Telescope eyepieces can be confusing for amateur astronomers. Maybe you're a new telescope owner looking to upgrade from the eyepiece that came with your telescope, or you're an intermediate user looking to expand your eyepiece options. This guide will demystify telescope eyepices, and explain what those numbers on your eyepiece mean for your viewing experience.
What Are Eyepieces?
Telescope eyepieces finish the work that your telescope stars. Your telescope's job is to reflect or refract light (or a combination of the two), while your eyepiece's job is to focus all that collected light into a single point that your eye can see. They come in many different shapes and sizes, and may look different (both in style as well as the view they provide), but they all do the exact same thing.
What Size Eyepiece Do I Need?
Most telescopes utilize 1.25" eyepieces. This measurement refers to the part of the eyepiece that fits into your telescope, measured in inches, meaning the eyepiece socket is 1.25 inches wide. Higher-end telescopes and eyepieces are available in 2" widths, but most of what you'll see in the consumer market is 1.25 inches, so we'll focus on that.
This 1.25" measurement is standardized, meaning you can pretty much use any 1.25" eyepiece in any telescope that accepts 1.25" eyepieces, regardless of manufacturer. You're not locked in to a specific brand when shopping for eyepieces.
You may also see some telescopes with eyepieces smaller than 1.25", and you should steer clear. These telescopes are considered toys, and usually lead to more frustration than observing.
What Does The Number On My Eyepiece Mean?
Some eyepieces say "6mm" or "14mm" or "20" or "32." This is the focal length of the eyepiece. Without going into too much detail, a larger number will result in a lower magnification when paired with your telescope, and a smaller number will result in a higher magnification.
For example, a 40mm eyepiece appears physically larger, and has a larger lens. However, when you look through a 40mm eyepiece, you'll see a wide, less magnified field of view. This is useful when looking at larger objects such as the moon.
On the other hand, an eyepiece with a smaller number, let's use 8mm as an example, the image will be much more magnified, with a much narrower field of view. This is useful for planetary viewing, like when you want to spot the bands of color on Jupiter.
There are also other things you'll notice. An eyepiece with a smaller number and higher power may appear dimmer. This isn't a problem, this is just how mangification works. Higher magnifications will trade light collected for making the image larger.
There's a minimum and maximum useful magnification for every telescope, and it varies. You may find that your Newtonian reflector pairs well with a 6mm eyepiece for planetary viewing, but your refracting telescope becomes unusuable with anything less than a 9mm. Likewise, you may find that one provides better wide-field views with the 40mm eyepiece, while the other performs better with a little more mangification.
The good news is that you can experiment with all the different brands and sizes available, and build your eyepiece collection so it best suits your viewing needs.
What is a Barlow Lens?
You may or may not own a Barlow lens, and you may or may not know what it does or how to use it. In simple terms, a Barlow lens will magnify what you normally would see, in combination with your chosen eyepiece.
Most Barlow lenses are 2x lenses, meaning they will double what you would normally see through your telescope and eyepiece. So if you have a wide-field eyepiece that provides a magnification of 50x, and a 2x Barlow lens, the resulting magnification would be 100x.
Barlow lenses are as versatile as eyepieces, meaning they can be used with any eyepieces from any brand, as long as they are the same size. Pair 1.25" Barlow lenses with 1.25" eyepieces. If you have five eyepieces of varying size, one Barlow lens will give you ten different magnifications to use with your telescope. Neat!
Eyepiece & Telescope Magnification
So how do you determine what magnification you're getting out of your telescope and eyepiece? There's a simple formula that you can perform to determine this:
Magnification = Telescope Focal Length / Eyepiece Focal Length
So, if you have an telescope with a 1000mm focal length, and a 10mm eyepiece, you're looking through your telescope at 100x power. The same eyepiece used in a telescope with a 50mm focal length would only result in a 50x power. This is why you can put the same eyepiece into two different telescopes and see very different views.
You've probably seen the term "Eye Relief" thrown around when referring to eyepieces, but what is it? Eye relief is the distance from the last lens on your eyepice to the surface of your eye. On larger eyepieces, such as the 40mm example we used earlier, eye relief will be longer, and it will be easier to view an object through it. On a smaller eyepiece, the eye relief will be much shorter, and it may be a little more difficult to view, since you'll have to get very close to the eyepiece to get a full field of view.
One factor to consider when choosing eyepieces is the resulting brightness. As a general rule of thumb, eyepieces with larger focal lengths will produce brighter images. This is a product of how the lenses and mirrors in telescopes and eyepieces work together. While you get wide-field views and lots of brightness with your larger eyepieces, you'll sacrifice brightness to make the image larger and more detailed when using smaller eyepieces.
You may have noticed something interesting on the bottom of your eyepieces. We're talking about the inside of the part that fits into the telescope. More often than not, your eyepieces will be threaded on the inside. This isn't a mistake, those threads make your eyepiece compatible with filters.
There are many different types of filters available, and they all have a slightly different purpose. Some may reduce glare, cut down on scattered light, others may adjust definition and resolution, and some others may simply favor certain color spectrums for astrophotography. There are a few basic filters that everybody should have in their kit.
You can go crazy with eyepiece filters. They're just as widely available as eyepieces, and many brands are available. Try out as many as you can, and build your kit so that it compliments your viewing experience.
As with most things, you'll probably be limited by your budget when it comes to shopping for new eyepieces. It's true that a budget eyepiece probably won't be as clear or sharp as one that costs several hundred dollars, but we have to be realistic about our needs.
A higher quality eyepiece will do a little to improve the viewing experience through a budget telescope, but not much. So you shouldn't go crazy with the credit card if your scope is one designed for beginners.
But there is good news! Any eyepieces you purchase can be used with a new telescope when you're ready to upgrade, as long as your new scope utilizes the same size eyepieces. So spend as much as you feel comfortable spending, especially if you plan to upgrade in the future.
What Eyepieces Do I Need?
A good setup for all-around observation is one high power eyepiece, one low power, and one medium power. Combine these three eyepieces with a good Barlow lens and you have different magnifications to use with your telescope.
You can have more than three eyepieces in your kit, but we don't recommend observing with less than three. Too few eyepieces will limit your viewing experience. Having more eyepieces will make your kit more versatile, allowing you to look at a wide range of objects at different magnifications. This is ideal, because some objects will look better at certain magnifications, and not so great at others.
Buy as many eyepieces as you feel like you'll utilize during your stargazing sessions. At least one low power, one high power, and one somewhere in between.
Hopefully you learned something from this guide. Keep your head up, and we wish you a great learning experience as you observe your way across the night sky!