Oculars (the correct term for "eyepieces") are probably the first accessory purchased by most telescope buyers. Additional oculars (beyond the one or two supplied with most telescopes) make visual observing much more enjoyable. Like camera lenses, they determine the field of view and magnification seen through your telescope. Having a proper selection of oculars adds greatly to the versatility of any telescope.

Before buying accessory oculars, it is wise to learn some basic facts about these expensive little magnifying lenses that slip into the focuser or star diagonal of your telescope. There are also some terms that it is necessary to understand. (Additional astronomy and telescope definitions can be found in the article "Definitions for the Amateur Astronomer.")

Focal length

Like telescopes themselves, oculars have a focal length, which is expressed in millimeters. In oculars, focal length is measured from the optical center of the ocular (called the rear nodal point) to the point of focus in space where you put your eye. Astronomers seldom talk about oculars in terms of magnification; they usually refer to them by their focal length, just as photographers do when talking about their lenses.

Focal length is important because it determines how much the ocular magnifies the image sent to it by the telescope. The shorter the ocular's focal length, the more it magnifies the image; while the image is larger at higher power, it is also dimmer and the actual field of view is smaller. The longer the ocular's focal length, the less it magnifies the image, but the image is brighter and the actual field of view is larger. Image brightness and field of view decrease as magnification increases.

Apparent field and actual field of view

Apparent field of view (AFOV) is the angular size of the light cone covered by the ocular. Good apparent field makes an ocular enjoyable to look through, more like looking through a porthole than a knothole. Apparent field is not the same thing as the actual field of view (FOV) of your telescope, which is the angular field of view (the area) visible through your telescope. The true field of view through your telescope probably seldom exceeds 1 degree and is often less. Actual field of view can be computed by dividing the apparent field of view of your ocular by the power of your telescope/ocular combination. For example, a telescope with a prime focal length of 1000mm with a 10mm ocular in place is operating at 100 power (1000/10=100). If that 10mm ocular happened to be a Plossl with an apparent field of 50 degrees, the actual field of view of the telescope would be .5 degree (50/100=0.5).

Exit pupil

The diameter of the pencil of light leaving the ocular of a telescope is called the exit pupil. The exit pupil should not exceed the maximum, dark adapted, diameter of the pupil of the viewer's eye, which averages about 7mm for young eyes in good condition. As we age, our maximum pupil size gradually decreases. Middle aged people frequently have about a 5mm maximum pupil size. This sets the minimum practical power for your telescope, which is whatever power produces an exit pupil equal to what your eye can accept. The formula for computing exit pupil size is simple. Divide the clear aperture of the telescope (in millimeters) by the magnification. For example, a 6" (150mm) scope operating at 50 power has a 3mm exit pupil (150/50=3).

Eye relief

Eye relief is the distance, usually measured in millimeters, from the ocular element to the eye. It is important, because viewing through an ocular with insufficient eye relief (only a few mm) is difficult. Eye relief is particularly important for eyeglass wearers. Fortunately, unless you have astigmatism, it is not necessary to wear your glasses when viewing through a telescope; the telescope itself is focused to your eye.

Magnification

Magnification is determined by dividing the prime focal length of the telescope (in millimeters) by the focal length of the ocular (in millimeters). In any given telescope, a shorter focal length ocular will give higher magnification and a longer focal length ocular will give lower magnification.

Ocular barrel diameter

There are three common ocular barrel diameters. These are .965", 1.25" and 2". 1.25" oculars are the size most often supplied with good quality astronomical telescopes. A few good telescopes are supplied with .96" oculars and conversely, some low quality telescopes are supplied with 1.25" oculars. The folks who distribute cheap telescopes are becoming wise to the fact that 1.25" oculars are a feature semi-informed buyers use to differentiate between "good" and "bad" telescopes and have begun to supply the larger size with their inferior scopes to deceive such buyers. The optical quality of an ocular is more important than its nominal diameter.

.96 Oculars

.96" oculars are used in many popular "department store" telescopes and some more advanced models, such the famous Celestron C-90 spotting scope. The primary advantage of .96" oculars is simply that they are smaller and usually less expensive than their larger brethren. They balance well in small telescopes and if they are of good quality they can perform well. Most experts consider them more difficult to look through than larger size oculars.

