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    Choosing and Using Astronomical Eyepieces
    Choosing and Using Astronomical Eyepieces
    Choosing and Using Astronomical Eyepieces
    Ebook686 pages5 hours

    Choosing and Using Astronomical Eyepieces

    By William Paolini

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    About this ebook

    A valuable reference that fills a number of niches including that of a buyer's guide, technical desk reference and observer's field guide. It documents the past market and its evolution, right up to the present day. In addition to appealing to practical astronomers - and potentially saving them money - it is useful both as a historical reference and as a detailed review of the current market place for this bustling astronomical consumer product.

    What distinguishes this book from other publications on astronomy is the involvement of observers from all aspects of the astronomical community, and also the major manufacturers of equipment. It not only catalogs the technical aspects of the many modern eyepieces but also documents amateur observer reactions and impressions over the years, using many different eyepieces.

    Eyepieces are the most talked-about accessories and collectible items available to the amateur astronomer. No other item of equipment commands such vigorous debate, or has evolved into such a remarkable array of forms and functions. 'Choosing and Using Astronomical Eyepieces' provides a vast amount of reference material to point readers towards the best buys and the right eyepieces for different kinds of observing.
    LanguageEnglish
    PublisherSpringer
    Release dateAug 23, 2013
    ISBN9781461477235
    Choosing and Using Astronomical Eyepieces

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      Book preview

      Choosing and Using Astronomical Eyepieces - William Paolini

      Part 1

      Background

      William PaoliniThe Patrick Moore Practical Astronomy SeriesChoosing and Using Astronomical Eyepieces201310.1007/978-1-4614-7723-5_1© Springer Science+Business Media New York 2013

      1. Introducing the Astronomical Eyepiece

      William Paolini¹ 

      (1)

      Vienna, VA, USA

      Abstract

      This chapter will introduce you to the astronomical eyepiece—its historical beginnings, basic function, physical construction, optical construction, and optical design characteristics. You will also gain a deeper understanding of some of the more technical aspects of eyepiece performance parameters related to focal length, apparent field of view, eye relief, exit pupil, stray light control, and optical aberrations.

      This chapter will introduce you to the astronomical eyepiece—its historical beginnings, basic function, physical construction, optical construction, and optical design characteristics. You will also gain a deeper understanding of some of the more technical aspects of eyepiece performance parameters related to focal length, apparent field of view, eye relief, exit pupil, stray light control, and optical aberrations.

      Historical Beginnings

      In the early 1980s amateur astronomers were treated to a new kind of eyepiece that would eventually revolutionize the way observers think about both eyepieces and observing. This bold new design was the Tele Vue Optics Nagler. For the first time amateur astronomers had access to an eyepiece where the view was so wide, and so well corrected, that the term spacewalk was coined for the views it provided. Since then, the amateur astronomer community’s attention on the eyepiece effectively skyrocketed, and the eyepiece remains today one of the most actively discussed topics among observers.

      As we examine many of today’s truly exciting marvels of eyepiece technology, it is easy to overlook the eyepiece’s very humble beginnings. When was the very first eyepiece conceived, and who was the first to use one? How many lenses did the first eyepieces use and how sharp were the views they provided? If we take a journey through time, and examine the archeological discoveries related to optics, we find many lens-like objects that existed as far back as 2500 b.c., more than 4,000 years before the first documented telescope!

      You can begin your journey with a visit to the Louvre Museum in Paris or the Egyptian Museum in Cairo and see these ancient optic-like artifacts as polished convex crystal lenses used for the eyes in Egyptian statues. Move forward to the period from 500 b.c. to 700 b.c. and you will find what is today called the Nimrud lens. This artifact appears to be an actual plano-convex lens discovered in an area of the world that was ancient Assyria. Then you can find other clues from history that may have, or did have, optical applications:

      A polished magnifying crystal found on Mt. Ida in Crete that can magnify up to 7× clearly and up to 20× with distortions from 500 b.c.

      Egyptian hieroglyphs depicting the use of glass lenses in 500 b.c.

      The Roman philosophers Seneca and Pliny who wrote about magnifiers and burning glasses during the first century a.d.

      The Persian scientist Ibn al-Haytham publishing a seven-volume book dedicated to optics called Kitāb al-Manāẓir (The Book of Optics) between a.d. 1011 and 1021.

