Ask someone to close their eyes and picture in their mind a telescope. Or better yet, ask a child to take a crayon and draw on a piece of paper a picture of a telescope. More likely than not, the image of a long tube pointed at the skies with an observer peering through the opposite end is the result. The refractor is the intuitive concept of a telescope.
Historically, the refractor is the earliest design for telescopes, with the earliest examples appearing in the Netherlands in 1608. Spectacle makers Hans Lippershey and Zacharias Janssen are two of the early creators of the design. Within a year or two Galileo created his own improved refractor design, pointed his telescope into the night sky, and history was made.
Conceptually, a refractor is a system of lenses with an objective lens system to gather light and an eye lens system to focus the light gathered by the objective lens into an image for the observer. Hence, the intuitive mental picture of a long tube with an objective lens pointed at the sky and an observer viewing through the opposite end focusing the eyepiece.
Inch-for-inch, refractors are regarded by the astronomy community as the best performing telescopes. Sharp, pin-point star images, and high contrast planetary images are their hallmark. Four inch or larger diameter refractors are the primary instruments of lunar and planetary observers. Astrophotographers have adopted apochromatic refractors as their go-to telescopes, producing the sharp and contrasty images that rival professional observatories.
In today’s world of refractor telescopes, there are achromats and apochromats, terms created to differentiate the levels of color correction of the respective lens systems. Grade school science demonstrates that sunlight passing through a prism separates sunlight into its constituent colors. In optical systems, such as telescopes, all types of optical glasses exhibit some degree of separation and dispersion of light into primary and secondary colors.
A classic achromatic refractor uses a two lens objective, with one lens made of crown glass, and the other lens made of flint glass. With the lenses ground with proper curves and using glasses with different refractive indices, the result is a telescope that can bring to focus two of the three prime colors of light, typically red and blue wavelengths. So conceptually, the achromat can produce an improved image of a distant object over a single lens objective of the type used by Galileo. In practice, there are some color errors, or chromatic aberrations, that creep into the visual performance of an achromatic refractor, especially at shorter focal lengths. The primary colors do not focus at the same point, resulting in a color fringe around Moon, planets or bright stars. This chromatic aberration also results in diminished sharpness and definition in the telescope image. Many classic early refractors typically have focal ratios of f/11, f/15, or f/20 or greater to minimize chromatic aberrations and become “color-free”. Even at long focal lengths, achromats can display chromatic aberrations, where the secondary colors of yellow and purple wavelengths do not come to focus.
This technical discussion gives the impression that achromats are flawed. That couldn’t be farther from the truth. Today’s achromatic designs use different types and combinations of Extra-low Dispersion, or ED, glass to lower the chromatic aberrations to a minimum. Modern designs of achromats have minimized the false color to a high degree. One current builder of high quality refractors has taken advantage of new glass technology and our light polluted skies. With a combination of ED glasses, altered lens curves, and adjusting the air spacing between the two lenses of the doublet, the color error in the violet wavelengths match the not-quite – black-really-deep-purple light polluted background sky, resulting in an excellent performing telescope that produces sharp and crisp images with the color error conveniently and cleverly hidden in the background. There are specialized long focus 102 mm achromatic refractors on the market with f/11 focal ratios or greater specifically designed for viewing the Moon and the planets. These modern “planet killers” take advantage of low dispersion glasses and long focal length to provide images that rival apochromatic refractors at one third the cost.
Sometime in the 1980s, the apochromatic refractor became commercially available to the amateur astronomy market. Sophisticated designs appeared using combinations of two, three, or four lenses of exotic glasses. Advanced telescope owners found themselves developing and reveling in a new vocabulary that included terms such as fluoro-crown, lanthanum, fluorite, FPL-51 or -53, APO – triplet, and Petzval. These apochromats offered improved sharpness, contrast, and color correction over the classic achromat refractors, while at the same time creating a more portable telescope with f-ratios of f/8 or less. But this optical performance improvement is costly. Apochromats are typically 2-10 times more expensive that an equivalent aperture achromatic refractor. Apochromats get expensive very quickly as the size increases. A typical 80 mm apochromat retails in the $700 range. A 102 mm triplet can easily cost over $2,500. 130 mm apochromats range fall into the $4,000-$5,000 realm, and you can buy a car with the money needed for larger apochromats.