I once asked Alan Hale (then President of Celestron International) why he persisted in supplying .96" oculars with the C-90 and he told me that he personally preferred the smaller size oculars in the (physically small) C-90 telescope. (Note: the new G-3 astro telescope version of the C-90 comes with a 1.25" ocular.) I prefer the larger 1.25 size oculars and one of the first things I have done with my C-90 telescopes is adapt them to this size ocular. The selection of .96 oculars is reasonable, but not as large as that of 1.25" oculars.

1.25" Oculars

The most common ocular size for astronomical telescopes is 1.25" diameter. These are supplied with the great majority of "serious" amateur telescopes and are one of the hallmarks of a high quality instrument. Many manufacturers offer comprehensive lines of 1.25" oculars. The principle advantage over .96" oculars is a larger opening into which to peer. The principle advantage over 2" oculars is lower weight. The large 2" oculars and associated 2" star diagonals are simply too heavy for many amateur telescopes. 1.25" oculars are also less expensive than 2" oculars and more readily available.

2" Oculars

The big 2" oculars are wonderful to view through, but for the reasons mentioned above, they are best reserved for the larger sizes of amateur telescopes. 2" oculars are most commonly found in medium to long focal lengths (17mm and up) and they are excellent for wide sky views. Shorter focal lengths (higher magnifications) are usually reserved for the 1.25" size oculars. On the amateur market, 2" oculars are less widely distributed than 1.25" oculars and the selection is smaller. The focusers or visual backs of most advanced amateur and professional telescopes are designed for 2" oculars and they can be adapted to many other amateur telescopes. These big oculars are usually excellent performers, the limiting factors being weight, expense and availability. The latter is slowly improving.

Ocular designs

There are many different designs, or optical formulas, for oculars. Like camera lenses, inside of every ocular you will find a group of individual lens elements. Oculars usually consist of two or more elements. Two element optical designs are called "achromats" (or achromatic). They focus the long and short wavelengths of visible light, red and blue, to a common plane, but green light is not properly focused and typically creates a noticeable fringe of color around bright objects. Two element oculars are to be avoided; their only advantage is low price. Some common names for, or descriptions of, two element optical formulas are Ramsden, Huygenian and just plain "achromat" These cheapies are usually found in the .96" size.

Since it requires at least three elements to focus all three primary colors of light (red, green and blue) to a common point, all but the cheapest and most inferior designs have at least three lens elements. Oculars designed to the Kellner formula have three elements, as do the similar Modified Achromat (MA) and Super Modified Achromat (SMA) types. (The modification appears to be the addition of a third lens element.)

Kellner type oculars generally have an apparent field of view of about 40 degrees, with moderate eye relief. They have good center sharpness, but exhibit some evidence of field curvature and astigmatism. They are most useful for terrestrial, planetary and lunar viewing. These are the least expensive practical oculars and are offered in focal lengths from about 6mm to 40mm. I have found the moderate 16mm to 25mm focal lengths to generally be the most useful in the .96" Kellner oculars I have owned. Kellner and other three-element ocular designs are available in .96", 1.25" and 2" diameter oculars.

In practice, a forth element is usually required to "bring it all together" and produce sharp views without intrusive aberrations. (All ocular designs have some residual aberrations.) Four element oculars are very common and generally perform very well. Orthoscopic and Plossl oculars are common types that feature four elements. The Orthoscopic design is asymmetrical and the Plossl is a symmetrical design.

The Ortho is a good design for oculars used to view bright objects. It has very good center sharpness with only a small amount of field curvature and astigmatism. Perhaps because of its asymmetrical design, it is less susceptible to internal reflections (ghost images) than the Plossl design. Orthos have an apparent field of about 45 degrees and more eye relief than Kellners. I have owned Orthos in both .96" and 1.25" size and found them a very good choice for planetary and lunar viewing. Orthos are most common in focal lengths from about 4mm to 25mm. I have found the 9mm to 12mm focal lengths most useful in the telescopes I have owned.

The Plossl design has become the mainstay of the ocular business and probably represents the best value in terms of performance and price in medium and long focal lengths. Plossls are most common in the 1.25" and 2" sizes, but I believe there is at least one company offering .96" Plossls. It is a well corrected design that uses four elements in two symmetrical pairs. Plossls typically have about 50 degrees of apparent field.