      The English friar Roger Bacon writing of the magnification properties of lenses and their possible use as corrective lenses in his Opus Majus in 1262.

      Venetian glass makers producing disks for the eyes in the 1400s.

      Nicholas of Cusa from Germany using concave lenses to correct near-sightedness in 1451.

      As can be seen, the actual history of making and grinding optics, including the convex and concave lenses that are the basis of the telescope, dates back much further than the earliest recorded use of telescopes in the time of Galileo. However, although all these discoveries of optic-like lenses and crystals, burning glasses, and magnifying lenses may lead the imagination to the possibilities that telescope-like devices could have existed long before we believe, none of them actually points firmly to use of a combination of optics to produce an actual refractive telescopic system with both objective and eyepiece.

      As intriguing as all these historical clues may be to imagining someone stumbling upon a secret ancient telescope, it was not until the early 1600s that we have the first confirmed use of optics for a telescope, invented by a Dutch-German optician named Hans Lippershey. Once this new invention was revealed, the speed at which copies of this device made it to all parts of the globe, enthralling culture after culture with its almost magical capabilities, is testimony that this was most likely the real genesis of both the eyepiece and the telescope.

      This very first telescope used a singlet convex lens with a focal length of approximately 600 mm for an objective, and a singlet concave lens of approximately 200 mm focal length for an eyepiece. Together they produced a telescope with only 3× magnification. Then, in 1608, having perfected his invention, Lippershey applied for a patent calling it the Dutch Perspective Glass.

      History does not record if Hans Lippershey ever turned his invention towards the heavens, but we do know that another individual did just that when, in 1609, this exciting new technology fell into the hands of the Italian physicist Galileo Galilei. Galileo enthusiastically embraced the new technology, carefully scrutinizing its operation, and then worked feverishly to improve the design and exploit its power for discovery. His efforts resulted in a telescope with a 37 mm diameter plano-convex objective that had an extended focal length of 980 mm, and the use of a 22 mm diameter plano-concave eyepiece with a shorter 50 mm focal length. Together these provided a six-fold boost in performance over the original telescope design, increasing its magnification from 3× to almost 20×.

      From this point forward, the eyepiece and telescope revealed the universe as never seen before, and over the next four centuries the eyepiece has blossomed from a single lens mounted in a paper or parchment tube to our current modern marvels of technology using highly complex multi-element designs, exotic rare earth glasses, coatings that have their layers deposited atom by atom, and in some cases even the incorporation of image-intensifying electronics. Compared to its humble beginnings, the eyepieces of today truly bear little resemblance to the simple single-element designs that dominated the first half century of its existence.

      Moving from Galileo with his first telescopic discoveries using the simple Lippershey/Galilean plano-convex lens eyepiece through the golden era of astronomical discovery (1600s–1800s) we find that the many discoveries of the period were all made with the most simple of eyepieces. The Lippershey/Galilean was of course the very first eyepiece and used extensively by Galileo. Within a few years, the Kepler eyepiece was invented; again only a singlet lens but having a double-convex design.

      Then, in 1671, a major advancement was made with the development of the Huygen eyepiece. This design used two optical elements and provided a much improved color-corrected field of view. It was this eyepiece design that remained in use throughout all of the major discoveries made by the great astronomers of the classical period of astronomical discovery. The next advancement in eyepiece design, the two-element Ramsden, was invented near the end of this classical era of visual astronomy and therefore could only have participated in discoveries near the end of the period.

      Today, many amateur astronomers sometimes refer to the time-honored designs such as the Huygen or the Ramsden as junk eyepieces. This could not be further from the truth, as these eyepiece designs, especially the Huygen, were what was used for a vast majority of the major visual discoveries in astronomy, a distinction none of the modern designs can claim. The following is an accounting of prominent astronomers who made major visual discoveries during the times when the Lippershey/Galilean, Kepler, and Huygen designs were the most technically advanced available:

      Thomas Harriot, an English astronomer and mathematician, using a telescope in 1609 makes the first drawing of the Moon.

