What’s a Telescope’s Field of View and How To Calculate it?

There are a lot of terminologies to learn when you start a new hobby and astronomy is no exception. In particular, there are a lot of unfamiliar words and phrases related to the hardware you’ll be using. In this article, we’re going to talk about field of view in astronomy and how that impacts your observing experience.

So what is it? Put very simply, the field of view is how much sky you can see, as measured in degrees. For example, if we had eyes in both the front and the back of our head, our field of view would be 360 degrees because we’d be able to see everything around us.

Since we only have eyes at the front that look forward, this is impossible. More realistically then, the average human eye, without the help of binoculars or a telescope, has a field of view of about 210 degrees. In other words, we can typically see everything in front of us and a little way to the sides.

In astronomy, the field of view usually refers to how much of the sky we can see through either binoculars or the eyepiece of a telescope.

Why Is It Important?

It’s important to remember that the field of view has nothing to do with magnification. You can observe the Moon with two different sets of binoculars that both have a magnification of 10x. If one has a larger field of view than the other, it simply means you can see more of the surrounding sky. The Moon itself will appear exactly the same through both pairs of binoculars.

So why is it important? Arguably, there are three primary reasons.

Firstly, unless you’re using a computerized GoTo telescope, you’re probably star-hopping. That means you’re using a star chart to hunt down your target. This typically involves starting with a bright star and then hopping across to other stars until you reach your destination.

If you’re star hopping, it’s important to know your field of view, otherwise, you won’t know which stars will be visible through your eyepiece. Get the field of view wrong and you could get lost looking for the next star to hop to.

There are two other reasons why your field of view is important – and they both come into play once you have your target in your sights.

As a general rule, the higher the magnification you’re using, the smaller your field of view. The smaller your field of view, the fewer stars you can see. If you’re observing a nebula, galaxy or a small globular cluster – objects that, unlike stars, aren’t sharply defined – it can sometimes be hard to tell if your view is properly focussed.

In this situation, you’d focus upon the stars that appear nearby. By adjusting the focus so that the stars appear sharp, you can be certain your target is properly focussed too. So the larger your field of view, the more stars are visible to use as reference points.

The final reason is largely aesthetic. If you’re using high magnification, your target can dominate your view. If your intention is to study the object close-up, then this might be fine, but otherwise, the view loses a lot of its visual appeal. 

It’s almost as though you lose the context of what you’re looking at. If you stand just inches away from a tree, you might only see the bark and a few leaves. You lose the context of seeing the tree as a whole and all the surrounding trees in the forest.

Likewise, using a high magnification to split a double star might achieve that goal, but you could also lose a sense of what you’re actually observing. The experience is heightened by seeing the surrounding stars that appear nearby. The sense of depth is greater and, depending upon your aesthetic sensibilities, your experience could be much more meaningful as a result.

Calculating The Field of View Through Binoculars

Unlike telescopes, you can’t remove the eyepieces from binoculars and increase the magnification. As a result, both the magnification and the field of view are fixed and cannot be changed. (There are some binoculars that allow you to zoom in and out, but these are typical of lesser quality.)

Most people own binoculars but not everyone is aware of their specifications. Some might know they own 10×50 binoculars but not really know what that means.

Binoculars are made to all kinds of specifications but 10×50 binoculars are very common. The first number denotes the magnification, while the second indicates the diameter of the objective lens in millimeters. (The objective lens is the larger lens – the “wrong” end if you will – and not the eyepieces you look through.)

So 10×50 binoculars have a magnification of 10x and an objective lens 50mm wide. The larger the first number, the more magnification. The larger the second number, the lighter the binoculars can gather and, theoretically, the fainter the objects you can see.

You might, therefore, think that the larger the numbers, the better. This isn’t necessarily true, as binoculars with large objective lenses can be heavy and often require a tripod for comfortable use. For this reason (not to mention cost) many manufacturers limit their binoculars to a maximum of 20×80. These are like having a small telescope for each eye!

Many binoculars are marked with a second set of numbers, close to one of the eyepieces and often below the magnification and objective lens size. For example, my Celestron UpClose G2 7×35 binoculars also have the specifications of 9.2/483 ft/161m.

In this case, the first number is the field of view, in degrees. The second and third numbers indicate that if you were looking at an object from 161 meters away, you’d see 483 feet of its width and height. These numbers could be in imperial or metric measurements.

Not all binoculars are marked with the field of view. If you only have two numbers (eg, 483 ft/161m) the key to calculating the field of view lies with the first number.

If it’s in imperial measurements (ie, feet) then dividing that number by 52.4 will yield the field of view in degrees. In this example, dividing 483 by 52.4 gives us roughly 9.2 degrees.