The old telescope salesman explanation of an achromat refractor is a “color-free image”, meaning no color fringing in the telescopic image. The telescope salesman pitch for an apochromat is “this time, I’m serious! It’s color-free!”. Depending on the sensitivity and sensibility of the observer, the image improvement of an apo – chromat over an achromat can be either a slight or a day-night difference. A lot depends on the sensitivity and sensibility of the refractor owner. From a technical viewpoint, there is no question that the apochromatic refractor offers the best telescopic images over an achromatic refractor. In fact, as discussed later in this chapter, an argument can be made that an apochromat is inch-for-inch better than every other telescope design, whether refracting, reflecting, or catadioptric.
Countless astronomy books and websites over the years have presented valid arguments that the Newtonian reflector represents the most “bang for the buck” in telescopes. Originally designed by Sir Isaac Newton in 1668, this telescope represents a simple design using either a spherical or parabolic main mirror to collect light and a flat diagonal mirror to reflect the collected light to the eyepiece and the observer. By limiting the number of optical surfaces that need to be accurately figured and polished to two, the cost for a Newtonian telescope can be minimized. In contrast, a doublet refractor needs four optical surfaces, and a triplet apochromat needs six optical surfaces to be accurately ground, figured, and polished. The Newtonian design, being reflective in nature, has no chromatic aberrations and can be constructed from less expensive materials. During the explosive growth of Newtonian telescopes in the 1980s, with John Dobson’s unique implementation of the design, Newtonians could be made from Pyrex glass, cardboard tubes, and plywood.
Sounds like the perfect telescope. An 8 in. Newtonian reflector on a Dobsonian mount can be easily acquired for the price of a 3-in. achromat refractor. That is a lot of performance for the money.
But there is a catch. Ask any telescope dealer, any telescope salesman, or any member of your local astronomy club, “What is the most common telescope problem?” Nine times out of ten, owners of Newtonian telescopes are unable to align the optics of their telescopes. The main mirror is tilted, the diagonal is not centered, the tilt of the diagonal needs adjustment, or a combination of all three leads to frustration for the telescope owner, resulting in a telescope that ends up in the closet. Whereas refractors and some catadioptric designs do not need col – limation alignment under normal use, Newtonian telescopes require frequent maintenance. This alignment skill is easily acquired by some, while others find the alignment task difficult. Some telescope owners take collimation in stride, while others find it a nuisance.
Fig. 2.6 The Maksutov-Cassegrain Telescope. Courtesy of Adam Chen
Catadioptric telescopes are a category of telescopes that combine lens and mirror technology to produce compact and transportable instruments. With a clever combination of a lens and mirrors, the incoming light path is folded upon itself, and any optical aberrations of the reflecting surfaces can be corrected by the refracting lens.
Catadioptrics are available in two popular forms: Schmidt-Cassegrain telescopes, or SCT and the Maksutov-Cassegrain telescope. Maksutov-Cassegrains appeared on the commercial market in the mid-1950s, and SCT’s burst into popularity in the 1970s. As a family, catadioptrics offer a high degree of optical performance and low maintenance in a small compact package.
Although SCTs are available in sizes from 5-in. to massive 14 and 16 in. apertures, the most popular and best-selling telescope since the 1970s has been the 8-in. SCT. The 8-in. SCT is the basic core product for not one, but two major telescope companies. The reason for this popularity stems from the all-around versatility that the design provides. As an analogy, the 8-in. SCT is the Olympic Decathlon Champion of telescopes. The Olympic Decathlon gold medalist doesn’t excel in any singular competition, such as the 100 m dash or the high jump, but is able to perform 10 different events better than anyone else can perform those same events. Versatility and a high level of performance are outstanding attributes of an 8-in. SCT. The 8-in. aperture allows for light gathering for deep sky objects, the typical f/10 focal length enables high magnification for lunar and planetary observations, numerous attachments are available for photographic use, and the compact size ensures frequent usage. But it’s not perfect for any singular use. Newtonian telescopes are available at a cheaper price in larger sizes for deep sky light gathering. Refractors produce images that are of better quality in contrast and sharpness. Many amateur astronomers tend to own two, three, or more telescopes in order to optimize their viewing. But if a person is only going to own one single telescope that is capable of handling multiple astronomy tasks, the 8-in. SCT is the likely choice.