      Galileo, starting in 1610, discovers topographical features on the Moon, that Venus appears in phases like the Moon, that the Milky Way is composed of individual stars instead of being a nebula, moons around Jupiter, markings on both Mars and Jupiter, that the Sun rotates, and unusual bodies close to Saturn (e.g., its rings).

      Giovanni Cassini, an Italian astronomer who, in 1665, discovered the oblate aspect of Jupiter and in 1675 discovered the division in Saturn’s ring system that now bares his name.

      Christiaan Huygens, a Dutch astronomer who, in 1671, discovered the first of Saturn’s moons and also invented the first compound eyepiece, the two-element Huygen eyepiece that greatly improved color correction compared to previous designs.

      Edmund Halley, an English astronomer who, in 1705, accurately predicted the comet of 1682 that now bears his name would return in 76 years.

      Charles Messier, a French astronomer who, in 1774, published a catalog of over 100 deep sky objects.

      William Herschel, an English astronomer who discovered the planet Uranus in 1781, then in 1781–1821 published catalogs of over 800 binary star systems and over 2,400 deep sky objects.

      Johann Gottfried Galle, a German astronomer who, in 1846, made the first visual confirmation of the existence of the planet Neptune (Galileo noted it as a star).

      Asaph Hall, an American astronomer who, in 1877, discovered the two moons of Mars.

      As can be seen from the list above, even as late as 1877 when Asaph Hall discovered the moons of Mars using the 26″ Clark refractor currently operating at the U. S. Naval Observatory in Washington, D.C., the Huygen design eyepiece was the standard. So when today’s amateur astronomers use their modern ultra-high technology eyepieces to view the many celestial objects from such famous lists as the Messier catalog or the Hershel catalog, they need to realize that all these wondrous celestial objects were discovered and cataloged for future generations using eyepieces no more advanced than that of the humble two-element Huygen.

      A270483_1_En_1_Fig1_HTML.jpg

      Fig. 1.1

      Huygen eyepiece from the U. S. Naval Observatory’s 26 in. Clark refractor (the telescope used by Asaph Hall in 1877 to discover the moons of Mars). Inset of 1.25″ Meade 7 mm RG Ortho is for reference and to scale (Clark eyepiece from the U. S. Naval Observatory collection. Image by the author)

      Basic Function

      At its heart, just what is an astronomical eyepiece? One can think of an eyepiece as really nothing more than a specialized magnifying glass. The main objective of any telescope, whether it be the primary mirror of a Newtonian or the large glass lens at the front of a refractor, focuses the image the telescope produces at what is called the focal plane of the telescope. Then, when you insert the eyepiece into the focuser of a telescope, the eyepiece’s job is to magnify this image produced by the telescope at that focal plane. At its simplest, the eyepiece is merely used as a specialized magnifying glass to observe the image produced by the telescope at the main objective’s focal plane. So the eyepiece’s function is not to improve the image a telescope can produce in any way, but instead its function is to magnify the telescope’s image as best it can with the least amount of aberration and distortion possible.

      A270483_1_En_1_Fig2_HTML.gif

      Fig. 1.2

      The eyepiece’s relation to the telescope’s main objective (Illustration by the author)

      Beyond the simple, however, the astronomical eyepiece is in reality very much more. For the visual astronomer, the astronomical eyepiece is nothing less significant than the user interface of the telescopic system. The eyepiece is therefore the most critical component for the visual observer because it enables the observer to connect to the celestial objects the telescope reveals in a thoroughly personal and engaging way. As such, its importance can’t be understated, bringing an array of functions and capabilities to the telescope that no other part of the system can accomplish. As testimony to this, all one has to do is to re-visit the 1980s when that new small company called Tele Vue Optics introduced the first high-quality mass-produced ultra-wide field eyepiece. This one offering transformed the telescope experience from a mostly porthole view of the universe into something much more exciting. As with the Dobsonian revolution that brought large aperture (aperture is the diameter of the main lens or mirror of the telescope) telescopes within reach of the amateur, with the advent of the Nagler technological breakthrough the amateur astronomer community similarly exploded with an enthusiasm that more than a quarter of a century later feverishly continues.

