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 well to understand 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 applied to oculars, and telescopes, that it is necessary to understand.

Focal length

The focal length of a telescope is the linear length of the light path (usually measured in millimeters) from the first active optical element to the point of focus. In a Newtonian reflector, this is from the surface of the primary mirror, to the surface of the secondary mirror, to the point of focus inside the focuser tube that holds the ocular. In a catadioptric telescope like a Schmit-Cassegrain, it is measured from the corrector plate (front lens), down to the primary mirror, up to the secondary mirror (attached to the center of the corrector plate), and back down through the hole in the center of the primary mirror to the point of focus behind the telescope and in front of the ocular.

Oculars also have a focal length, which is measured in millimeters. In oculars, this 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. 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--the greater the power, but the dimmer the image and the smaller the actual field of view. The longer the ocular's focal length, the less it magnifies the image--but the brighter the image, and the greater the actual field of view. Astronomers seldom talk about oculars in terms of magnification, they refer to them by their focal length, just as photographers do when talking about their lenses.

Telescope focal ratio

Focal ratio is the telescope's photographic "f-stop." It is computed by dividing the focal length (in millimeters) by the clear aperture (in millimeters). Thus, the ubiquitous 8" (200mm) aperture telescope with a focal length of 2000mm has a focal ratio of f/10 (2000/200=10). Focal ratio is not particularly important for visual astronomy, since all telescopes with the same clear aperture capture the same amount of light, and focal length, not focal ratio, determines magnification and field of view. For astronomical telescope purposes, f/5 is a "fast" (comparatively short exposure) focal ratio, f/10 is a medium focal ratio, and f/15 is a "slow" (comparatively long exposure) focal ratio.

Apparent field and Actual field of view

Apparent field 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 tube. Apparent field is not the same thing as the actual field of view 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 far 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 is 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. The elderly are generally down to about 4mm. 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.

Short, medium, and long focal length oculars, and Magnification

For purposes of reference, I consider oculars with focal lengths from 1mm to 14mm to be short focal length oculars, from 15mm to 30mm to be medium focal length oculars, and from 31mm on up to be long focal length oculars. Magnification is determined by dividing the prime focal length of the telescope (in millimeters) by the focal length of the ocular (in millimeters). But a short focal length ocular will give higher magnification, and a long focal length ocular will give lower magnification, with any given telescope.

Ocular barrel diameter

There are three common ocular barrel diameters. These are .965 inch, 1.25 inch, and 2 inch. 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, like the famous and excellent 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, though, consider them harder to look through than the larger size oculars. I once asked Alan Hale (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 fairly extensive, but not as large as that of 1.25" oculars. Celestron offers one of the best .96" ocular lines.

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 are simply too heavy for many amateur telescopes, particularly models with less than 8" of clear aperture. 1.25" oculars are also less expensive than 2" oculars, and much 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 the 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 the .96" and 1.25" oculars. The visual backs of most professional telescopes are designed for 2" oculars, and they can be adapted to many amateur telescopes as well. The big oculars are usually excellent performers, the limiting factors being weight, expense, and availability. The latter is slowly being corrected.

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 two element optical formulas are Ramsden and Huygenian. 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 Modified Achromat designs are available in both .96 and 1.25" size.

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 short focal length (high magnification) oculars. It has very good center sharpness and 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, superior to most others, 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 has become the mainstay of the upscale ocular business, and probably represents the best value in terms of performance and price. 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 very well corrected design, which uses four elements in two symmetrical pairs. Plossls have about 50 degrees of apparent field. If well made, Plossls are bright and contrasty, with a flat field, and excellent sharpness. They offer better eye relief than the types discussed above, and are available from many suppliers in a wide range of focal lengths for the standard 1.25" size, typically from about 6mm up to 45mm. Plossls are useful for almost all purposes, from short focal lengths designed for planetary views and splitting double stars, to long focal lengths designed for spectacular deep sky views. 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. A 50-60mm Plossl in 2" diameter is spectacular for wide field deep sky viewing.

