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Latest Revision: December 12, 2003
DISCLAIMER: All this information is accurate to the best of my knowledge; if there are any omissions or errors, please let me know. This document is intended to be an overview, not the end-all-and-be-all of a given topic. If you want to find out more about a specific gadget, accessory, or thingy, consult the references listed in each section.
There are many aspects of eyepiece designs. The three that are most important to the typical amateur are:
Of these three, only field of view is easy to estimate by just looking at the specifications of an eyepiece -- a given kind of eyepiece usually has a given field of view (perhaps varying a few degrees from focal length to focal length).
Optical quality is influenced by a number of factors. The effect of optical design -- the arrangement of the lenses in the eyepiece -- on optical quality is discussed in more detail in the section on Common Eyepiece Types. Some other "quality" details to look for:
In general, higher prices (for a given optical design) give you better mechanical and optical quality.
Most manufacturers publish eye relief figures for their eyepieces, but these can easily be off by several mm in practice. One useful rule for eye relief: for a given eyepiece design, eye relief decreases as focal length decreases (i.e., as magnification increases). In many popular designs, such as inexpensive Plössls, high power eyepieces may have an unacceptably short eye relief. This can be compensated for by switching to a different design (some lines of eyepieces, such as the Vixen Lanthanum series, are specifically optimized for high eye relief at all focal lengths), or by using a lower-power eyepiece and a Barlow lens (see the "Accessories" section for more information).
First, a few definitions:
The basic formula here is very simple:
P = FLT / FLE
where both focal lengths must be expressed in the same units (e.g., mm). Since the telescope focal length is fixed for a given scope, you can see that high powers are associated with short eyepiece focal lengths, low powers with long ones. Most eyepieces have focal lengths between 5mm and 60mm.
Most telescopes are defined in terms of their aperture and focal ratio, f, rather than focal length. The relationship between the two is:
FLT = f * A
where A is the clear aperture of the telescope. To give a concrete example, a person with a 200mm (8") f/10 telescope -- typical of Celestron and Meade Schmidt-Cassegrains -- has a telescope with a focal length of 2000mm. This would thus yield 40x with a 50mm eyepiece (a fairly long focal ratio), and 200x with a 10 mm eyepiece.
You can also see that by combining the equations, you get
P = f * A / FLE
For a given aperture, shorter focal ratio scopes require shorter focal length eyepieces to achieve a given magnification. Thus, an 8" f/6 Newtonian would need much shorter focal length eyepieces to get to high power than an 8" f/10 SCT. Conversely, the f/10 telescope needs much longer focal length eyepieces to attain low powers.
A common way is the "star drift" method. This involves pointing the telescope to a star on (or very near) the equator, e.g., Delta Orionis (one of the "belt" stars), turning off your clock drive if you have one, and measuring how quickly the star appears to travel across the field of view. A star on the celestial equator travels a full 360 degrees in 24 hours, or 1/4 degree in 1 minute. A somewhat cruder method, but useful for a rough estimate, is to take the manufacturer's spec for field of view and divide by the power of the eyepiece. This makes two assumptions: (a) the manufacturer's specification (of both field of view and of focal length) is accurate, and (b) the angle seen is small enough that small-angle approximations (i.e., sin x and tan x both ~ x) hold.
You need to have several different eyepieces of differing focal length to get a useful range of powers. However, the range of focal lengths you'll want to consider will depend somewhat on you and on your telescope.
A practical lower limit on focal length is set by the magnification limit of your telescope. As a rough approximation, the maximum magnification usable on a typical scope is 2x / mm of aperture. Since P = A * f / FLE,
2 = f / FLE
where the eyepiece focal length here is that producing 2x / mm of aperture. Thus for an f/10 telescope the smallest focal length eyepiece you'd normally employ is 5 mm, regardless of aperture. At higher magnifications, resolution does not increase -- you become limited by diffraction -- and so higher powers do not gain you much.
A practical upper limit is set by the size of your pupil. If your pupil is smaller than the exit pupil of the telescope, not all the light exiting the telescope will enter your eye. Telescope exit pupils are inversely proportional to magnification, so lower powered eyepieces produce larger exit pupils.
