Eyepieces

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.

Eyepieces -- Contents:

1. What should I look for in an eyepiece?
2. How do I determine the power of an eyepiece?
3. How do I determine the field of view (amount of sky) seen through an eyepiece?
4. How do I know which focal length(s) to get?
5. Common Eyepiece Types (or, "what should I get?")
6. What accessories should I consider?
7. Where can I find out more?

What should I look for in an eyepiece?

There are many aspects of eyepiece designs. The three that are most important to the typical amateur are:

1. Optical quality. The quality of an eyepiece is determined by a number of factors, including eyepiece design (number of lenses and their placement), and mechanical quality (smoothly polished glass; presence or absence of coatings, etc.)
2. Field of view. Measured in degrees, this is the apparent width of the portion of sky seen through the eyepiece. The values cited in manufacturer's specs (and in the section on common eyepiece types) are for the apparent field of view; for the actual amount of sky seen through an eyepiece, this must be scaled by the magnification used. Wider fields of view show more objects (or more of a large object) at a given magnification, and are more comfortable generally. Field of view depends principally on optical design; typical values range from 30 to 85 degrees.
3. Eye relief. This is simply the amount of space between the front lens of the eyepiece and the observer's eye when the full field of view becomes comfortably visible. Eyepieces with low eye relief can be difficult or even impossible for people with glasses to use, and are uncomfortable even for people without.

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:

• Coated optical surfaces. Good eyepiece lenses have antireflection coatings -- preferably multiple coatings -- on all air-to-glass surfaces, inside and outside the eyepiece barrel. These coatings reduce reflective "ghosts" in the field of view, and also help preserve image contrast -- important for seeing faint objects. Most eyepieces, even inexpensive ones, are coated to some extent, but the best ones usually have extensive, thorough coatings throughout.
• Smooth "polish" on the lens elements. This is hard to judge without an actual test under the stars, since any roughness present is microscopic. But, as with smoothly polished mirrors and telescope lenses, this lack of microscopic roughness also makes a big difference. Occasionally, you can stumble across an old eyepiece that lacks modern features like coatings which still performs well on planets and other demanding targets. Odds are such eyepieces were very finely prepared with attention paid to lens smoothness.
• Blackened lens edges and internal surfaces to reduce scattered light in the optical path.

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).

How do I determine the power of an eyepiece?

First, a few definitions:

1. P: Power or magnification
2. FL_T: Telescope's focal length
3. FL_E: Eyepiece's focal length

The basic formula here is very simple:

P = FL_T / FL_E

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:

FL_T = 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 / FL_E

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.

How do I determine the field of view (amount of sky) seen through an eyepiece?

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.

How do I know which focal length(s) to get?

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.

Lowest Focal Lengths / Highest Powers

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 / FL_E,

2 = f / FL_E

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.

Highest Focal Lengths / Lowest Powers

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 = FL_E / 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.

Common Eyepiece Types (i.e., "what should I get?")

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.

What accessories should I consider?

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:

1. They increase the versatility of an eyepiece set. Having a 2x Barlow and 30mm and 20mm eyepieces is like having four eyepieces -- 30mm, 20mm, 15mm, and 10mm. With a good choice of eyepieces and Barlow lens you can keep the cost of your eyepiece collection down.
2. Barlow lenses don't affect the eye relief of an eyepiece, meaning that you can get high magnifications without the small eye reliefs of most high-power eyepieces. Owners of short-focal-ratio scopes like large Dobsonians find this aspect of Barlows to be very useful, since they have difficulty getting high powers otherwise.
3. The Barlow lens effectively increases the focal ratio of a telescope; a 2x Barlow makes an f/6 telescope behave like an f/12. Since most eyepieces have less edge distortion at longer f/ratios, an eyepiece + Barlow combination may have a sharper looking field of view than a single eyepiece of equivalent magnification.

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:

• Telecompressors or compressor lenses. They could be called "anti-Barlow" lenses, since they decrease the focal ratio of your telescope. They generally do not work as well for visual use as Barlows do -- most such lenses are used for photography with long-focus (f/10 or so) instruments rather than for visual observing.
• Coma correctors. These are designed to compensate for aberrations common in the fields of view of large, fast (f/5 or faster) Newtonians. Expensive (over $200), but possibly worth it if you have such a scope.
• Filters of all types. Light pollution filters, discussed in detail elsewhere in the GATFAQ, and
  color filters for planetary observing, are important to many observers. Eyepieces usually come with threads in the barrel to accommodate filters.
• Guiding eyepieces, useful for astrophotographers. Guiding eyepieces usually have a crosshair or reticle pattern embedded inside, so that tracking stars is easier; a few are illuminated, making it easier to see the reticle pattern. Otherwise, these are like regular eyepieces of fairly conventional design (older types like Orthoscopics are commonly employed for this purpose).

Where can I find out more?

There are many other places to get information.

• Phil Harrington, Star Ware. An extremely useful book on telescopes and telescope accessories, it covers many of the topics in the GATFAQ and in extensive detail. Try to find the most recent edition possible for the most current information.
• Both Sky & Telescope and Astronomy publish reviews of eyepieces and related accessories on a fairly regular basis. Some starting places:
  • Astronomy, June 1998: Buying guide to eyepieces. Unlike the S&T articles below, concentrates more on accumulating a large collection of manufacturer's specs for a range of eyepiece designs than on test reports -- a nicely complementary approach.
  • Sky & Telescope, July 1997: Test reports on Barlow lenses.
  • Sky & Telescope, April 1996: Test reports on various low to medium priced eyepieces.
  • Astronomy, June 1993: Buying guide to eyepieces available at the time, similar in focus to the more recent article. Many eyepieces in this article are still available or are only
    slightly different from more recent (early 1998) models.