Eyepieces – Characteristics, Aberrations and Design
An eyepiece (or ocular) is a combination of small lenses mounted in a tube, used to view and magnify the image formed by a telescope or other optical instrument. Eyepieces come in many different designs, but most incorporate an eye lens that is closest to the eye and a field lens that is closest to the objective.
Contrary to many people’s expectations, the eyepiece does not produce an image. It just converts the conical beam of radiation from the objective into a parallel beam, and the eye then focuses this beam on to the retina to produce the image that we see when looking through the telescope.
Take care with the choice of eyepieces. At least three will be necessary: one giving low power (wide views of star clusters and nebulae), one moderate power (general views) and one high power (for more detailed views, particularly of the Moon and planets).
Each eyepiece has a unique set of characteristics that will determine its applicability to a particular task. Read on to discover what you need to know to assemble a perfect eyepiece set for your own telescope and your own needs.
The magnification provided by a telescope is calculated by dividing its focal length by the focal length of the eyepiece used with it. Thus, the magnification can be changed simply by changing the eyepiece; eyepieces come in standard sizes with a simple push-fit to facilitate this. Longer focal length eyepieces deliver lower magnifications than shorter focal length eyepieces do.
A 2,000 mm focal length telescope using a 20 mm focal length eyepiece will yield 2,000 / 20 = 100x. The eyepiece’s focal length is always engraved on the side, along with the type of eyepiece or a letter indicating this (e.g., KE 25 mm = Kellner 25 mm focal length). Common low-power eyepieces range from 40 mm to 20 mm focal length, medium power from 19 mm to 12 mm and high power from 11 mm to 4 mm.
Field of View
This is probably the most talked about aspect of eyepieces today. The field of view defines how much of the sky you see through your telescope, and there are two definitions to be mentioned: the apparent field of view and the true field of view.
The apparent field of view is how wide the eyepiece’s field of view appears when observing through the eyepiece (in other words, how large the “porthole” looks as you observe through the eyepiece). Since a single human eye’s vision is approximately 140° from left to right, the larger the eyepiece’s apparent field of view the more natural it appears compared to our normal unaided eye’s vision.
An eyepiece at the least expensive end of the scale would have an apparent field of view of 35° to 45°, a wide-angle eyepiece would have a field of view of 55° to 65°, while for an ultra-wide-angle eyepiece it might be up to 85°. Of course, the prices would correspondingly range from $ 20 to $ 60, through $ 80 to $ 300, and to $ 200 to $ 600.
In addition to how engaging the apparent field of view makes the view through the eyepiece, its size also affects the true field of view (the actual angular extent of the sky that the observer can see when using a particular eyepiece-telescope combination).
The true field of view is obtained by dividing the apparent field of the eyepiece with the magnification resulting from that eyepiece. When an eyepiece with a shorter focal length is used and the overall magnification increases, the true field of view will decrease.
The value of the true field can be found directly by timing the passage of a star across the centre of the field of view from one edge to the other, with the telescope drive switched off. The diameter of the true field, in arcseconds, is given by true field = 15t cosD. The parameter t is the time taken for the star to cross the eyepiece’s field measured in seconds, and D is the declination of the star measured in degrees. Ignore the negative sign of southern declinations and repeat several times to get an average.
The eye relief, another important eyepiece specification, especially for those who wear glasses, is the distance the observer’s eye can be from the ocular’s eye lens while observing. Eye relief typically ranges from about 2 mm to 20 mm, depending on the construction of the eyepiece.
Eyeglass wearers generally need at least 15 mm of eye relief to find an eyepiece usable at all, while those without can get by with much less (eye relief as low as 10 mm can be perfectly comfortable). Note that it is often not necessary to wear eyeglasses when observing, since the eyepiece can simply be refocused to accommodate the needs of an observer’s vision. Where an observer will need to wear eyeglasses is if the eyeglass prescription corrects any astigmatism.