Celestron's Ultima oculars generally resemble Plossl designs, with an extra element in the middle to suppress ghost images and aid correction. These premium oculars are offered in focal lengths from 5mm to 42mm (1.25" size only). Meade's similar 5-element Super Plossl oculars are priced about like the Celestron Ultimas (which is to say considerably higher than standard Plossls), and are available in focal lengths from 6.4mm to 40mm in 1.25" size, plus 56mm in 2" size. Both are very high quality ocular lines. I personally prefer 1.25" Plossls, Ultimas, and Super Plossls in focal lengths from 10mm to 36mm.

The Erfle is a 5 element design which features a very wide apparent field of view of about 60 degrees. These are usually found only in 1.25" and 2" size. Offering a similar apparent field in both sizes are the premium 6 element designs called Super Wide Angle by the Meade Instrument Company, which are apparently intended to fulfil the same role as traditional Erfles. Erfles have very good edge to edge sharpness, although center 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 to 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.

Tele Vue's premium priced Radian oculars also have a 60 degree apparent field of view, but are clearly designed for high magnification purposes. They range from focal lengths of 3mm to 14mm, in 1.25" size only. Tele Vue'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. These are medium focal length oculars.

There are some premium specialty oculars which feature extremely wide apparent fields of view, and long eye relief. Prominent among these are the Tele Vue Nagler and Meade Ultra Wide Angle (8-element) designs. Naglers have an astonishing apparent field of 82 degrees, the Meade Ultra Wides, 84 degrees! These 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" Tele Vue Nagler series). These are very expensive oculars to buy, but for the person who does a lot of high magnification viewing, it is probably worth it.

Other premium ocular designs incorporate special extra low dispersion glasses incorporating rare earth elements to improve performance. An example of these would be the Vixen Lanthanum-LV line of oculars, all of which have an eye relief of 20mm, regardless of focal length. This is achieved by incorporating a barlow lens into the design 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 oculars are available in focal lengths from 2.5mm (!) to 25mm.

Ocular recommendations

As we have seen, the focal length of your telescope, as well as the focal length of the ocular, together determine both the field of view and the magnification of that view. For reasons of time and space, it is impossible to give specific ocular recommendations for telescopes of every focal length. What I can do is give recommendations for four common telescope prime focal lengths with which I am personally familiar: 900mm (common for 4.5" reflectors and various 80mm refractors), 1000mm (the Celestron C90/G-3, C102, and G-8N, among others), 1200-1250mm (the Criterion 4000, Meade ETX-90, Questar 3.5, and various 6" reflectors), and 2000mm (the classic Celestron C8, Meade LX-10/50/200, and others). The magnification and field of view figures will be identical for all telescopes of the same focal length, using the same ocular, regardless of clear aperture. However, the power/inch and exit pupil figures given below apply only to the specific clear apertures mentioned. For simplicity, I have taken the liberty of rounding off the figures. Even if your telescope is not one of these four focal lengths, the following recommendations should give you some idea of how to choose oculars. All oculars referred to below are the common 1.25" barrel size.

For most purposes, four oculars will suffice for general viewing. One will usually be a short focal length/high power ocular, for detailed planetary and lunar viewing, as well as splitting some double stars, on nights of good seeing. The second would be a short-medium focal length/medium power ocular, for detailed views of deep space objects, splitting double stars, and planetary and lunar viewing on nights of less than optimum seeing. The third will be a long-medium focal length/medium power ocular, which is the most versatile ocular of all--and the focal length usually supplied with most new telescopes; this is also the ocular to choose for most terrestrial viewing. The last of the four will be a long focal length/low power ocular for wide sky views and aiming your telescope. If you are on a very tight budget, you can probably get by with three, eliminating one of the medium focal length oculars, but since these two are the most versatile oculars of all, you are paying a heavy price to save a modest amount of money. I will confine most of my basic recommendations to medium priced designs (Orthos and Plossls), in commonly available focal lengths, for economic reasons. Feel free to purchase more expensive designs with greater apparent fields and/or eye relief if you feel comfortable doing so. I have used apparent field and eye relief figures published by Tele Vue and Celestron.

