Last updated on
4/11/
2006

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Eyepiece Basics

Understanding these telescope eyepiece properties is important when choosing a set of eyepieces:

  • Magnification means just what you might expect - enlarging the object you are observing to see more detail. Eyepieces are described in terms of their focal length, and a shorter focal length means more magnification, which is the opposite of camera lenses. (The reason is that camera lenses are self-contained optical systems, but telescope eyepieces are only a part of the total optical system - your scope’s primary mirror/lens combination is the main ingredient and it is a constant.) The magnification of an eyepiece is equal to the telescope focal length divided by the eyepiece focal length, being careful to keep both in the same units. For most 8” f/10 SCTs, the scope’s optical focal length is 2,030 mm (or about 80” even though the scope is physically much shorter than that, which is why the design is so popular). So the 25mm eyepiece that the manufacturers provide with most 8” SCTs, has a magnification of  81, or 81X.
  • Field of View means the area of sky a particular eyepiece/scope combination shows you.  All things being equal, a higher magnification (shorter focal length) eyepiece shows less of a field of view, which again is the opposite of camera lenses.  The point here is that I said “all things being equal”, and in fact they are not.  Once you decide you need a particular magnification of the image delivered by a scope’s optics, you have chosen a particular eyepiece focal length. For that focal length, different eyepiece designs provide a wider or narrower Field of View (FOV) - more complex (and thus expensive) designs show more sky (FOV) at a particular magnification (focal length).  The true Field of View provided by a particular eyepiece depends on what optical system (scope) it’s used on, so eyepiece manufacturers use the term “Apparent Field of View” to indicate the “wide-angleness” of the eyepiece design irrespective of the scope itself. Knowing the Apparent Field of View of an eyepiece together with the characteristics of your scope, you can calculate the actual Field of View of an eyepiece. The Actual FOV is the Apparent FOV divided by the magnification of the eyepiece, and that magnification depends on the focal length of your scope. A “wide-angle” eyepiece like a 40mm will typically show a larger Actual FOV than a “telephoto” eyepiece like an 8mm. But I had a Meade 15mm Plossl with a 52° apparent field of view and an Orion Lanthanum Superwide 8mm with a 65° apparent field of view, and the actual fields of view of these two eyepieces were fairly close to each other (0.38° and 0.26° respectively).  Of course, to accomplish this the Orion has a much more sophisticated design and thus it is about twice as expensive as the Meade. Click on the FOV Calculator button above for a discussion of some applications you can find on the Web, that will do this calculation (and illustrate your field of view) for you.
  • Eye Relief is the distance away from the eyepiece your eye can be and still see the whole field of view (this term is also used for binoculars).  Eye relief  is largely an issue for those who wear eyeglasses - an eye relief of less than 13mm or so will not allow you to wear eyeglasses and see the whole field of view, although this depends to some extent on the thickness of your eyeglasses.  Of course, if you don’t need eyeglasses because of astigmatism you can just take your glasses off when looking through the telescope eyepiece, but after a while this can become annoying (especially if it’s cold outside and you’re wearing a hat). Generally, manufacturers create better eye relief by adding a barlow lens to a plössl design and using rare-earth (lanthanum) in one or more of the glass elements (these are called “lanthanum” eyepieces by most of their manufacturers).  Eye relief is somewhat related to the eyepiece focal length and field of view - it is difficult to create an eyepiece with high magnification, a wide field of view, and still have good eye relief.  Fortunately, at very high magnifications you don’t so much need a wide field of view.
  • “Contrast” means what you might expect - the ability of the eyepiece to reveal low contrast  details (i.e. details that are at almost but not quite the same brightness relative to each other) in an object. Wide field-of-view eyepieces tend to achieve the wide field with designs that use more individual lenses in the eyepiece, and this greater number of air-glass interfaces within the eyepiece leads to more internal light scattering, even with multi-coated lenses. This tends to reduce contrast. Note that “contrast” is a qualitative term - there is no commonly used definition to give an eyepiece a quantitative measure of its contrast.

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Here are a few useful formulas.  Be very careful to keep all dimensions in the same units, and because eyepiece focal lengths are quoted in millimeters I recommend you use millimeters for all dimensions:

               Scope Focal Ratio = (focal length of scope) ÷ (aperture of scope)
               Magnification = Scope Focal Length ÷ Eyepiece Focal Length
               Actual Field of View = Apparent FOV ÷ Magnification

               (An 8” f/10 SCT has a focal length of 2030mm.)
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