Plössls, Kellners and orthoscopic eyepieces typically have short eye relief below a focal length of around 12-15 mm. Eyepieces with good eye relief tend to have smaller fields of view, and vice versa. In general, more expensive eyepieces made from glass that incorporates the rare-earth element lanthanum offer a fixed eye relief regardless of the focal length. The Pentax XW and Tele Vue Delos are two such lines, where the eye relief is a generous 20 mm, no matter the focal length of the eyepiece.
Exit Pupil (telescope-dependent)
This is a measurement of the fully illuminated circle of light that comes from the eyepiece attached to a telescope. To calculate the exit pupil in millimeters, divide the diameter of the telescope’s objective (in millimeters) by the magnification of the view. For example, if a telescope has a 150 mm (6 inch) objective and a magnification of 50x, the resulting exit pupil is 3 mm. The exit pupil varies each time you change eyepieces.
It is best to use an exit pupil that does not exceed the size of your fully dilated pupil. Most observers under the age of 30 have a maximum pupil dilation of about 7 mm, after 30 minutes of total dark adaptation. For older observers, maximum 5 mm dilation is common.
Eyepieces come mounted in several different-sized barrel diameters. Standard sizes are 0.965 inches (24 mm), 1.25 inches (31 mm) and 2 inches (51 mm), with 1.25 inches by far the commonest. For some reason, eyepiece barrel sizes are always denominated in inches, even outside the U.S.
The sub-diameter size 0.965-inch eyepiece is most often found on inexpensive telescopes, in particular from those imported from Japan and China. These eyepieces are typically orthoscopic, with limited fields of view (about 30°) and poor eye relief. Although Plössl eyepieces are mostly used with the 1.25-inch barrel size, some manufacturers supply them for 0.965-inch barrels as well.
Most eyepiece types are available in the American standard size of 1.25-inch diameter. The larger barrel provides room for multi-element lenses, which offer excellent optical quality, roomy fields of view and comfortable eye relief.
Finally, the jumbo size 2-inch barrel is used with large Dobsonian reflectors and quality apochromatic refractors. These oculars provide ultra-wide fields of view and accordingly, with few exceptions, are available only in longer focal lengths. A telescope with a 2-inch focuser accepts 2-inch eyepieces, and, with an adapter, 1.25-inch models.
Different eyepiece designs come to focus at different points, and because of this, you may have to rack the focuser in or out a significant distance when changing eyepieces. What you want are parfocal eyepieces, which you can simply swap in and out without having to refocus.
Most eyepiece series are parfocal within the series. Parfocal eyepieces normally have to be bought as a set from a single manufacturer and will be more expensive than equivalent non-parfocal eyepieces bought separately. However, the extra expense is usually worthwhile because of the increased ease of use of the telescope.
To achieve the same thing, but much more cheaply, you can adjust your eyepieces to be parfocal by adding parfocalizing rings. These are available from various manufacturers, or you can build your own. To parfocalize your eyepiece collection, you first determine which of them requires the most in focus. With that eyepiece focused sharply on a medium-brightness star, you lock the focuser.
You then add a parfocalizing ring to each of your other eyepieces and physically slide them in and out in the focuser until they reach focus. At that point, you slide the parfocalizing ring down to the top of the focuser and lock the set screw (or set screws).
Physical Size and Weight
The physical size and weight of an eyepiece can also be important considerations, as these two characteristics affect many aspects of usability.
Physical size becomes an issue when you plan to use an eyepiece on a small telescope with little physical clearance or if you plan to install two identical eyepieces in a binoviewer. For example, the body diameter of many ultra-wide eyepieces is so large that even if you adjust the binoviewer so that the eyepieces touch, you will not be able to look through both of them simultaneously.
Weight may be an issue with Dobsonian telescopes, where changing from a lightweight eyepiece to a very heavy one may cause balance problems. A solution to this is purchasing weight rings that slide onto the eyepiece’s barrel and lock with a set screw, allowing you to increase the weight of your lighter eyepieces.