If well made, Plossls are bright and contrasty, with a flat field and excellent sharpness. They offer marginally better eye relief than Orthos and are available from many suppliers in a wide range of focal lengths in 1.25" diameter, typically from about 6mm up to 45mm. In addition, Plossl oculars are usually parfocal within a given manufacturer's line, which means that when one is in focus, they are all in focus. The longer focal length Plossls provide adequate eye relief for eyeglass wearers.

Plossls are useful for most purposes, from short focal lengths designed for planetary views and splitting double stars, to long focal lengths designed for spectacular deep sky views. A 50-56mm Plossl in 2" diameter is spectacular for wide field, deep sky viewing. Plossl oculars are available from Celestron (Omni), TeleVue, Meade (Super Plossl), Orion (Sirius), Vixen and others.

The Erfle is a 5-element design which features a wide apparent field of view of about 60 degrees. These are usually available only in 1.25" and 2" size. Meade QX 1.25 and 2" oculars are probalby the best known Erfles today. Offering a similar apparent field in both sizes are the premium, 6-element designs called Super Wide Angle by the Meade Instrument Company, which fulfil the same role as traditional Erfles.

Erfles have a nice, flat field from edge to edge, although sharpness is slightly inferior to the Orthoscopic and Plossl designs. They also feature long eye relief, especially important for eyeglass wearers. This combination of features makes these oculars excellent for wide sky viewing of open clusters, starfields, etc. They are usually found in focal lengths in the 18mm to 32mm range. I have used 1.25" Erfles in 24mm and 32mm focal lengths, where they give a greater field of view than Plossl type oculars. In 1.25" diameter, the Meade Super Wide Angle oculars are available in focal lengths from 13.8mm to 24.5mm and the QX in 15mm and 20mm focal lengths.

Some modern ocular designs incorporate more elements than Plossls, Orthos and Erfles for improved correction of aberrations, increased eye relief and a wider apparent field of view. The upscale TeleVue Radian, Tele Vue Panoptic, Celestron X-Cel, Celestron Ultima, Burgess/TMB Planetary and Meade 5000 Series ocular lines (to name a few of the better known brands) are priced higher than standard designs and are available in a variety of focal lengths.

Some premium ocular designs incorporate extra low dispersion glasses with rare earth elements to improve performance. The Celestron X-Cel, TeleVue Nagler and TeleVue Radian are examples of well known ocular lines that incorporate various types of ED glass elements in their design. The Vixen Lanthanum-LV oculars, all of which have an eye relief of 20mm, regardless of focal length, use a Barlow lens in front of the main 5-element group, one of which is made of glass incorporating the rare earth element Lanthanum. The total number of elements in these oculars reportedly varies from 6 to 8, depending on focal length and Barlow design. These Lanthanum-LV oculars are available in focal lengths from 2.5mm to 25mm.

TeleVue's Radian oculars have a 60 degree apparent field of view and are primarily intended for high magnification purposes. They range in focal length from 3mm to 14mm, in 1.25" size only. TeleVue's premium Panoptic oculars have a 68 degree apparent field and are available in both 2" and 1.25" barrel diameters. There are three focal lengths in the 1.25" size: 15mm, 19mm and 22mm. Celestron Ultima LX oculars have an apparent field of 70 degrees and are offered in focal lengths from 5mm to 32mm.

There are some premium specialty oculars that feature extra wide apparent fields of view and long eye relief. Prominent among these are the TeleVue Nagler, Meade Ultra Wide Angle (8-element) and Celestron Axiom LX designs. Naglers have an astonishing apparent field of 82 degrees, the Meade Ultra Wides 84 degrees and the Celestron Axiom LX 81 degrees! The Meade and Nagler ultra-wide designs are primarily used for short focal length (high magnification) applications, because short focal length necessitates short eye relief and tiny openings in conventional ocular designs. Focal lengths run from 4.7mm to 14mm (1.25" Meade Ultra Wide Angle series) and 4.8mm to 16mm (1.25" TeleVue Nagler series). Celestron offers their Axiom LX line in a wider range of focal lengths, ranging from 7mm to 31mm. These are all expensive oculars to buy, but for the person who does a lot of high magnification viewing, they may be worth it.