If the first number is metric, you should divide it by 16 to get the field of view. For example, my Orion 15×70 Astro Binoculars have the specifications 77m/1000m. Dividing 77 by 16 gives us a field of view of 4.8 degrees.

As a general rule, a good 10×50 binocular should give you a field of view of between six and seven degrees. My Orion 10×50 Explorer binoculars have a field of view of six and a half degrees.

Calculating the Field of View Through a Telescope

Telescopes, of course, function differently from binoculars because you can change the eyepieces. Any time you change the eyepiece, you also change the magnification and – yes, you’ve guessed it – the field of view.

Unfortunately, as a result, calculating the telescope field the view is a little more complicated. There are a couple of methods to do this, but the following is the simplest – and even that’s a two-step process.

• Step 1. Calculating the Magnification

This first part is the easiest and almost all amateur astronomers know how to calculate the magnification of their eyepieces already. Of course, if you want to see anything through a telescope, you have to look through an eyepiece, but in order to for the light to reach your eye, it first has to enter the telescope through the other end.

This “other end” is either called the objective lens or the aperture, depending on what type of telescope you have. If you have a refractor, which has a glass lens at one end and the eyepiece at the other, then this lens is called the objective lens.

If, on the other hand, you have some variant of a reflector, you’ll have a tube with an open end that points up to the sky and the eyepiece on the side of the tube. At the closed end of the tube is a mirror that reflects the incoming light off a secondary mirror and up to the eyepiece.

If you have a reflector, then the open end of the telescope is called the aperture.

Regardless of whether you have a refractor or a reflector, all telescopes have what’s known as a “focal length.” This is basically the distance that light travels from the objective lens (refractor) or the aperture (reflector) to the eyepiece.

You can typically find the focal length specified on a sticker on the tube of the telescope. If not, you may need to look for the telescope’s specifications online. So, for example, my Celestron NexStar 130SLT reflector telescope has a focal length of 650mm. In other words, light travels 650mm (65cm) from the aperture to the eyepiece.

All eyepieces also have a focal length, which is marked either on the barrel of the eyepiece or on the top. Like the focal length of a telescope, this is also measured in millimeters. Unlike a telescope, the focal lengths of eyepieces are very short and will typically range anywhere between 3mm and about 40mm.

So to calculate the magnification of your eyepiece, you divide your telescope’s focal length by the focal length of the eyepiece you’re using. For example, one of my longest eyepieces has a focal length of 20mm, so that’s 650 (telescope focal length) divided by 20 (eyepiece focal length) equals a magnification of nearly 32.5x.

My shortest eyepiece has a focal length of 3 mm, which gives me a magnification of nearly 217x. Obviously, an eyepiece will give you a different magnification, depending upon which telescope you’re using it with. Similarly, it’s a good idea to have a small selection of best eyepieces to give you a range of magnifications when you’re observing.

• Step 2. Calculating Your True Field of View

Here’s where it gets a little complicated, telescope field of view calculator. Your eyepiece has what’s known as an Apparent Field of View value. This is the number of degrees of sky your eyepiece would show you if you held it directly up to your eye, without the use of a telescope. (Obviously, it wouldn’t be very effective – you need a telescope to magnify and focus the view!)

The true field of view is the number of degrees your eyepiece shows you when you use it with your telescope. To calculate this, you divide the apparent field of view by the magnification.

For example, I have a zoom eyepiece that can vary its focal length from 24mm to 8mm. When set to the focal length of 24mm, it gives me a magnification of 27x and has an apparent field of view of 60 degrees. Dividing 60 by 27 gives me a true field of view of about 2.2 degrees, or about the width of four full Moons.

Compare that to the view when it’s set to a focal length of 8 mm. This gives me a magnification of about 81x and an apparent field of view of 40 degrees. Dividing 40 by 81 gives me 0.49 degrees, or a little less than the width of the full moon.

Unfortunately, unlike the focal length, not all eyepieces will have the apparent field of view specified anywhere on it. You’ll probably need to either refer to the eyepiece or telescope’s original packaging or try looking it up online.

If you’re unable to find the specifications, taking the apparent field of view as 50 degrees will usually suffice.

Which Field of View is Best?

Think back to the reasons why the field of view is important; with a larger field of view, you see more of the sky through the equipment you’re using. With a smaller field of view, you see less of the sky. As a general rule, the lower the magnification the larger the field of view and the higher the magnification, the smaller the field of view.

Arguably then, leaving aside the aesthetics, it really depends upon what you want to observe. Small objects, such as galaxies, most globular clusters, planetaries nebulae and even planets themselves, can be easily observed with a small field of view – as long as there’s a nearby star to focus on.

Star clusters and nebulae often require a wider field of view because they’re typically larger and are best observed at low magnification.

At the end of the day (or night) regardless of what you’re observing or the equipment you’re using, there’s just one rule you should always bear in mind: enjoy the view!