Good-quality eyepieces are mechanically sound and obviously well built, and the construction of the housing and the barrel provides the first impression. For housings made of solid materials that are of uniform color throughout, such as Delrin, and wear over time will simply show as a depression. If the housing is made of a coated metal, however, time will wear these surface coatings away, and the bare metal will eventually show.
Modern eyepiece barrels are constructed from polished or anodized aluminium or base metals. Barrels made of chrome or nickel-coated brass are typical of older eyepieces. No matter the material used, after many years of constant insertion into the focuser the barrel will wear, revealing the color of the underlying metal. If this is an issue for you, choose an eyepiece with a solid metal barrel that is uncoated and only polished.
Check the inside of the barrel for a well-darkened and even appearance. Eyepieces with poor blackening inside the barrel usually result in ghost images around bright objects. A nice additional touch is a set screw that ensures your eyepiece stays in place, and some sort of eye guard to block external light from entering your eye.
Before purchasing an eyepiece, make sure that the barrel is threaded for filters on the field lens end (some oculars are also threaded for filters on the eye lens end). Filter threads are generally standardized for 1.25-inch and 2-inch eyepieces, but there are exceptions. Meade eyepieces and filters, for example, use a thread that is not standard.
Depending on the design of the eyepiece, the image quality will become poor at some angular distance from the optical axis. This is why manufacturers introduce a field stop, a simple ring inside the barrel, which defines the edge of the field of view. (However, even with top-notch eyepieces, the image quality will still tend to be poorer at the edge than at the centre.)
The field stop gives a sharp edge to the field of view and, when the eye is suitably placed to observe through the eyepiece, this should be visible. If the field stop is badly designed, the edge of the field of view appears “mushy” and this detracts from the viewing pleasure.
Untreated lenses reflect up to 10-percent of the light passing through them. Good-quality eyepieces are “multicoated”, indicating that at least one lens surface was treated with multiple layers of anti-reflective coatings. Better-quality eyepieces are “fully multicoated” – an indication that all air-to-glass surfaces are multicoated.
Magnesium fluoride (MgF2) is probably the most widely used thin-film material for optical coatings. Coated lenses allow more light to pass through the eyepiece, so giving somewhat brighter images. More importantly, they will reduce the amount of light that is scattered within the eyepiece so that the contrast will be higher.
You can easily check if an eyepiece is adequately coated – if you hold it under a light, you will see blue or purple reflections (and not white) on the lenses.
Performance in Fast Telescopes
The focal ratio (f) of a telescope is a measure of the “speed” of the instrument to capture images of celestial objects in terms of exposure times on photographic plates or on a CCD chip. A “fast” telescope is one with a small focal ratio of around f5 or less, whilst a “slow” telescope has a focal ratio of f10 or higher.
In “fast” telescopes, the light cone from the objective converges over a shorter distance than in “slow” instruments. Individual light rays arrive from significantly different angles, which is difficult for any eyepiece to handle without showing aberrations (especially near the edge of the field of view).
In an f5 or faster telescope, only premium eyepieces of modern design are capable of providing a good image across a wide apparent field of view. Inexpensive wide-field eyepieces will invariably provide very poor performance near the edge of the field.
There are several types of zoom eyepieces available, usually with focal lengths in the range of 7 to 21 mm or 8 to 24 mm. They remove the necessity of changing eyepieces of various focal lengths in order to vary the magnification.
Zoom eyepieces have been around for many years, but they have not achieved widespread popularity among amateur astronomers. (Perhaps because zooming is perceived to be associated with many budget binoculars and telescopes.) The other problem is that adjusting the magnification usually requires a small adjustment to the focus as well.
The apparent field of a zoom eyepiece narrows from a generous 60° at 8 mm focal length to a somewhat small 40° at 24 mm, but if you can accept this restriction, a single zoom eyepiece can replace a drawer full of regular Plössl eyepieces. Zoom eyepieces also go well with the binocular viewer – a device that splits the single beam of light from the telescope’s objective and diverts it into two identical eyepieces.
