Telescope eyepieces are the most important accessory for any visual telescope setup because they determine the magnifying power at which you're viewing. Change the eyepiece and you can bring an object closer or widen the field of view to observe a larger swath of sky. Telescope eyepieces are available in a wide variety of designs, sizes, and prices, and these options can be confusing for beginners, as well as for more seasoned amateur astronomers. However, learning a bit about telescope eyepieces will allow you to choose the right design and focal length (part of what determines the magnification) for your particular telescope and observing habits to help you get the most out of your telescope. Let's start with the fundamentals.
To make any telescope eyepiece work, it must first fit the focuser. The standard sizes for focusers are .965″, 1.25″, and 2″. This is a dimension which directly corresponds with the diameter of the telescope eyepiece barrel. The size .965" was, at one time, the standard for smaller, more inexpensively made telescopes, but it has started to fall out of fashion. This change in trend means that .965" accessories, including eyepieces, have become increasingly hard to find and selection is limited. However, OPT maintains a complete selection of .965” telescope eyepieces.
No matter what size telescope focuser you may have, there are adapters available to make any eyepiece work with your focuser and eyepiece. For example, you can adapt a .965″ focuser to accept a 1.25″ eyepiece. Adapters are simply a nice tool to help you make the most of the telescope eyepieces and equipment you may have collected over the years and offer you more choices. Do not discard an old eyepiece because it seems “out of date” – you may have a hidden gem!
One defining characteristic of telescope eyepieces is their focal length. They may have a series or brand name, but you'll find that they all clearly list what the focal length is in their name, in millimeters. There is some simple math that will help you choose the best telescope eyepieces for your particular telescope. Thankfully, there are some rules of thumb that give us a few guidelines to live by. You can typically use 40-60x for every inch of telescope aperture, depending upon telescope design. Due to sky conditions, light pollution, and limits of most telescope optics, you have a maximum power cap of about 450x, no matter how large your telescope might be, and a minimum of 50x before you relegate your telescope to a giant finderscope. Keep in mind that these are approximations and averages. If you happen to have amazing optics and steady skies, you can push that power cap much higher. Let's consider some scenarios. We'll use an 11" SCT with a focal length of 2800mm as the sample telescope and round to the nearest whole number. What's our power limit? Eleven inches of aperture x 60 = 660x. We usually observe in our backyard in the middle of suburbia, so we have to stick with a limit of about 450x.
To find the focal length of telescope eyepiece that would give us this maximum power, divide the focal length of the telescope 2800mm by the maximum power we want to achieve. So 450x = a 6mm eyepiece.
Suppose we get a fancy new 10mm eyepiece for a present and we want to know what power that results in. Divide the focal length of the telescope 2800mm by the focal length of the eyepiece, so 10mm = 280x.
Once you have the math down, you'll want to aim for a range of magnifications from low to high to help locate objects. You'll want a low power,wide-field telescope eyepiece to observe very large objects, like the Orion nebula or Andromeda galaxy. You’ll want a high-power telescope eyepiece for lunar and planetary observing. With at least one medium power telescope eyepiece in between for generalized observing. Remember that relative to any one telescope, magnification and brightness are opposites. So using low-power telescope eyepieces is a great way of seeing the faintest objects with your system, and high-power views are best for bright objects. You should also familiarize yourself with a few other terms that you'll often see associated with telescope eyepieces.
The first is eye relief (ER), which is the distance your eye must be from the eyepiece lens to achieve focus. This is of particular importance to eyeglass wearers, who will likely prefer longer lengths to be comfortable, such as something over 15mm. This is a matter of comfort, however, and that comfort level is different for everyone. There are many who like long focal lengths who do not wear glasses while observing. If you are a first timer or are not very familiar with this factor, it is better to err on the side of long than short. Short eye relief will have your eye very close to the lens to focus.
Apparent field of view (AFOV) is next. Dictated by eyepiece design and expressed in degrees, the apparent field of view is the width of the field as seen through the eyepiece. For example, you may have two eyepieces of the same focal length but very different AFOV. As this number increases, so does the width of the field. So an 82 degree eyepiece will allow you to see considerably more than a 60 degree eyepiece.
Both ER and AFOV are dependant on the design of the eyepiece and the focal length, so you will see some limitations. Other terms you may encounter are distortion and curvature of field. A good quality telescope eyepiece provides a flat field-of-view and sharp images from edge to edge. Types of optical glass are also a consideration. Lanthanum is an exotic and offers extra long eye relief with an excellent field of view. ED optical glass stands for Extra Low Dispersion, and it provides outstanding eye relief and image width, turning higher power eyepieces into a pleasure instead of a peep-hole. When choosing a telescope eyepiece, be sure to look for multi-coatings, internal baffling, and lens edge blackening to prevent reflections and internal threading to accept filters.
Now let's take a look at telescope eyepieces' designs.
The orthoscopic design consists of a cemented triplet matched to a single plano-convex eye lens. While this might seem a little technical, orthoscopic eyepieces are about as close to perfect as they can get. They offer excellent eye relief, nearly non-existent chromatic or spherical aberration, have a fairly flat and wide field of view (40 to 50 degrees), and have little enclosed reflection, thanks to their internal multi-coatings. Superior high contrast is one of the reasons the "ortho" has remained a favorite of astronomers for over 125 years. Well suited to the beginner, orthoscopic eyepieces are also prized by discriminating observers for their legendary performance on high-power planetary observing.
Kellner eyepieces are a 3-lens design that offer good image quality at a budget price. Designed by Carl Kellner in 1849, this eyepiece endures because its achromatic doublet format corrects chromatic aberrations. With a field of view of about 40 to 50 degrees, the Kellner is a good choice for small to medium aperture telescopes with a focal ratio of f/6 or longer.
Crafted by Georg S. Plossl in the 1890s, the Plossl eyepiece design consists of two sets of doublets, in a 4-element design, which provide an approximate 50 degree field of view. Although it has a relatively short eye relief, the Plossl eyepiece delivers quality views of deep space and planetary subjects at all focal ratios. It remains one of the most highly regarded of all telescope eyepiece styles, and it's considered an excellent performer for all-around use.
The Erfle consist of five or six elements and was created solely for an ultra wide field of view, such as 60 to 70 degrees. At low powers, it provides impressive deep sky views – huge swatches of sky with contrasting backgrounds and pinpoint stars. Galaxies and nebulae are framed by the fields in which they reside. However, at high magnification, the image sharpness can become distorted at the edges with some telescope types and may require corrective measures. For expansive views, 5 to 6-element designs nearly double the FOV over basic eyepieces, while ultra-wides (consisting of 6 to 8-elements) deliver panoramic scenes and extremely high quality images.
New designs in telescope eyepieces have pushed the boundaries of technology, with 82 degree,100 degree and even 120 degree fields-of-view. Innovative telescope eyepiece manufacturers like Tele Vue and Explore Scientific are constantly developing new designs to increase your viewing pleasure.
Specialty telescope eyepieces will include measurement and illuminated reticle eyepieces used for tracking and guiding.