The exit pupil associated with a given eyepiece is given by:
Exit pupil diameter = FLE / f
Typical exit pupils are around 7 mm in dark adapted young adults, gradually decreasing to ~5mm with age. For the same f/10 telescope considered earlier, and a young observer with a 7 mm pupil, the maximum focal length of the eyepiece should be 70mm. For older people, or people in places that are fairly brightly lit, 50 or 60mm might be a better upper limit. Somewhat larger eyepieces can be used to increase field of view, but image brightness may suffer.
Once you've established these limits, you can fill in the range however you like. In general, particularly for inexperienced observers or people with poor atmospheric stability, the low end of the power range (i.e., the high end of the focal length range) is more important. You really only need one good high-power eyepiece, and if you're principally interested in large, dim objects like galaxies and nebulas you may be able to forego that for a while.
You will note that for scopes with a high f-ratio, say f/15, it's hard to get very low powers; conversely, it's hard to get very high powers with an f/4 scope -- not many places make good eyepieces with 90mm and 2mm focal lengths, respectively. Under these circumstances, you might want to look at accessories like Barlow lenses or telecompressors to alter your scope's effective f/ratio.
There are many different brands of eyepieces out there. Fortunately, most of the ones readily available nowadays fall into a relatively small number of optical designs, each with their own benefits and drawbacks. In general, with eyepieces (even more so than with telescopes per se), you get what you pay for. Under the right circumstances, you can get some real bargains, however.
|Optical Design||Typical Price
|Huygens (or Huygenian)||<$30||
This is a two-element (lens) design, the simplest widely sold with telescopes. It typically has a very narrow field of view (often less than 30 degrees) and very poor color correction and other optical aberrations. Huygens are very hard to buy simply because they're so poor compared to other, only marginally more expensive designs. I include them here because, unfortunately, they are commonly supplied with very cheap telescopes.
If you do have such an eyepiece, hang on to it despite what I said above. There are no cemented lens elements in a Huygens, so it's a good choice for solar projection -- you won't run the risk of frying the cement in a more expensive eyepiece and ruining it.
Ramsdens are another two-lens design like the Huygens, but a bit less prone to the worst problems. Ramsdens sometimes make feasible eyepieces on very long-focus (over f/15) scopes, but they still have narrow fields of view (often around 30 degrees), and on most other scopes their performance is even worse. Again, these are primarily seen nowadays only with very inexpensive telescopes. As with Huygenians, these make decent eyepieces for solar projection.
Kellners are a 3-element design--the front (eye) lens is a compound doublet, like the objective of an achromatic refractor. This design greatly improves on the performance of 2 element designs and also increases the field of view to 40-45 degrees. Kellners are still prone to some annoying visual defects, notably ghost reflections, and tend to work poorly on relatively short focus telescopes (they work fine above f/10). Kellners are probably the least expensive eyepieces you might want to consider actually buying.
These are 3-element designs similar to the Kellner but optimized in various ways. The best known examples are the Meade MA (Modified Achromat) and the Edmund "RKE". The Meade MAs are commonly sold with less-expensive Meade scopes; the RKE is an Edmund Scientific invention sold with most of their scopes, as well as separately. Although not quite in the class of highly expensive eyepieces, MA and RKE eyepieces make good choices for observers on a limited budget; they normally perform a bit better than standard Kellners, have similar fields of view (around 45 degrees) and in a few cases approach the performance of inexpensive 4-element designs.
Orthoscopics are a 4-element design that was popular for many years. They outperform Kellners and other 3-element designs and are widely used for medium- to high-power views of planets and other small objects. The field of view is similar to that in good 3-element designs; around 45 degrees. Because of the popularity of the Plössl design, Orthoscopics aren't as popular as they used to be, but good ones are comparable in overal quality to good Plössls.
Plössls are the current "sexy" eyepiece, much as Orthoscopics were several decades ago. Well-designed Plössls are characterized by good fields of view (around 50 degrees) and good sharpness over the field. Plössls tend to have significantly better sharpness than 3-element designs and thus make a good basic choice for fast (~f/5) scopes that would have troubles with less expensive designs. Inexpensive Plössls, such as the Adorama and Sirius brands, sell for around $50, but many observers consider the somewhat higher priced Meade and TeleVue Plössls to be of better quality and worth the ~$80 per eyepiece.