Possible Aberrations and Distortions in Eyepieces
Because eyepieces can suffer from the same aberrations and distortions as telescopes, in this chapter you will find some of their more common problems. Each eyepiece design corrects for optical aberrations, but the degree of success will vary with the focal ratio of the telescope.
Just as an observer’s eyes can be astigmatic, so can eyepieces. Severe astigmatism manifests as oddly shaped stars, even when in sharp focus. Rather than small points of light, stars appear as ovals or crosses. Most eyepieces suffer from some degree of astigmatism, but the problem typically appears when an eyepiece is used in a telescope of a shorter focal ratio than the eyepiece design allows.
Axial Chromatic Aberration
Chromatic aberration is nearly extinct in modern eyepieces thanks to the use of achromatic lenses. It causes in-focus stars to separate into a small spectrum of colors around the fringe of the star point. The color fringing from this aberration occurs throughout the field of view; if the center of the view is color-free, then this is not an axial chromatic aberration but is instead lateral transverse chromatic aberration.
Lateral or Transverse Chromatic Aberration
Commonly referred to as lateral color, it is mostly seen in the off-axis region of the eyepiece’s field as a red, green, blue or yellow fringe on the limb of the Moon or bright planets. Most eyepieces will only show minor amounts of lateral color, and in most cases, it is caused by observing an object that is too low in the sky.
In all but the cheapest oculars, this aberration is rarely due to the optics of the eyepiece. It is instead caused by the misalignment of the optics of the telescope, or due to a particular design of the telescope. Coma makes a star appear as a small comet shape, with the tail always pointing directly away from the center of the field of view.
If star images near the field’s center are in focus when those near the edge are not, and vice versa, then the eyepiece suffers from field curvature. This is a familiar effect for owners of Schmidt–Cassegrain telescopes, because the focal plane of such telescopes is not flat but strongly curved. More expensive and complex eyepieces such as the Naglers tend to be better corrected for this aberration than cheap, simple ones.
Examine the intra-focal and extra-focal image of a bright star. If the rings are noticeably easier to see on one side of focus compared with the other, then you are probably picking up some spherical aberration. Like most other aberrations, spherical aberration can also come from the telescope’s objective and is not solely a characteristic of the eyepiece.
Angular Magnification Distortion
This is a common flaw found in lesser eyepieces, which is readily detectable when viewing either terrestrial sights or large, bright celestial objects such as the Moon. This distortion is usually characterized by a warping of the scene in a way that is similar to the effect seen through a fish-eye camera lens. It occurs when the eyepiece’s focal length is not constant across the field but varies slightly.
Radial or Rectilinear Distortion
With rectilinear distortion, the magnification increases toward the edge of the field and this causes straight lines to appear curved outward, even when the image is stationary. Just as the angular magnification distortion discussed above, rectilinear distortion is usually distracting only during daylight tests. It is not apparent during observations conducted at night.
Timeline of Eyepiece Design
There are a wide variety of eyepiece designs, or optical formulas, differing in complexity and hence image quality. Early eyepiece models are named for their original designer, famous opticians with last names such as Huygens, Ramsden, Kellner, Plössl or König. Models that are more recent are named after corporations…
Lippershey/Galilean (invented 1608)
This was of course the very first eyepiece, used extensively by Galileo. It consisted of a single divergent (plano-concave) lens, which provided an enlarged, upright image. The design is still in use today, in very cheap binoculars and opera glasses.
Kepler (invented 1611)
Within a few years, the Kepler eyepiece was invented; again only a single lens but having a double-convex design. It gave a wider, albeit inverted view but still suffered from aberrations galore.
Huygens (invented 1662)
This is a basic eyepiece consisting of two simple plano-convex elements, commonly used on small refractors and microscopes. The eye relief is good, the angular field of view is narrow and the eyepiece is relatively free from chromatic aberration (only when used with telescopes of f10 or longer focal ratios).
Huygens eyepieces have the letter “H” on the barrel; for instance, an H25mm indicates a Huygens eyepiece with a 25 mm focal length. If you own such an eyepiece, it is probably a good idea to replace it with a modern design.