Maksutov-Cassegrain, or Maks for short, are highly popular smaller telescopes in the 3.5-6 in. diameter range. Costing from $400 to an astronomical (pardon the pun!) $4,000, these little gems of the telescope world are available in every configuration conceivable. Maks tend to have long focal lengths in the f/12 to f/15 range, which helps to improve contrast but limits the field of view. Available on simple manual mounts, to equatorial mountings, to high tech computerized go-to systems, Maks are as versatile as their SCT counterparts. Due to the compact size, Maks make excellent travel scopes.
The natural question that arises from all these discussions of telescope design is: What telescope is ideal? Recalling the car analogy, it depends on the type of observing.
For observing the Moon and planets, the answer is clear. Refractors offer the sharpest, clearest, and most contrasty images. But why? A discussion is needed on the difference between unobstructed and obstructed optics. As seen in Figs. 2.4, 2.5, and 2.6, the Newtonian and the catadioptric designs share a common attribute of a secondary mirror that is centrally located along the light path. These secondary mirrors allow for the folding of the light path to direct the incoming light to the eyepiece. In the case of the Newtonian, the secondary diagonal serves to guide the light to the side of the telescope to the focuser. In both catadioptric designs, the secondary mirror fold the light path creating a very compact telescope that enables portability. The commercially available Newtonians have secondary mirrors that measure approximately 30-35 % the diameter of the primary mirror. Most SCTs and Maks the central obstruction of the secondary mirror constitutes 40 % the diameter of the primary mirror. The main impact of the central obstruction is a decrease in sharpness and contrast in the telescope image. What is the difference in performance between the unobstructed refractor and an obstructed design? The best analogy is to compare the familiar standard definition TV versus HDTV. A good SCT or short-focus Newtonian will offer a good lunar image like that of a standard 480i digital TV image, but a good ED achromat or apochromatic refractor is like watching a 1080p high-definition TV.
Any of these telescope designs can be used to view the Moon. If the reader of this book currently owns a telescope, proceed to the following chapters and find the Apollo landing sites and enjoy. To those looking to purchase a telescope, the following recommendations are offered based on telescope technology, decades of amateur astronomy field experiences, and telescope retail experience:
• For lunar and planetary observing, an 80 mm-to-102 mm refractor is the top choice. These refractors are light, portable, and low maintenance telescopes that offer sharp contrasty images of the Moon and planets. Remember, the reason that large telescopes exist is to act as light buckets and gather the faint distant light of stars, galaxies, and nebulas. The Moon is bright. Really bright. Light gathering is the least of your problems in lunar observing. And when aperture fever occurs, and a larger bulkier telescope is acquired, the refractor takes the role of the easy-to-use, grab-and-go scope. No wasted money here.
• As discussed earlier, it’s hard to argue against an 8-in. SCT. Yes, the lunar images are a little soft compared to a 102 mm apo refractor, but the overall versatility cannot be denied.
• A long focus f/10 or greater Newtonian has a central obstruction approximately 25 % of the primary diameter, and comes very close to refractor image quality. These telescopes are difficult to find in the commercial market, but there are some of these gems on the used market.
• 90-125 mm Maksutov-Cassegrains should also be considered, especially when small size is a requirement.
Fig. 2.7 The Author’s Collection of Telescope Eyepieces. Courtesy of the author
Telescope design is only half of the optical story. The rest of the story are the eyepieces at the focus end of the telescope. And here again, there’s another zoo filled with strange and wonderful denizens that are vying to complete the optical train. Kellner, Abbe, Konig, Brandon, and Nagler are all names of optical designers who have lent their names to their eyepiece designs, and are now an accepted part of the astronomy vocabulary. There are 0.965, 1.25, and 2 in. eyepieces. Some designs have been around for over a century, and some designs are less than a decade old. And most have valid use in astronomy.
Fig. 2.8 (a) Huygens. Courtesy of the author. (b) Ramsden. Courtesy of Adam Chen
Two of the oldest and simplest of the compound eyepiece designs are the Ramsden and Huygens. Originating from the 1700s, these eyepieces serve as historic curiosities. Occasionally, an antique Ramsden will show up at swap meets, eBay, or even antique stores. The Huygens eyepieces are still supplied in 0.965 in. size on cheap beginner telescopes sold at department stores and big-box stores. Both designs are flawed, with narrow apparent fields, chromatic aberration, and short eye relief.