The short focal length (high power) ocular

Unless you become a planetary specialist, this is the ocular you will use the least. Be careful not to exceed the 50 power-per-inch of clear aperture rule. The fifty power/inch of clear aperture rule implies that the maximum usable power for any telescope is determined by the light grasp of that telescope. Magnification beyond 50 power per inch is called "empty magnification" because it produces fuzzy, useless views. The telescope does not have enough light grasp to illuminate the dim field of view. The theoretical maximum is 105 power for a 2.4" (60mm) clear aperture telescope (department store telescopes this size are often advertised as "375x"!), 175 power for a 3.5" (90mm) aperture telescope, 225 power for a 4.5" (114mm) telescope, 300 power for a 6" (150mm) telescope, and 400 power for an 8" (200mm) clear aperture telescope. In the real world of less than ideal seeing, powers well below 50 power/inch are usually the practical maximum; I recommend something closer to 30 power/inch for a basic high power ocular.

For the 900mm focal length telescope, a 7mm Ortho ocular will give 129x at 29 power/inch, and a 7.5mm Plossl will give 120x at 27 power/inch (based on a 4.5" clear aperture). Beware: these oculars only have 5mm of eye relief, which I find to be uncomfortable. Shorter oculars, unless of exotic (read expensive) design, are so hard to see through that they are virtually unusable. If you must wear glasses while observing, this might be a good time to splurge for a 6mm (150X), or 9mm (100x) Vixen Lanthanum-LV ocular, with their generous 20mm eye relief.

For the 1000mm focal length telescope, a 10mm Plossl will give 100x magnification at 29 power/inch (3.5") or 13 power/inch (8"). This represents a very practical choice for a telescope with a 3.5" or larger clear aperture. The eye relief is 7mm. To save a few dollars, at the cost of 1mm of eye relief, try a 9mm Ortho (111x). If you have some extra money in your pocket, or if you must wear glasses, the long eye relief 6mm (167x), or 9mm Vixen Lanthanum-LV oculars might be good choices.

For the slightly longer 1200-1250mm focal length scope, a 7.5mm Plossl will give 160-167x and 27 power/inch in a 6" aperture telescope. In a 3.5" aperture scope, the power/inch figure goes up to 48, which is pushing it. A 10mm Plossl will give 120-125x magnification at 34-36 power/inch for a 3.5" clear aperture, and only 21 power/inch for a 6" clear aperture. This is probably a better choice for the smaller aperture telescope. For either, a 9mm Ortho (139x) will suffice, and the 9mm Lanthanum-LV remains a superior alternative choice.

In the longer 2000mm focal length telescope with an 8" clear aperture (f/10 focal ratio), a 7mm Ortho will give 287x at 36 power/inch; a 7.5mm Plossl will yield 267x at 33 power/inch. Remember, both of these oculars have only 5mm of eye relief. This would be another occasion where a longer eye relief ocular, like a Vixen Lanthanum-LV, might be a worthwhile investment. A 9mm Lanthanum-LV would give 222x at 28 power/inch (as would a 9mm Ortho, but with only 6mm of eye relief ).

The 1st (shorter) medium focal length ocular

For the shorter of the two medium focal length oculars, a Plossl design will usually work splendidly. These 15-30mm oculars are very versatile, and will get much use. Globular clusters, planetary nebula, and the like are common deep sky subjects for this class of ocular.

Our 900mm telescope will require a somewhat shorter focal length ocular than the longer focal length telescopes to perform the same duties. A 15mm Plossl will give 60x at only 13.3 power/inch for a telescope with a 4.5" clear aperture. The 15mm Plossl has an eye relief of 10mm, which is adequate for those who don't have to wear glasses while telescoping; there is a 15mm Lanthanum-LV for those who do.

The 1000mm medium focal length telescope will yield 59x at 17 power/inch (3.5" aperture) when used with a 17mm Plossl. This is a very fine combination for much deep sky viewing, as well as for nice lunar views, and some terrestrial viewing. The 17mm Plossl is a decent ocular to look through, at least for people who don't wear glasses, with 13mm of eye relief. The much brighter 8" aperture telescope of this focal length would probably be better served by a 15mm Plossl, which would give a magnification of 67x at only 8.3 power/inch. Again, for eyeglass wearers, substitute the long eye relief 15mm Lanthanum-LV.

The 1200-1250mm telescope gives 71-73x with a 17mm Plossl, at about 21 power/inch (3.5" aperture) or 12 power/inch (6" aperture). The 17mm Plossl is a fine ocular for a telescope of this focal length.