Ramsden (invented 1782)
Another basic telescope eyepiece consisting of two simple elements, with equal focal lengths. While it has a decent field of view, the Ramsden eyepiece suffers from chromatic aberration and poor eye relief, so the Kellner eyepiece is usually preferred. The Ramsden design is named after the English optician Jesse Ramsden.
Kellner (invented 1849)
It is essentially a Ramsden eyepiece with one element replaced by an achromatic doublet. Like the Ramsden, the Kellner is prone to ghosting, caused by internal reflections within the eyepiece.
Kellners are the lowest priced eyepieces of any real usefulness to amateur astronomers and work well with slow telescopes such as SCTs, where they can deliver sharp images (at least in the center of the field of view).
Plössl (invented 1860)
Named after the Austrian optician Simon Plössl (1794–1868), this design features two close-set pairs of doublets for the eye lens and the field lens. Plössl eyepieces have an apparent field of view of about 52° and are available in different qualities, from budget to deluxe. They are suitable for both planetary and deep sky viewing and are available in focal lengths from about 5 mm to 40 mm.
Orthoscopic (invented 1880)
Designed to give good definition, little geometric distortion and comfortable eye relief, the orthoscopic eyepiece consists of two elements – a single eye lens that is normally plano-convex, and a cemented triplet that is usually symmetrical.
This arrangement produces an apparent field of view of 40º to 50º, which is smaller than other popular designs such as the Plössl. Planetary observers looking for good performance at high magnification sometimes prefer orthoscopic eyepieces.
Monocentric (invented 1883)
Invented by Hugo Adolf Steinheil around 1883, this solid eyepiece design (now largely obsolete) consists of three elements cemented together, and therefore completely free from ghosting. The narrow field of view of around 25° restricts the usefulness of the monocentric eyepiece to only planetary observing.
König (invented 1915)
The König eyepiece is a three-element concave-convex doublet and a convex-flat positive singlet. It allows for high magnification with excellent eye relief, and modern improvements typically have fields of view of 60° to 70°.
Erfle (invented 1921)
The Erfle, granddaddy of all wide-field eyepieces, was originally developed for military applications. The design consists of three lenses, at least one of which is a doublet. It provides an apparent field of view of over 60º, but at high powers, the eyepiece suffers from astigmatism and ghost images.
Kaspereit (invented 1923)
This modification of the Erfle adds a sixth lens, to create a design of three corrected doublets. The Kaspereit expands the apparent field of view to 68° and provides an eye relief of about 10 mm.
Brandon (invented 1949)
A variant of the König eyepiece, this design was widely used during World War II in U.S. Army optics. The Brandon design differs from the König by using three high index glass types and four different lens radii. The Brandon offers crisp images and overall performance is comparable to the Plössl on deep sky objects, but superior on the planets.
RKE (released 1977)
The RKE eyepiece, designed and marketed by Edmund Scientific Co., has an achromatic field lens and double convex eye lens, a reversed adaptation of the Kellner eyepiece. Off-axis aberrations (lateral color and field curvature) are very well corrected, and the field width is slightly larger than that of the classic Kellner eyepiece.
Tele Vue Nagler (released 1979)
Considered by many to be the Rolls-Royce of eyepieces, the Nagler has an apparent field of view of 80° or more, making it suitable for activities such as searching for comets or novae. The eyepiece offers excellent eye relief, and is relatively free from coma and field curvature.
It comes in six subtypes, all consisting of a negative doublet field lens, which increases magnification, followed by several positive groups. These combine to have long focal length, and form a positive lens. The Nagler’s weight of 0.5 kg (1.1 lb) and bulky size are probably the only disadvantages.