Fig. 2.9 Kellner design. Courtesy of Adam Chen
In the mid-1800s, the Kellner eyepiece was developed by replacing the singlet eye lens element of a Ramsden with a achromat doublet. This resulted in a better performing design with a wider field, better color correction, and less spherical aberrations. When used on long focus telescopes, Kellners still produce a reasonably good image. The main shortcoming of the Kellner is ghosting when looking at bright objects. So Moon watchers beware! The design exists today under various names, including Modified Achromat, RKE, and modified Kellner. These eyepieces are a good economical alternative for those on a budget.
Fig. 2.10 orthoscopic. Courtesy of Adam Chen
Fig. 2.11 The Plossl orthoscopic. Courtesy of Adam Chen
Fig. 2.12 The Konig orthoscopic. Courtesy of Adam Chen
An examination of the patents for the Abbe, Plossl, and Konig eyepiece designs describes all three as orthoscopic designs. The term “orthoscopic” means free from distortion. In common astronomy vernacular, the term orthoscopic has evolved to become synonymous in name with the Abbe design.
The Abbe has stood the test of time. Since the 1950s, through the growth of amateur astronomy in the 1960s and 1970s, the Abbe design has been highly regarded for sharp and high contrast images. The classic “volcano top” Abbe ortho – scopic, so named for its distinctive beveled shape, are well-known and are highly desired. These Abbe eyepieces have been made by an optician from Japan named Tani-san, whose retirement in 2013 brought an end to decades of Circle T volcano top eyepieces. But don’t fret, Abbe orthoscopic eyepieces are available through other sources. The Abbe orthoscopic eyepiece was held up as the pinnacle of eyepieces until the advent of new revolutionary wide-angle and high eye relief eyepieces in the 1980s. Today, dedicated lunar and planetary observers still insist on Abbe eyepieces today.
The Plossl eyepiece has a interesting reputation in the amateur astronomy world. In the 1960s, the Plossl only existed as a rare and mysterious eyepiece, commercially available from a small vendor in Europe. Then in the 1980s, Plossls suddenly became widely available, to the point now where it is so commonplace that the eyepiece is considered mediocre by average backyard astronomer. Nothing could be further from the truth. Although poorly manufactured examples exist, a premium Plossl is an extraordinary eyepiece, versatile in lunar, planetary, and deep sky use. There exists a number of variants of the design in which an extra lens or two to the system, somewhat blurring the definition of a Plossl, but these variants tend to be of high quality and offer high performance.
Not as widely available as the other orthoscopics, the Konig design has its fans. Depending on the implementation, the Konig potentially offers a wider field-of – view than the Abbe or Plossl. In practice, the Konig achieves a wider field, but with a slight sacrifice of edge of field sharpness, and slightly shorter eye relief.
A variant of the Konig design is the Brandon eyepiece. Chester Brandon developed his eyepiece design during his time at the Frankford Arsenal in Philadelphia. The eyepiece design was widely used during World War II in U. S. Army optics. The Brandon design differs from the Konig by using three high index glass types and four different lens radii. When marketed in the 1950s as a high priced premium eyepiece, the Brandons were priced at $15.95. My how times have changed, with the price of Brandons and many premium eyepieces in the three-digit range. With sharp crisp contrasty images, the Brandon reputation exceeded that of any other 1950s eyepiece, and is still the go-to eyepiece for many lunar and planetary observers today.
Fig. 2.13 (a) The Erfle. Courtesy of the author. (b) Modern Wide-Angle. Courtesy of Adam Chen
During World War II, a number of scopes used for spotting or for aiming weaponry by the American armed forces were equipped with Erfle eyepieces. In the 1950s and 1960s, war surplus Erfle eyepieces became available for amateur astronomers seeking wide-angle views through their telescopes. With an apparent field of view of approximately 60°, these large eyepieces created a demand for wide field eyepieces. That wide-angle demand has grown over the years, to the point now where the consumer demand for ever wider fields of view drives the eyepiece industry. The Erfle does not display as sharp an image in the center of the field as the orthoscopic eyepiece, and a degradation of the image occurs in the outer third of field. Newer modern wide-field designs use exotic glass types and different lens configuration and curves to correct the edge degradation, while at the same time providing a wider field of view.