      Physical Construction

      With our perspective on the history and basic function of the eyepiece, it’s time to examine the major components of the eyepiece’s construction. Examining any modern eyepiece one can see that it is made of several distinctly different sections. Each of these sections is important to the eyepiece and serves a distinct function for the eyepiece. The major components of any eyepiece are:

      The housing or mount

      The barrel

      The shoulder

      The optics

      The field stop

      The eyeguard (optional)

      A270483_1_En_1_Fig3_HTML.jpg

      Fig. 1.3

      Major components of an eyepiece (Illustration by the author)

      Of the major components, the two most obvious parts are the housing (sometimes called the mount) and the barrel (or the top and bottom of the eyepiece). The housing is typically composed of metal, Delrin, or some other convenient hard polymer/plastic material. The brand name and focal length of the eyepiece are typically imprinted or engraved into the housing, and sometimes the coatings used are indicated on the housing as well (e.g., fully coated, multi-coated, or fully multi-coated). On the outside of the housing, some eyepiece manufacturers place rubberized panels or engrave a diamond pattern into the surface to allow secure gripping of the eyepiece. Also attached to the eyepiece housing, or integrated as part of the housing, is some type of eyeguard. The function of the eyeguard is to shield the observer’s eye from stray light that may be in the distance around the observer. Eyeguards are typically a rigid or foldable rubber shield, and in some cases they can be a mechanical feature that is raised or lowered as the observer needs.

      The second most obvious part of the eyepiece, the barrel, is typically chromed or nickel-plated brass, polished or anodized aluminum, or some other base metal. In some rare instances the barrel is stainless steel or, in the case of vintage eyepieces, an uncoated brass. Older eyepieces typically have a barrel that is entirely smooth, whereas on more modern eyepieces manufacturers alter them with a feature called an undercut or taper. This safety feature is designed so the focuser can catch the eyepiece should it not be tightened securely in place or accidentally positioned where it could fall out of the focuser.

      The edges of the inset can be sharp (called a full undercut) or have a gentle bevel (called a beveled or tapered undercut). The purpose of the beveled design is to reduce the likelihood of the undercut getting stuck on the focuser’s set screw or compression ring when inserting or removing the eyepiece from the telescope. A further improvement to this safety design is the tapered barrel, where there is no machined section but instead the entire barrel gently angles inward so that the part of the barrel closest to the housing is smaller in diameter than the bottom of the barrel. Amateur astronomers generally have mixed feelings about this small feature and can sometimes have very passionate opinions about it. The lines are generally divided between those liking the feature and others strongly disliking it, as it often makes the eyepiece difficult to remove from the focuser.

      A270483_1_En_1_Fig4_HTML.jpg

      Fig. 1.4

      Barrel features left to right: smooth, undercut, beveled undercut, tapered (Image by the author)

      Inside the eyepiece housing (and sometimes in the barrel) are the optics of the eyepiece. These optics, sometimes referred to as the optical assembly, are a combination of lenses of different sizes with surfaces of different curvatures that are grouped and spaced to the specific design of the optician. The individual lens types that any eyepiece uses have a generic name based on the direction of their surface curves and overall shape. The illustration below shows the basic lens types. In the vast majority of all eyepieces, the curves on the lens surfaces are spherical, with their shape being the radius of the circle that their curve defines. Although there is sometimes mention of non-spherical shapes for lens surfaces (called aspherical), this is very rare and usually adds cost to the design due to the complexity in grinding a lens without a uniform shape. (Note that the Leica Vario ASPH Zoom and the TMB Aspheric Ortho are two examples of eyepieces using at least one lens with an aspheric surface.)

      A270483_1_En_1_Fig5_HTML.gif

      Fig. 1.5

      Typical lens types in an eyepiece’s optical assembly (Illustration by the author)

      Within the eyepiece’s optical assembly, the lens closest to the eye is commonly referred to as the eye lens, and the lens furthest from the eye is referred to as the field lens (see illustration below). Also inside the eyepiece, either in the housing or in the barrel, depending on the optical design, is a fixed circular opening or diaphragm that is located at the eyepiece’s focal plane. This is called the field stop of the eyepiece. Ideally, the field stop is located at or very near the shoulder of the eyepiece. The shoulder is where the barrel meets the housing of the eyepiece. This shoulder is also where the eyepiece comes to rest when it is inserted into the focuser of the telescope.