The 2000mm focal length, f/10 telescope also does well with the 17mm Plossl. In this case, the power is up to 118x, and the power/inch figure is 14.7.

The 2nd (longer) medium focal length ocular

This is the ocular most commonly supplied with a new telescope. For high quality telescopes with medium focal ratios (around f/10), it is very often a 26mm Plossl. This merely shows that the manufacturers know what they are doing, because this combination is probably the most versatile of all. It is useful for a variety of deep space and terrestrial viewing. For short focal length telescopes, of course, a shorter focal length ocular is required.

Our 900mm telescope will do well with a 20mm Plossl. This arrangement gives a magnification of 45x at 10 power/inch. The actual field of view is 1.1 degree, and the eye relief is 14mm. This makes a good replacement for the inexpensive 24-25mm ocular often supplied with these telescopes.

The 3.5", 1000mm telescope is typically supplied with a 24-28mm ocular. If your telescope came with some economy ocular, or none at all, I would recommend purchasing a 26mm Plossl. It has an eye relief of about 22mm, and gives such a telescope a magnification of 38x, at 11 power/inch. The actual field of view is 1.3 degrees. The 8", 1000mm telescope is usually supplied with a 20mm Plossl, which gives a magnification of 50x at 6.3 power/inch. The actual field of view of this fine combination is 1 degree.

The 1200-1250mm telescope usually comes with the same focal length ocular as the 1000mm scope. And my advice is also the same: a 26mm Plossl. In this case the magnification is 46-48x at 13-14 power/inch for a 3.5" clear aperture. For a 6" clear aperture, the power/inch figure is 7.8. The actual field of view is just over 1 degree.

The 2000mm, f/10 (8") telescope is also typically supplied with a 26mm Plossl. This ocular provides 77x magnification at 9.6 power/inch. It makes a fine all-around ocular, and you should buy one if you don't already have it. The actual field of view is .65 degree.

The long focal length (low power) ocular

This is the ocular to use when you are trying to locate objects in the night sky. It is also the best choice for wide sky views of open clusters and star fields. It gets a lot of use in my telescope.

Once again, the 9000mm focal length telescope requires a somewhat shorter ocular to provide views similar to its longer focal length brethren. In this case, a 32mm Plossl will provide excellent wide sky views. 28x at 6.25 power/inch is what you get with a 32mm Plossl in one of these scopes. The exit pupil is a bright 4mm, and the eye relief is 22mm. The actual field of view is approximately 1.8 degrees. Train this combination on the famous "double cluster" for a breathtaking view.

Our 1000mm telescope can also provide a wide sky view with a 32mm Plossl. The magnification is 31x. The power/inch runs 8.9 for a 3.5" aperture scope, and 3.9 for an 8" aperture scope. The actual field of view with a 32mm Plossl is 1.6 degrees. The exit pupil is 2.9mm for a 90mm scope, and 6.45mm for a 200mm scope. The latter is close to the theoretical 7mm usable maximum for young eyes. If you are a middle-aged person, you will probably not be able to benefit from that much light, so a 26mm Plossl, at 38x and 4.8 power/inch, with a 5.26mm exit pupil, will deliver all the light your eye can use. The actual field of view with a 26mm Plossl is 1.3 degrees.

The slightly longer 1200-1250mm telescope has a magnification of 30-31x with a 40mm Plossl. This means 8.6-8.9 power/inch with a 3.5" aperture, and a 2.9mm exit pupil. With a 6" aperture you are looking at about 5.1 power/inch, and a 4.9mm exit pupil. The actual field of view is just under 1.5 degrees. This ocular has very long eye relief of about 31mm, which is great for eyeglass wearers, and probably a little too long for everyone else. (I find that my eye is an uncomfortably long distance from the eyepiece.)

The 2000mm, f/10 telescope also does well with a 40mm Plossl. It does even better with a Rich Field Adapter (focal reducer), or 2" ocular of 50mm focal length or longer, but that is another story. For our purposes, the 1.25", 40mm Plossl gives a magnification of 50x, at 6.25 power/inch. The exit pupil is 4mm in diameter. In this case, the long focal length necessarily reduces the actual angle of view, which is down to .92 degree. Nevertheless, there are many beautiful subjects to be examined with this combination.