Like almost every other thing money can buy, eyepieces come in varieties of qualities, size, and design. What many observers, especially beginners, fail to realize is that a good telescope eyepiece is crucial for scope performance. So they may not bother replacing the budget ones that came with their telescope.
The problem with neglecting your telescope eyepiece is that no matter how good your telescope is, the entire optical system will be ruined with a poor eyepiece. Unarguably, the eyepiece is the most important on a telescope. That’s why observers are always delightfully surprised with the massive improvement when they replace a poor eyepiece with a quality one. Fortunately, it doesn’t have to be prohibitively expensive as there are several great brands on the market at an affordable price.
Or maybe you already know the importance of the eyepiece and you just want a new one for an older telescope. No problem. This telescope eyepiece guide will take you through everything to note when looking to get a replacement.
Choosing A Brand
Unless in very rare cases, you’re not limited to using a particular brand of the eyepiece. For instance, you can use a Celestron eyepiece with a Meade telescope and vice versa. The important thing is to ensure that the eyepiece barrel size matches your focuser barrel size. Most eyepieces these days come in 1.25,” and that has become almost an international standard though some of the older scopes come in one inch or, the larger format 2″ eyepieces. Even when the scope is larger, they should come with an adapter to fit into 1.25″ eyepieces. If in doubt, check before you buy.
To enjoy a richer viewing experience, it’s advisable you have several magnifications ranges at your disposal: High magnification eyepiece to see the moon and planets, medium magnification for brighter deep-sky objects such as star clusters and galaxies, and low magnification for very large objects.
All telescopes have a specific focal length (distance from the center of the lens or curved mirror to the object it is focusing on), and it is the best indicator on how powerful the telescope is. You’ll typically see the figure printed or engraved somewhere around the eyepiece focuser. The number should be somewhere between 400 to 3000mm and depends on the type of telescope and aperture.
Eyepieces also have focal lengths. To calculate the magnification of a piece, divide the focal length of the scope by the focal length of the eyepiece. For example, a 1000mm focal length scope matched with a 25mm eyepiece will deliver 1000/25 = 40 power (or 40x) use a 10mm eyepiece on the same scope and get 100x and so on.
In essence, you can achieve different levels of magnification with the same telescope by changing eyepieces. One way to do this is to have three eyepieces (high, medium, low) or have two eyepieces and a Barlow lens. The Barlow lens is attached to an eyepiece to doubles or even triple its magnification.
Note that there is a limit to how large a focal length eyepiece you can use for each telescope. Exceed it and light emerging from the eyepiece will scatter around the dilated pupil of your eye and be wasted.
Field of View
In very basic terms, the field of view refers to the circle of sky that’s visible through the eyepiece. Usually, the higher the magnification of the optical device, the smaller the field of view.
Every eyepiece has its apparent field of view, which is measured in degrees (°). An eyepiece with a larger apparent field captures greater expanses of the sky than a smaller one and this figure ranges from about 45° for simpler eyepieces to 60° and above for widefield ones.
Aside from the apparent field of view, observers may consider the true field of view as being more important. The true field of view is obtained through the eyepiece and is the angular diameter of the area in the sky that can be seen when the eyepiece is attached to the telescope so that it adjusts the magnification.
You can determine the true field of view by dividing the apparent field by the magnification of the eyepiece. Taking our example for calculating magnification above, if magnification is 40x with a 40° apparent field eyepiece, so you get 40°/40x giving a true field of 1°
The term “eye relief” when used with optical instruments (such as telescopes, microscopes, or binoculars) refers to the distance from the last surface of the eyepiece within which the viewer’s eye can get the full viewing angle. If the viewer’s eye is outside this distance, they get a reduced field of view.
Ideally, eye relief should be a bit long for the viewer’s comfort especially for those that must wear eyeglasses with the telescope. In such cases, they will need about 15mm of eye relief to enable them to see the entire field of view. For those who don’t wear glasses, very short eye relief is also not advisable to avoid your eyelashes touching the eyepiece lens and staining it with oil or sweat.
Choosing an Eyepiece Type
Choosing the right telescope eyepiece can seem somewhat daunting as there are so many types available on the market. The final decision will depend on a number of factors including:
- How much magnification you want which is largely dependent on the kind of objects you wish to observe.
- Your budget – Determine how much you are willing to spend. You don’t necessarily have to buy the most expensive one to get good results. The important thing is to know what you are looking for. The available choices range from the three-element pieces (made with three lenses inside) to those with up to eight elements.
- Using glasses – If you wear glasses for simple farsightedness (people that can’t focus on close objects) or nearsightedness (people that can’t focus on far objects), you won’t need to wear them while using the telescope. Simply adjusting the focuser will give a clear view. However, for more serious eye problems like astigmatism, keep your glasses on at low power magnification but take them off at high powers.
- How much of optical imperfections you can tolerate.
- How wide of a field of view you want.
Once you’ve gotten the above sorted, here are a few pointers to help you choose.
You will come across names like SVBONY, Orion Plössl, and Celestron, or modern versions like Nagler and Lanthanum. In all of these pieces, there will be some deviation from the ideal form or optical aberrations though this will be better corrected in some designs when compared to others. Generally, expect better quality with pricier brands.
There are several highly sophisticated eyepieces to choose from. Some of them having multiple optical elements. Some have up to eight optical elements and are as close to being perfectly neutral as possible. They also have a wide field of view and great eye relief (the comfortable distance required between the user’s eye and the eyepiece for proper use). This kind of lenses makes high-power lunar and planetary viewing without compromising on eye relief. Some of them may feature rare-earth glass elements that help reduce optical aberrations further. Although they can be pricey, veterans regard them as the best in eyepiece design. Such as
Other high-performing eyepieces come in four-element designs, e.g. Orthoscopic or the Plössl. In fact, a Plössl is well recommended as it can deliver well-corrected, wide fields of view with good eye relief better still when it is anti-reflection multicoated.
Although a little bit more pricey the Telescope Accessory Kit from Celestron and Orion are quite good value for money giving you a the wide variety of eye pieces and bringing down the cost compared to buying items separately.