Telescope Eyepieces: All you Need To Know, But Were Afraid To Ask!

Have you ever wondered why some telescopes are better than others? The answer lies in their lenses.

Telescopes come in two types: refractors and reflectors.

Refractor telescopes use glass lenses to focus light from distant objects onto eyepieces. Reflector telescopes use mirrors instead.

Telescope Eyepieces All you Need To Know, But Were Afraid To Ask!

Refractors are generally considered superior to reflectors because they produce sharper images.

They also allow you to see more detail in dimmer conditions. However, both types of telescope require good optics.

This means that the quality of the lens or mirror determines whether you get a clear image.

But what also determines whether you get a clear image is the quality of the eyepiece itself.

In this article, we look at telescope eyepieces in closer detail and provide you with answers to all the questions you have wanted to ask but were afraid to before.

Let’s dive in.

What Is A Telescope Eyepiece?

A telescope eyepiece is an optical device used to magnify distant objects. It consists of a pair of lenses that are held together by a frame.

The lenses are called oculars (pronounced oh-kay-lays). A single eyepiece can be attached to one or multiple telescopes.

The first type of eyepiece was invented by Galileo Galilei in 1609.

He created a simple design consisting of two convex lenses separated by a small gap.

His invention revolutionized astronomy, as it allowed people to view stars and planets without having to travel to faraway places.

The second type of eyepiece came into existence around 1750 when Edmund Halley designed a doublet for his reflecting telescope.

Doublets consist of two identical lenses arranged side by side. This allows them to work together to create a brighter image.

Today, there are many different designs of eyepieces available on the market.

Some are made specifically for astrophotography while others are meant for general viewing purposes.

Types Of Telescope Eyepieces

Types Of Telescope Eyepieces

There are three main types of telescope eyepieces: Newtonian, Schmidt-Cassegrain, and Cassegrain.

Newtonian Eyepieces

Newtonian eyepieces are the most basic form of the telescope eyepieces. They consist of only two lenses.

One lens focuses on incoming light rays, and the other lens bends those rays so that they fall on your eye.

This makes Newtonian eyepieces very easy to make. However, they don’t offer much magnification power.

Schmidt-Cassegrain Eyepieces

Schmidt-Cassegrain eyepieces are similar to Newtonian eyepieces except that they have four lenses.

These lenses are arranged in pairs, each pair focusing on incoming light rays.

When light passes through the first pair of lenses, it is bent 90 degrees.

Then it goes through the next pair of lenses which bend it another 90 degrees. Finally, it enters your eye.

Because of this arrangement, Schmidt-Cassegrain eyepieces offer higher magnification powers compared to Newtonian eyepieces.

Cassegrain Eyepieces

Cassegrain eyepieces are named after French astronomer Laurent Cassegrain. Cassegrain eyepieces use three mirrors instead of two lenses.

Each mirror reflects incoming light rays onto your eye. The advantage of using mirrors is that they reflect more light than lenses do.

Therefore, Cassegrain eyepieces produce clearer images than Newtonian or Schmidt-Cassegrain eyepieces.

Wild Field Eyepieces

To get above the 50° field of view, you’ll need to buy one of these types of eyepieces.

The cheapest of the wide-field eyepieces is the Erfle eyepiece

However, the downside is they often suffer ‘ghosts’ at the edges of certain images, so they will not work well for looking at faraway planets, however, they do give over 60° of view.

Telescope Eyepiece Magnification

You might be wondering how strong the magnification will be in your eyepiece?

This is relatively easy to work out. You need the focal length of your telescope and the focal length of your eyepiece.

Divide the first by the second, and this will give you the magnification calculation.

For example, if the focal length of your telescope is just 700mm but your eyepiece is 25mm, you have a magnification that equals 700mm divided by 25mm and this will result in 28x.

The smaller the eyepiece in this equation, the more magnification you will get.

How Do You Choose Your Telescope Eyepieces?

How Do You Choose Your Telescope Eyepieces

Choosing the right eyepieces depends on several factors such as:

Magnification Power

You need to choose eyepieces that will allow you to see the details of the object.

Magnification power refers to how much the image appears larger than what it actually is.

We have given you a guide above to work out what the magnification power will be on your eyepiece.

Aperture Size

The aperture size determines the amount of light entering the eyepiece. A smaller aperture means less light gets through.

Field Of View

When choosing an eyepiece, you also need to consider its field of view (FoV). The FoV is the area of space that can be viewed at one time.

It’s measured in arcminutes. For example, if you’re looking at Saturn with a 50mm eyepiece, then the FoV would be about 1/3rd of a degree.

If you were looking at Mars with the same eyepiece, the FoV would be 0.5 degrees.

If you’re planning to photograph a wide range of objects, then you’ll probably need a large focal length eyepiece.

On the other hand, if you’re just going to focus on a specific object, then you may not need a long focal length eyepiece because it will give you too much distortion.

Eye Relief

Eyepieces are designed to focus the image outside of the glass surface, and the image should hit the focus on your pupil.

The distance between the surface of the eyepiece and where the image is focused is called eye relief.

If this is large enough, glasses wearers can leave their glasses on.

Simply put, the greater the eye relief of a telescope, or any viewing distance viewing tool for that matter, the further away from the eyepiece you can stand whilst still seeing a decent image.

It’s important to find a comfortable position for eye relief, too much or too little can be a pain, so you need to decide what is best for you.

Exit Pupils

The exit pupil measure is the size of the image that is formed at the eyepiece.

The adult human eye, dark adjusted, has a diameter of 5-7mm, so any exit pupil larger than this wastes light.

The eyepiece exit pupil is worked out by dividing up the aperture of the telescope by the lens magnification.

You should not reduce the magnification to try and increase image brightness if you find the exit pupil is bigger than the pupil of your eye.

When it comes to eyepieces, the size of your scope determines the magnification of your image, so a smaller aperture usually only benefits from a lower magnification eyepiece, but a larger magnification can still generate a good exit pupil size.


Finally, as with all space equipment, eyepieces come in different price ranges.

Generally speaking, cheaper eyepieces tend to have lower-quality optics.

So, when buying new eyepieces, always check the manufacturer’s warranty before purchasing them.

Also, keep in mind that the longer the focal length, the more expensive the eyepiece becomes.

So, if you plan to buy a telescope with a long focal length, then you need to budget accordingly.

Final Thoughts

We hope this article has helped you learn all you need to know about telescope eyepieces and has answered your questions on how to pick the perfect one for your telescope.

Remember, you don’t need multiple eyepieces, but always buy one that suits your magnification needs and that has enough eye relief for you so that it is comfortable.

Use and care for them as much as you would the whole telescope, and keep exploring space as much as you can with them!

There’s still so much to discover!

Gordon Watts