      The purpose of the field stop is to limit light rays that are outside of the design parameters of the eyepiece from entering the field of view. The field stop also provides the distinct circular outline where the image in the field of view ends when observing through the eyepiece (called the apparent field of view, or AFOV).

      A270483_1_En_1_Fig6_HTML.jpg

      Fig. 1.6

      The apparent field of view (AFOV) of an eyepiece (Illustration by the author. Lunar astrograph courtesy of Mike Hankey, Freeland, MD, USA—www.​mikesastrophotos​.​com)

      Limiting the AFOV of the eyepiece with a field stop is a critical part of its design parameters. If the field stop is removed to allow additional light rays and widen the AFOV, then this additional view is likely to be less sharp and show significant levels of aberration in this extended portion of the AFOV. So while some ambitious amateurs may want to widen their eyepiece’s field stop to attain a larger AFOV, doing this will most likely result in poor image quality near the edge of the field of view.

      Since the field stop is located at the eyepiece’s focal plane, when an observer holds his or her eyepiece up to a light and looks through it, he or she will see a bright field of view bordered with a sharply defined edge. This edge of the view appears sharp because the field stop within the eyepiece is at the focus point of the eyepiece. So when one looks through an eyepiece, it is like looking through a magnifying glass that is focused on the area between the circle of the field stop within the eyepiece. If there are any imperfections in the edge of the physical field stop, then the observer will clearly see those imperfections at the edge of the field of view when observing. It is therefore wise to be careful when handling or inspecting the inside of the eyepiece so as to not damage the sharp edge of the field stop.

      Since the eyepiece’s function is to magnify whatever is located at its field stop, when the eyepiece is inserted into the telescope the function of the focuser becomes to move the eyepiece so the image produced at the focal plane of the telescope is precisely positioned at the field stop of the eyepiece. When this happens, then the eyepiece can clearly magnify the image the telescope is forming at that location. Eyepieces should ideally locate their field stop at their shoulder (e.g., where the barrel meets the housing of the eyepiece, which is also where the eyepiece comes to rest when it is inserted into the focuser of a telescope). This is considered the ideal location because the magnification factor of a Barlow lens accessory assumes that the field stop is located at the shoulder of the eyepiece. If it is not, then the magnification factor will change slightly.

      When the field stop is physically located at the same position on different eyepieces, then these eyepieces are said to be parfocal. This means that when one eyepiece is placed in the focuser and the image is brought into focus, then all the other eyepieces will automatically be in focus when they are placed in the focuser as well. Again, at its simplest, this demonstrates how the eyepiece is a specialized magnifying glass that is magnifying the image projected by the telescope when this image is positioned at the field stop of the eyepiece.

      A270483_1_En_1_Fig7_HTML.gif

      Fig. 1.7

      Eye lens, field lens, field stop, and shoulder locations on the eyepiece (Illustration by the author)

      Optical Construction

      The optical construction, or optical assembly, is the heart of any eyepiece. This assembly typically contains multiple lenses, arranged and grouped into a specific optical prescription, more commonly called the optical design of the eyepiece (e.g., Kellner, Ortho, Plössl, König, Erfle, Nagler). This design then directly controls how the eyepiece magnifies the image produced by the telescope and influences such things as how far your eye needs to be from the eyepiece to see the view, how wide or narrow the view appears, how sharply stars appear in different parts of the view, and other characteristics as well.

      Given the impact the optical design has upon the perceptions of the observer, there have been hundreds of optical designs developed over time to address many different goals and needs. However, the predominant designs offered commercially to observers are more limited and generally fall into a much smaller number of design types. These optical designs are also sometimes referred to by the name of their inventor, and all of them evolved from the very first astronomical eyepiece in the early 1600s that was the Lippershey/Galilean design (a negative concave singlet lens) used by Hans Lippershey and Galileo.

      The table and illustration that follows represents a broad overview of these major optical designs. The Elements column of the table indicates the number of individual glass elements within the eyepiece. Since multiple elements can be cemented together to form a single group composed of multiple elements of glass, the Groups column indicates how many groupings the individual elements are arranged into. Finally, the Arrangement column indicates the layout of the groups. As an example, a 1-3-2 arrangement means that the first group in the eyepiece, or the eye lens, is a single element of glass as the first group (e.g., the first 1 in the 1-3-2 sequence). The second group in the eyepiece (e.g., the middle 3 in the 1-3-2 sequence) is a group composed of three lenses cemented into a triplet group. Finally, the last group, which is the field lens (e.g., the last 2 in the 1-3-2 sequence), is two lenses cemented into a doublet group.