Fig. 2.14 The Nagler and related ultra wide angle eyepieces. Courtesy of Adam Chen
The quest for an ever wider field-of-view exploded with the introduction of the Nagler eyepiece. Competitors quickly followed, with virtually every telescope company offering their version of an over 80° field-of-view eyepiece. The ante was raised again with the introduction 100° and even wider field-of-view eyepieces in recent years. These eyepieces contain seven or more lenses in their complex designs in an effort to provide wide fields without sacrificing sharpness at the edge of the field. These eyepieces are not cheap, with many exceeding the cost of many telescopes! These eyepieces are outstanding for deep sky and wide field applications. However the complex design, high number of optical surfaces, plus the inevitable, although slight, light absorption caused by the amount of glass in the light path, these ultra wide angle eyepieces do not offer the same level of sharpness and contrast as the simpler orthoscopic designs.
Fig. 2.15 The Barlow lens. Courtesy of Adam Chen
While not an eyepiece, a Barlow lens is a useful addition to every eyepiece case. A Barlow lens is a negative lens system placed along the light path between the objective and the eyepiece that increases the effective focal length of the telescope, therefore increasing the magnification. Typically, Barlows double (2x) or triple (3x) the magnification. Newer focal extenders using three or four lens elements are available to quadruple (4x) or quintuple (5x) the focal length. These accessories are useful in three ways. A single Barlow lens effectively doubles the number of magnifications available in an eyepiece collection. The use of a focal extender also allows longer focal length eyepieces with their higher eye relief to be used at higher magnifications for eyeglass wearers. The use of a Barlow lens can improve the off – axis edge sharpness of some eyepiece designs. The Barlow lens and related focal extenders are also useful for astrophotography.
Some discussion is needed on the subject of zoom eyepieces. In the 1960s, the zoom eyepiece earned a reputation for mediocre optics and was not worth the money. Today’s zoom eyepieces deserve some attention. Improvements in lens coatings, the introduction of high index glass, and improved manufacturing has yielded a modern zoom that is worthy of a spot in an observer’s eyepiece case. Although still narrower in field-of-view at longer focal lengths, and wider at shorter focal lengths, the performance has been greatly improved. A zoom eyepiece will not take the place of an eyepiece collection for critical observing, but serves the role for quick look situations, or when showing the night sky to children whose short attention spans don’t allow for the changing and refocusing of conventional eyepieces to change magnification. Viewing the Moon through a 102 ED refractor with an 8-24 mm zoom eyepiece served as an inspiration for the title and subject of this book.
The driving criteria for eyepiece selection for lunar and planetary observing is sharpness and contrast. The rule of thumb for selecting the right eyepieces for viewing the Moon is “the simpler the better”. The classic Abbe, Plossl and Brandon designs are the preferred choices. There are more esoteric lunar and planetary eyepiece designs based on the monocentric design, or on proprietary designs. These are not discussed here due to their low availability.
There is an Achilles heel to the three classic orthoscopic designs. The older eyepiece designs perform best in longer focal length telescopes. In the era in which these designs originated, telescopes had long focal lengths, typically f/10 or greater. At f/20, even the lowly Ramsden design performs well. But many of today’s telescopes have much shorter focal lengths, often f/6 or shorter. The classic designs suffer from loss of edge sharpness because of the steeper angle of the light cone from the objective as it enters the eyepiece. Modern designs take into account the shorter focal length telescopes of today. The design rationale for many of the updated configurations of the Plossl design has been to widen the field-of-view and improve performance with short focal length telescopes.
The recommendation for the ideal eyepiece for lunar observing is as follows:
• For telescopes with a focal length of f/7 or greater, the Abbe or Plossl designs will perform at the highest level.
• For telescopes with focal lengths of less than f/6, the modern variants of the Plossl are suggested.
• For observers who must wear glasses while viewing, there are some proprietary eyepiece designs that provide 20 mm of eye relief. These tend to be premium eyepieces that use exotic lens configurations and glasses, and therefore are not cheap. But they are recommended for eyeglass wearers.
• Consider using a Barlow in combination with a low – or medium-power eyepiece in order to obtain higher magnifications. The comfortable eye relief from this combination is often preferred by both eyeglass wearers and non-eyeglass wearers. The classic orthoscopic high power (4 mm and 6 mm) eyepieces are notorious for their near-pinhole sized eye lens.