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First, why do you need more than one, may be your initial question? As you may recall in Part 1 of this series, a key element in any type of telescope is it’s ‘focal length’. This is the actual distance between the primary optic, be it a lens or a mirror, and the eyepiece focuser, an adjustable mount where you insert your eyepiece. As discussed before, each and every time the light from the object you are observing passes through an optical piece of glass, you loose a little light. The more light from the object, the better the image.
Since we know what the focal length of the telescope is, we use this to determine the magnification of the object that we are observing. This is done by dividing the focal length by the focal length of the eyepiece. For example, if the focal length of the telescope is 1000 millimeters (mm), and the focal length of your eyepiece is 50mm, then the magnification would be 20 times, or 20x. Typically, telescopes will come with two eyepieces, allowing for magnifications of about 30x and 90x or so.
Most inexpensive telescopes sold at department and toy stores have very cheap optics and use eyepieces that have a focuser diameter of 0.965 inches. These should be avoided! Not only are the optics very cheap, or poor quality, but you are very limited in the variety of eyepieces you can acquire. While some companies do make an adapter which allows you to mount a standard 1.25″ eyepiece in such telescope focusers. Larger telescopes may be built to use even wider, 2″ eyepieces. However, just as with the cheap optics, the focusers for such inexpensive telescopes are also low-grade.
Just as you want to have a stable mount for your telescope to reduce unwanted movement, so too for the focuser. Plus, just as you want a mount that can be adjusted and moved smoothly and steadily to track an object, likewise for the focuser as well. The ‘action’ must be very good as you make slow, short turns of the adjustment knob to focus the object into the best possible view. Having an extra adapter could throw off the precision and the ‘action’.
Two other important factors to consider before we get into the specifics of the different types of eyepieces; field of view and eye relief. Field of view is essentially the angular amount of sky you will be able to see with your telescope. Eye relief is the distance between the eyepiece itself with your own eye. In both cases, more is usually better for observing.
The most common types of eyepieces for telescopes are the following: Orthoscopic, Huygens, Ramsden, Monocentric, Kellner, Plössl, Erfle, and Nagler. These vary in design, using different numbers and types of optical elements. The first four are most often used with refractor telescopes for planetary observations. They typically consist two to four optical elements, have narrower fields of view and longer focal lengths for higher magnifications.
The Huygens and Ramsden designs are very old and typically are non-achromatic, meaning that their glass elements are not treated with an optical coating to reduce distortions associated with magnification. However, they do come in handy in some applications, such as image projection, commonly used for solar observations. A blank ‘card’ is mounted away from the eyepiece such that the Sun’s image is cast upon it, necessary since direct viewing of the Sun is dangerous and could cause blindness.
The Orthoscopic and Monocentric designs are more modern types of eyepieces used for planetary and general astronomy. Both consist of four elements. The Orthoscopic uses two ‘singlet’ and one ‘doublet’ set of lenses. The Monocentric has one singlet and one triplet set of lenses. The doublets and triplets are optical elements glued together, reducing any distortions caused by air gaps between lenses while providing the extra corrective qualities. The Monocentric is the favorite of planet watchers.
The other four types: Kellner, Plössl, Nagler and Erfle, are generally used for reflector and catadioptic, or compound, telescopes. They work best for both general astronomy and more specifically, deep sky observations. These will have four or more optical elements and are usually have an ‘achromatic’ coating to limit distortions. The most popular among these are the Plössl, Kellner and Erfle. They offer good, overall optic characteristics, including wide fields of views, needed for deep sky observing. Nagler is the best, but also the heaviest and most expensive of the group.
Before we wrap this up, I should also mention the Barlow lens. This is an optical device that increases the magnification. Most on the market double the magnification, though some companies do make 1.5x and 3x Barlow lenses. Again, this is an adapter that is attached to the focuser and then an eyepiece is inserted onto the Barlow lens. Most telescopes come with one.
This has been a brief review of telescope eyepieces. You can read whole books on just them. The key points I am attempting to provide you with are; 1) buy the best you can afford and 2) buy what you need for you purposes. If you goal is to check out planets or the craters of our Moon, then a refractor telescope with some of the first group of eyepieces is best. If you intend to observe deep sky objects, like galaxies, or faint ones like comets, then a reflector is the best type of telescope to buy and acquire a collection of the second group of eyepieces. Or, split the difference and purchase a catadioptic telescope, such as the popular Schmidt-Cassegrain design and again, go for Group #2, with maybe one or two from Group #1 for planetary work.