      Besides the numbers and groupings of lenses, numerous other factors in an eyepiece’s optical design produce its performance characteristics. The number of glass elements, their groupings and arrangements provide the basics to help identify an eyepiece’s design. However, many of the designs developed throughout history have similar or the same elements, groups, and arrangements. The other defining characteristics that influence the optical characteristics of an eyepiece are the radius of the curves on each surface of a lens element, the type of glass used for each lens and its index or refraction, and the spacing between the groups. All these factors blend together to make each eyepiece’s optical design, and performance characteristics, unique.

      A270483_1_En_1_Fig8_HTML.gif

      Fig. 1.8

      Historical eyepiece designs. For illustrative purposes of lens types used, curves/sizes/spacing not to scale (Illustration by the author)

      Optical Design Characteristics

      When reviewing the performance of any eyepiece, the lens prescription of the optical design directly controls seven major characteristics important to observers. When opticians create or modify an optical design, they will optimize the subset of characteristics they are most interested in, while balancing the others to provide the best mix of characteristics to meet their design goals for the eyepiece. The primary characteristics that observers should understand about any eyepiece they intend on using are:

      Focal length

      Apparent field of view (AFOV)

      Eye relief

      Exit pupil behavior (i.e., eye position sensitivities)

      Internal reflections and ghosting (controlled with internal light baffles and antireflection surfaces)

      Aberrations (includes distortions)

      Focal Length

      The focal length is probably the first characteristic that one looks at when considering an eyepiece. The reason this is so important is that the focal length of the eyepiece will determine the magnification it will produce when placed in a telescope. The magnification the eyepiece produces is unique to each telescope because it is dependent on the focal length of the telescope. The way it is calculated is to simply divide the focal length of the telescope by the focal length of the eyepiece. As an example, if the telescope has a focal length of 1,000 mm and the eyepiece has a focal length of 20 mm, then the magnification of this eyepiece with this telescope is 1,000 ÷ 20 = 50×.

      For a line of eyepieces to provide a good range of magnifications in any telescope, they must be available in a range of focal lengths. A full range is usually considered to have the longest focal length be between 32 and 40 mm, and the shortest focal length near 4 mm, with multiple others available between those two end points. For a line of eyepieces with 1.25 in. barrels, it is typical to see the following focal lengths being available: 40 mm, 32 mm, 24 mm, 16 mm, 12 mm, 9 mm, 7 mm, 5 mm, 4 mm. For eyepieces with 2 in. barrels, they typically only focus on the longer focal lengths as the larger barrel can accommodate long focal lengths with wider AFOVs. Therefore, 2 in. eyepieces are usually found in focal lengths as long as 56 mm, with some rarer ones having up to 100 mm focal lengths.

      Some eyepiece lines, however, are not made in a full range of focal lengths, and many amateur astronomers often wonder why a manufacturer does not extend the line to a full range. What these observers do not realize is that the available range of focal lengths that an eyepiece line can accommodate is often directly related to the optical design type used by that eyepiece line. As an example, the time-honored Erfle design, which observers have been using for many decades, is rarely ever seen in a focal length shorter than 16 mm. Similarly, the very popular Tele Vue Panoptic line of 68° AFOV eyepieces is only available in focal lengths as short as 15 mm. The reason that shorter focal lengths are not produced for these designs is that at focal lengths shorter than approximately 15 mm, the resulting eye relief of these designs becomes too short to be effectively used by an observer. Every optical design is therefore not able to support all the focal lengths that may be desired by the amateur astronomer.

      Because an eyepiece line may not have available eyepieces in a shorter focal length that may be needed by an observer, this does not mean that the observer needs to look for another brand or line of eyepieces. To attain shorter focal lengths using designs such as the Erfle, the Panoptic, and others that are not available in shorter focal lengths, all an observer need do is to simply use a Barlow lens with those eyepieces. When an observer finds they enjoy a particular line of eyepieces not made in shorter focal lengths, with the incorporation of a quality 2× or 3× Barlow lens he or she can easily attain these shorter focal lengths (and without shortening the eye relief of the eyepiece).