Takahashi LE (released 1980)
Takahashi LE eyepieces are another company-proprietary design, consisting of five optical elements in three groups (a modified Erfle). The LE line includes four short focal-length ED models, as well as five non-ED oculars. LE-ED eyepieces range from 2.8 mm to 7.5 mm, while non-ED focal lengths span 12.5 mm to 30 mm, all parfocal with 1.25-inch barrels. There is only one eyepiece with a 2-inch barrel, which provides 25 mm of eye relief and has a focal length of 50 mm.
Tele Vue Wide Field (released 1982)
Precursors to the Panoptic line, these eyepieces are a modified Erfle design with much better edge performance. The various focal lengths of the Tele Vue Wide Field were introduced from 1982 through 1984: 15 mm, 19 mm, 24 mm, 32 mm, and 40 mm.
Tele Vue Panoptic (released 1992)
The Panoptics are a 6-element hybrid, which features apparent fields slightly narrower than those of the Naglers, about 68° versus the Naglers’ 80° range. They come in seven focal lengths, with 1.25-inch barrels for the 5 mm, 19 mm, and 24 mm versions. The 22 mm may be used in either 1.25-inch or 2-inch focusers, while the 27 mm, 35 mm, and 41 mm fit 2-inch focusers only.
Pentax XL/XW (released 1996/2003)
These are premium eyepieces, with seven lenses in five groups. All Pentax XL eyepieces have a 65° apparent field of view, except de 28 mm, which has only a 55° field. The line was discontinued in 2004, but remains widely used and available. The 2-inch 40 mm Pentax XL typically sells for $ 220 used, if in good condition.
The Pentax XWs are available in eight focal lengths from 3.5 mm to 40 mm. Each has a 70° apparent field of view and 20 mm of eye relief. Images are sharp right out to the edge of the field, even in fast instruments, and they are easily the match of orthoscopics on the Moon and planets.
Meade 4000 SWA/UWA (released 1997/2003)
At about 70-percent the price of the Pentax XL line, in 1997 Meade introduced its super-wide-angle (SWA) eyepieces. Available in six focal lengths ranging from 16 mm to 40 mm, these 6-element eyepieces with 67° fields offer exquisite wide-angle images at low to medium powers.
The ultra-wide-angle (UWA) line is an 8-element design, with an 84° apparent field. There is little off-axis color visible and good edge correction through all. As a word of caution, keep in mind that Meade uses a different filter thread than any other manufacturer. As a result, non-Meade filters do not fit on Meade eyepieces.
Williams Optics UWAN (released 2006)
A 7-element hybrid with an 82° apparent field, but only 12 mm of eye relief. The line comes in three focal lengths (4 mm, 7 mm and 16 mm), all with 1.25-inch barrels. For a much lesser price, the UWANs offer a real alternative to high-end designs from other manufacturers.
Tele Vue Ethos (released 2007)
Like most Tele Vue products, they are rather pricey, but you do get what you pay for. The Ethos sports an apparent field of view of 100°, more than 50-percent larger than an 82°. Focal lengths range from 3.7 mm to 21 mm and the eye relief is 15 mm. More recently, the company has produced the Ethos SX eyepieces of 3.7 mm and 4.7 mm focal length with an apparent field of 110°.
Takahashi-UW (released 2011)
This design has ten lenses in six groups and produces an apparent field of 90°. The line includes four eyepieces with focal lengths from 3.3 mm to 10 mm. Eye relief is somewhat small, only 12 mm (for an eyeglass wearer, anything less than 15 mm means not being able to see the whole field).
Tele Vue Delos (released 2011)
Tele Vue’s new eyepiece range, called Delos, has focal lengths from 3.5 mm to 14 mm. The apparent field of view is 72° (much smaller that that of the Ethos), but eye relief is a generous 20 mm. These eyepieces are very comfortable to use and perfect for star parties, where inexperienced observers may find it difficult to see the image through an eyepiece with small eye relief.
Explore Scientific 120° (released 2012)
The 9 mm focal length Explore Scientific has a hyper-wide apparent field of view, 120° across, the largest available in any commercial eyepiece. It consists of 12 optical elements arranged in eight groups, with 15 layer coatings on all glass-to-air surfaces.
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