The eyepiece of the telescope is what we use to observe astronomical objects and alter their magnification. Fundamentally, the eyepiece and the lens are the two components of the telescope that allow us to see the celestial objects in space. In this article, you will discover what the number on an eyepiece means.
Sometimes people call the eyepiece of the telescope a ‘scope’ which is actually gun terminology. They operate in the same way.
If you are new to your telescope, you might wonder what the numbers on each eyepiece mean. You may also be wondering why you have more than one. Keep reading to learn more about telescopes and their eyepieces.
What Is An Eyepiece?
Telescope eyepieces are an essential part of a telescope’s optical system They work by magnifying the image formed by the telescope’s primary mirror or lens. The eyepiece is placed at the telescope’s focal point, where the light rays converge to form an image.
When you look through a telescope eyepiece, you are essentially looking at a small, magnified image of the object you observe. The eyepiece consists of lenses that magnify and focus the image.
The image’s magnification depends on the eyepiece’s focal length and the telescope’s focal length. With higher magnification, the field of view gets smaller. As a result, you can see less of the sky at once.
Eyepieces come in different designs and specifications. Important specifications are the focal length and field of view. Eyepieces with a wider field of view allow you to see more of the sky at once. Conversely, eyepieces with a higher magnification give a narrower field of view.
An eyepiece’s component quality affects image clarity and brightness. Higher-quality eyepieces have better coatings to reduce glare and improve contrast. They are constructed with higher-quality materials and use precision manufacturing techniques. These factors reduce distortion and aberrations in the image.
Different types of eyepieces are available. If interested in the configurations available, refer to How Telescopes Work.
Why You Need More Than One Eyepiece?
An eyepiece is not a fixed component of any telescope. Eyepieces are interchangeable to achieve different levels of magnification. You want different levels of magnification for different uses of your telescope.
For example, a wider field of view is appropriate if you are just exploring space and looking for random things. In this case, you will require an eyepiece with a larger number.
You might be looking for something specific within a small area. You can do this with greater magnification albeit with slower movement. However, to locate an object in the sky it is best to use an eyepiece with a larger field of view. A wider field of view will make it easier to find targets. When the target is located, the eyepiece may be changed to provide more magnification.
Be aware that increasing the magnification too high may make the image less sharp. Each telescope has a maximum magnification that it will support.
At this point remember that different eyepieces provide different magnifications.
If you wish to discover more about telescope magnification and how to calculate it, please visit: How To Find Total Magnification.
What Does The Number Mean On Each Eyepiece?
So as you may have guessed by this point the numbers of your eyepieces will generally relate to their magnification. However perhaps not in the ways you expect.
Generally, the number represents what we call ‘focal length’. The distance (measured in millimeters) between the optical center of the lens and the point where the image is formed is called the focal length.
The light that has passed through the telescope enters the front of the eyepiece. The light is refracted (bent) inside the eyepiece to form an image at the eye point. Eye relief is the distance between the rear of the eyepiece and the eye point. Eye relief is an important property of eyepieces but is outside the scope of this post.
Now that we understand a little about eyepieces we can detail what the numbers mean.
The most important number on eyepieces is the distance between the rear of the eyepiece and where an image is formed. The longer the focal length, the smaller the field of view.
The higher the number on your eyepiece, the lower the magnification. A shorter focal length enables a larger field of view. The focal length does not actually dictate magnification but dictates the field of view.
In turn, the eyepiece’s physical size will change the magnification. A long focal length will make objects in view seem larger. Shorter focal lengths make objects smaller.
The take-away is that the smaller the number, the greater the magnification.
Field of View
An eyepiece should also have its field of view embossed on it.
An eyepiece’s field of view refers to the extent of the visible sky. It is an important specification to consider when selecting an eyepiece because it determines how much sky you can see at once.
The eyepiece field of view is affected by both the eyepiece’s focal length and the telescope’s focal length. The shorter the focal length of the eyepiece, the wider the field of view, but the smaller the magnification. Conversely, a longer focal length eyepiece will provide a narrower field of view but higher magnification.
What About A Barlow Lens?
A Barlow Lens is a confusing object in many ways but is a practical and economical solution. Most astronomers use Barlow Lenses for certain observations.
It doesn’t actually work as a lens on its own. Instead, it works in conjunction with an eyepiece.
Whatever eyepiece you have mounted on the telescope, the Barlow lens will multiply the magnification of that lens.
Barlow lenses come in various configurations. A common Barlow lens provides x2 magnification. This means that the optics will provide double the magnification of the eyepiece. It is possible to purchase Barlow lenses in x3, even x5, and in between.
You might be asking what’s the point of a Barlow lens. Why can’t you just buy an eyepiece that is double the magnification of the one you already own? You can do this, but it may not be that economical.
A Barlow lens is useful as you can switch between general navigation and combing through a smaller field of view seamlessly. A Barlow lens fits between the telescope and the eyepiece.
Let’s say you have three lenses of varying focal lengths.
Having one Barlow lens means you now have 6 different magnification levels from only three eyepieces. This saves you buying three more eyepieces which is economical and practical.
At this stage, you may think that Barlow Lenses are the bee’s knees. However, there is no free lunch. The is a price to pay for such convenience. As you introduce more ‘glass’ into the optic train, the quality of the image will be degraded to some degree.
Modern optical technology has improved so this doesn’t occur so greatly. A good Barlow lens should only dim a picture by around 3%. However, lower-quality Barlow lenses will decrease this much further.
Also, imperfections in the lens in the Barlow Lens will degrade the final image.
On the plus side, remember that a longer focal length means longer eye relief. This simply means the longer the eyepiece’s focal length, the further away your eye will be from it. As a result, you will have to strain your eyes more. A Barlow Lens allows you to use less powerful eyepieces.
A Barlow lens can provide increased magnification but also allow more comfort in this high-power viewing.
There you have it. The numbers on an eyepiece detail the focal length and field of view of that eyepiece. The shorter the focal length, the greater the field of view which allows for great magnification.
Put simply, the higher the number on the eyepiece, the less the magnification. At any focal length, different eyepieces provide different fields of view.
Hi, my name is Jason Anderson, and I am a Physics Professor. Ever since I was a kid, I’ve been fascinated with space, the universe, the moon, you name it. I spent hours and hours at the planetarium close to my hometown, wondering what else could be out in the universe.
Since then, I’ve been an avid stargazer and astronomer, and love nothing more than spending my time charting stars, observing planets, and finding constellations.
This is why I decided to start Telescope Guru. I only wish to share this fun pastime with the world. With this site, I hope to answer all of your questions relating to astronomy, telescopes, and stargazing.