      To illustrate, the Tele Vue Panoptic has 15 mm as its shortest available focal length; however this eyepiece operates at an effective focal length of 7.5 mm when used with a 2× Barlow, and operates at an effective focal length of 5 mm with a 3× Barlow. Incorporating a Barlow can therefore greatly extend the focal lengths of any eyepiece line. The illustration below shows how a single Barlow of the proper magnification factor can eliminate the need to purchase three additional eyepieces. Depending on the expense of the eyepieces, using a Barlow can save the observer from $100 to more than a $1,000 when expensive ultra-wide field eyepieces are being considered.

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      Fig. 1.9

      Comparison of two complete eyepiece sets with similar range of focal lengths. Using a 2.8× Barlow (top right) allows the four eyepieces shown with it to produce a similar range of focal lengths as shown by seven eyepieces (bottom). Using a Barlow can therefore reduce the number of eyepieces needed and save money (Image by the author)

      Apparent Field of View (AFOV)

      The apparent field of view (AFOV) of an eyepiece is probably the most talked about aspect of the eyepiece. AFOV is how wide the eyepiece’s field of view appears as you observe through the eyepiece. In effect, it is how large the porthole looks as you view. It is measured as the angle of the view from the furthest left to the furthest right of the field of view. Since the popularization of wide-field eyepieces with the introduction of the Nagler design by Tele Vue Optics in the 1980s, eyepieces with the widest AFOVs possible have become the rage among the vast majority of observers.

      The eyepiece’s optical design is what controls how large the AFOV of an eyepiece can be while maintaining an acceptably good image from center to edge. The eyepiece then uses the field stop as the physical device to limit or stop the AFOV to the specification of the optical design. The sharply defined edge to the field of view you observe through the eyepiece is actually this physical field stop device. Therefore, any damage to the field stop, or a field stop that is not properly formed to a smooth knife edge, will be immediately noticeable as an irregularity in the outer circle of the field of view. Extreme care should therefore always be exercised when inspecting or cleaning the field stop of an eyepiece. The table below lists the AFOV that is common for the major eyepiece designs.

      Today, larger AFOVs are generally considered more desirable by most observers, as they provide a more natural view, like that of the unaided eye, which is approximately 140° at its widest. However, some observers still feel quite comfortable with, and even prefer, AFOVs that are more constrained in size, as smaller AFOVs can reduce observing strain. Smaller AFOVs can make it easier to take in the entire view at a glance, reducing the effort needed to look around when the AFOV is large. Additionally, smaller AFOV eyepieces are many times less prone to aberrations and distortions off-axis, since their field of view is constrained. The 50° Plössl is an excellent example of such an eyepiece—easy to produce at high quality for a modest price with performance that is very good in all telescope designs and focal ratios.

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      Fig. 1.10

      The on-axis and off-axis regions of the field of view (Illustration by the author. Lunar astrograph courtesy of Mike Hankey, Freeland, MD, USA—www.​mikesastrophotos​.​com)

      Although the AFOV of an eyepiece is a hot topic among amateur astronomers and often the basis for recommendations, it remains a completely personal preference, unique to the likes and dislikes of each observer. New observers should therefore always experiment for themselves at a local astronomy club or organized evening star party to determine their AFOV size preferences for an eyepiece before they commit to any purchases.

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      Fig. 1.11

      Comparison of common AFOV sizes and how AFOV can affect the view: 120° – 100° – 82° – 70° – 52° – 42° (Eagle Nebula (M16) astrograph courtesy of Mike Hankey, Freeland, MD, USA—www.​mikesastrophotos​.​com. Illustration by the author)

      Eye Relief

      Eye relief is controlled by the optical design and is the measure of the distance from the center of the top of the eye lens of the eyepiece to the point above the eye lens where the eyepiece magnifies and focuses the image. Eye relief of an eyepiece is a very important factor to consider, as it affects comfort and ease of use. If an eyepiece has an advertised eye relief of 10 mm, then the observer will need to place his or her eye 10 mm above the center of the eyepiece’s eye lens surface to see the image magnified by the eyepiece. When the eye

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