before the clouds start gathering again.

Two Eyes Are Better Than One

We have two eyes, binoculars have two lenses, one for each eye. Telescopes have one lens and to focus on the image seen through that lens, we need to shut our other eye. Our brains get used to seeing in stereoscopic vision (two eyes) and paint an image onto our brain in relation to that stereoscopic image.

It’s actually your brain that collates the information that the eyes reveal to it, and as that’s the way we were designed, two is better than one. Plus any pair of binoculars will have a wider field of view than a single lens telescope which allows for more efficient sky scanning.  Seeing in stereo also helps us to recognise the relationship between astral and planetary alignment.

Which Binoculars Are Best For Sky Watching?

As we’ve just established, binoculars are far better for watching the skies than a telescope (unless you have access to an observatory) but as there are so many brands and types of binoculars available, which ones are going to suit your purpose best? We’ve already mentioned a few of the factors that are important for sky watching binoculars. Let’s get into the relevant information to help you make the right choice.

Sky Watching Binoculars – The Basics

We’ve already covered the magnification, objective lens diameter, exit pupil and the field of view but there’s a few more important features plus a few that although not 100% necessary, will enhance your enjoyment of your hobby. When it comes to magnification more is not always better as we said earlier, the magnification affects the clarity of the image, to some extent the brightness of the image, and how mobile you can be when using the binoculars.

The objective lens diameter affects the light input, the overall size and weight of the binoculars and both affect the exit pupil. We would recommend a good size for binoculars to start sky watching would be 10×50. These will not only be large enough to view interstellar objects but also small enough to hold comfortably for long periods of time. Plus they can be employed for other functions like birdwatching, wildlife studies etc.

Should Sky Watching Binoculars Be Waterproof?

As we live on a water filled planet (71% of the earths’ surface is water) and we live on one of the wetter islands on that planet ( 1308mm in 2020 according to the meteorological office ^[1] )    it’s inevitable that at some point in time you and your binoculars will get wet. We suggest therefore, it’s well worth investing in waterproof binoculars. There are various types of waterproof binoculars available but for watching the skies we would recommend binoculars with an IPX6 code to be sufficient.

Fog Proofing

The lens of binoculars can fog or steam up due to any sudden temperature fluctuations (like leaving a warm room to stand outside watching the sky at 2 am in February) to combat this, the manufacturers remove all of the air from the lens tubes and inject an inert gas to prevent any reaction due to the effects of temperature on moisture content.

As this gas is completely sealed inside the lens tubes, no dust or airborne debris can enter either, keeping the lenses clean and allowing a clear view of the night sky.

Binocular Types Explained

When it comes to any hobby or activity that employs the use of binoculars it pays to understand the types and differences between those types so we can choose the correct type of binoculars for our intended purpose. There are two main types of binoculars and they both have good and not so good points when it comes to sky watching. They are;

Porro Prism Binoculars

The classic inverted W or M shaped binoculars, Porro prism binoculars, have been around for the best part of 200 years. They have many good features including;

• Improved Image Quality
• Superior Depth Perception
• Wider Field Of View
• Clear Images
• Low Purchase Price

Due to the interior design of the prism arrangement, they are easily knocked out of alignment which if undetected can lead to health issues (eye strain, headaches and migraine) they also have the following bad features;

• Heavy
• Bulky
• Large
• Easily Damaged
• Difficult To Fully Waterproof

Roof Prism Binoculars

Easy to recognise due to the shape which resembles a capital H, roof prism binoculars look like they are of a simpler design. But looks can be deceptive and roof prisms actually have far more precise prism arrangements than Porros. These prisms also need a special coating to correct phasing issues which makes them more costly than Porros, however they do have many good features including;

• Superior Magnification
• Light Weight
• Durability
• Compact Design & Size
• Excellent Waterproofing Qualities

With that said they do have some bad features too, which include;

• Higher Production Costs
• Narrower Field Of View
• Less Image Clarity
• Higher Purchase Price Than Comparable Porro Prisms

Now that you have some idea of the pros and cons of both types of binoculars, you’ll be able to make a far more informed choice on the suitability of both for sky watching.

Image Stabilised Binoculars

A fairly modern entry to the domestic binocular market, I S binoculars have been used for many years by the coast guards and other more military operators. They allow for the employment of far higher magnification without the need for tripods or other supports. The internal gyroscopic mechanisms detect and correct any movement, slight or otherwise, and present a clean image at all times.

I S binoculars will be relatively more expensive than regular binoculars and many (not all) have batteries that will need recharging or replacing regularly. 

Are Zoom Binoculars Any Good For Sky Watching?

Zoom binoculars require a very high level of precision manufacture which is both costly and difficult to achieve. As a result they’re often disappointing, at least for skywatching. On a similar note, binoculars with built-in cameras are also not very good for use in the night sky.

Where In The Sky Should You Watch?

Now you are able to choose the perfect binoculars for sky watching, where should you watch and what can you expect to see? A good place to find out more about what to look at in the night sky is google sky maps^[2]. Here you can see what’s happening right now in your area (and further afield), for instance, you can type a certain planet into the search box and google sky takes you to where it can be seen in the sky. 

Or click on the “Our Solar System” icon and thumbnails of prominent solar system objects will appear. Once you decide which object you’d like to see, click on it and google will take you to its location in the night sky.

There are quite a few helpful sites to get information on what you’re likely to see from your part of the UK, you could try the Greenwich observatory site^[3] or Jodrell bank^[4] or any number of other sites dedicated to our night skies and the forever changing scenes. Plus there are also books, magazines and other publications to help you identify what’s showing in the sky near you.

If you just want to get outside and explore the night sky and you’re lucky enough to have a clear sky, just get out there and point your binoculars skyward. The first stop should always be the moon, great when it’s full but as it’s our nearest interstellar neighbour you’ll still get a pretty decent view when it’s in any of its phases. Then there are the many and varied constellations visible from wherever you happen to be viewing from.

Frequently Asked Questions