Many manufacturers have produced eyepieces that take the basic Plössl design and expand on it, usually for greater field of view, eye relief, or edge sharpness. These designs tend to breed like rabbits, mutating all the while, so it's difficult to give a good summary. Some well-known names: the Meade Super Plössls, the Orion Ultrascopics, and the Celestron Ultimas.
|Miscellaneous Mid-High Priced Eyepieces||$75-200||
This is a "catchall" for all designs in this price range that don't fit in simply as Plössl variants, and are not unusually wide in apparent field. Some smaller companies make eyepieces with designs that are similar in price to a high-quality Plössl or Plössl variant while being quite distinct in design. Possibly the most famous one now is the Radian, a series of eyepieces by TeleVue, that are so named because their field of view is almost exactly one radian (about 57 degrees). Like their wider-field eyepieces, the TeleVue Radians are highly optimized for field sharpness even at short f/ratios. Also in this category is the Lanthanum series, produced by Vixen, which is specifically optimized to have the same (or nearly the same) eye relief in all the eyepieces in the series, regardless of focal length.
|Generic "Wide Field"||$100-200||
"Wide Field" generally means eyepieces with fields of view of around 60 to 70 degrees. As with the Plössl variants, there are so many it's hard to keep track of the details. One example is the König, an old but fairly effective design with about a 60 to 65 degreee field of view. Good Königs are still sold by University Optics. Another older (circa World War II) design is the Erfle. Erfles tend to suffer from more edge distortion than newer wide field designs, and so aren't widely seen anymore. Among the newer variants, TeleVue's Panoptic series is highly regarded, much like the ultra-wide-angle Naglers. The Big Three vendors (Celestron, Meade, and Orion) all have similar, credible "house brands".
|Generic "Super Wide Field"||>$200||
Eyepieces in this class typically have fields of view over 75 degrees -- nearly twice that of a typical Kellner. The most famous example is the Nagler design, produced by TeleVue, which pioneered modern super-wide-field eyepiece design, at an admittedly premium price (usually over $250 per eyepiece). Naglers are popular among owners of large Dobsonians and other telescopes with large focal lengths. These eyepieces are optimized for wide fields of view; though the engineering (not to mention the view!) is superb, for targets requiring a narrow field, like planets, a highly-optimized eyepiece of different design may be able to outperform Naglers. As with the other wide-field designs, there are variants on the basic theme; the best known is Meade's "Ultra Wide Angle" series of eyepieces.
One possibly useful accessory is the Barlow lens. This is a device that effectively increases the focal length of the telescope it's used with, which (as described in the section on eyepiece power) increases the magnification proportionately. Typical magnification factors for Barlow lenses range from 1.8x to 3x, depending on design.
Barlow lenses have three big advantages:
On the downside, Barlow lenses introduce another optical element into the telescope/eyepiece "system", so they often degrade the image somewhat, particularly on short focal ratio (<f/6) telescopes. Owners of such large telescopes will often have to buy a premium Barlow optimized for such focal ratios. I have an inexpensive Celestron Barlow that is quite poor in my f/5 Newtonian, but works fine on a f/8 Newtonian owned by an astronomy club I once belonged to. A few observers find the effects of even good Barlows to be noticeable and avoid them. Nevertheless, Barlow lenses are still a very useful accessory for many people.
Another, related accessory is the Powermate, designed and sold by TeleVue. The Powermate is similar in principle to a Barlow -- it inserts into your focuser to increase the magnification of any eyepiece, and increases the effective focal ratio of the scope -- but it uses more lenses to correct some of the minor defects inherent in the conventional Barlow design. I have never used one, but many people who have tried one consider it to be significantly better than typical Barlow lenses. The Powermate comes in a wider range of magnification factors (including an almost-insane 5x), and a higher price (~$175) than conventional Barlows.
Other eyepiece accessories and eyepiece-like tools that may be relevant:
There are many other places to get information.