Understanding Telescope Eyepiecesby Chris BrownewellTelescope HistoryA telescope eyepiece is a lens or set of lenses that magnify the image from a telescope objective mirror or lens and transform it into a viewable image. Without this magnification, the image would be too large and distorted to use visually. The origins of telescope eyepieces are closely tied to the origins or telescopes themselves. Glass was discovered accidentally through the chance combination of ingredients in fires by the Phoenicians. The oldest artificial glass known comes from the graves of Egyptian kings from about 3500 BC. Crude lenses have been found in Crete and Asia Minor that date from about 2000 BC, but these were not clear enough to see through well. The ancient Greeks and Romans knew the facts that light could be refracted, reflected, and magnified, but they never made the connection with using glass to build magnifying instruments. During the 13th century Roger Bacon independently rediscovered magnification using glass lenses and their use in observing objects in the sky. For this he was imprisoned as a magician. By the end of the 13th century eyeglasses were being made.The first telescopes were probably built during the early 1500s by spectacle makers who put two lenses in line with each other by chance. As nobody at the time was interested in being brought before the inquisition for claiming to see amazing things, no one took credit for this invention until Johann Lippershey of Middleburg, Holland applied for a patent on it in 1608. His patent was denied on the grounds that the knowledge of how to build a telescope was too widespread. Galileo, learning of this new invention, constructed his own telescope. Galileo was the first to point a telescope at the moon, planets, and stars. The Galilean telescope, as Lippershey's original design is referred to today, uses a convex objective lens and a simple concave lens as the eyepiece. This design has a major fault. As magnification is increased, the field of view decreases very rapidly. Since the objective and eyepiece are simple lenses, it also suffers from chromatic and spherical aberrations. Because of the nature of light, all optical systems have to deal with various distortions of the image. When telescopes were invented, nobody had a very good idea about exactly what light was or how it behaved. When light is bent or 'refracted' by a simple lens, the different colors of light are brought to a focus at different distances from the lens. This phenomena in an optical system is called chromatic aberration and looks like a colored haze around an image. All of the curves ground into early telescope lenses were spherical since spherical optical surfaces are the easiest ones to make. Unfortunately, the focal plane from such a surface is curved and this causes spherical distortion. There are several other types of optical distortions, but these two are the most important where eyepieces are concerned. Eyepiece PioneersIn 1611, Johannes Kepler proposed the idea of using a convex lens for the eyepiece. This dealt with the problem of rapidly decreasing field with magnification, but it came at a price: the image produced is upside down. This design works well with a telescope of focal ratio f/40 or greater.The focal ratio of a telescope is the focal length, that is, the distance from the objective lens to the focus, divided by the diameter of the objective lens. As there were no methods for testing the optical quality of lenses in the 17th century, the focal ratio had to be large to compensate for any problems with the lenses. Telescope sizes in those days were referred to by the length of their tubes, rather than the diameter, with a longer telescope being better that a shorter one. Making the telescope lenses very long in focal length minimized the problems of chromatic and spherical distortion, but made the telescopes difficult to use. In 1654, Christiaan Huygens invented an eyepiece design using 2 plano-convex lenses with the curved sides toward the telescope objective. This design is an improvement over the Galilean and Keplerian designs and is still used today as original equipment in some department store telescopes. This design features a narrow field of view and short eye relief. What astronomers mean by eye relief is how far from the eyepiece your eye can be and still see the image comfortably. Eyepieces with short eye relief are unsuitable for observers who wear eyeglasses. Eyepieces that have long eye relief are more comfortable to use for most observers. In 1780, Jesse Ramsden introduced another 2-lens design that is still used today in cheap telescopes. This design has the same faults as the Huygenian, but is effective at focal ratios of f/7 or longer, as versus the f/8 or longer of the Huygenian. Unlike the Huygenian, the Ramsden is also suitable for using crosshairs with. All of the refracting telescopes made up to about 1750 used a simple objective lens made of crown glass. The invention of a new type of glass called flint glass and the discovery that its refractive properties were different from crown glass made possible the construction of achromatic lenses. An achromatic lens is a doublet where the front lens is made of crown glass and the rear element of flint glass. With an objective lens of this type, it is possible to build a telescope that is corrected to have no spherical or chromatic aberrations at f/15 or longer. This was a big improvement over f/40. Reflecting telescopes that did not suffer from chromatic aberration had been invented by Newton and Gregory in the 1600's using mirrors made of ground and polished speculum metal, but these mirrors were so hard to make and tarnished so quickly that these telescopes were inferior to even the poor refractors that were then available. It wasn't until Jean Foucault invented a method for silvering glass in the mid 1800's that reflectors really became a viable telescope design. The Science AdvancesIn the mid 1800's, several companies were founded in Germany that made tremendous advances in optical technology and gave us most of the eyepiece designs that are commonly in use today. In 1846 Carl Zeiss founded the firm that bears his name in Jena, Germany. In 1866 Dr. Ernst Abbe joined him as his head optician and who eventually succeeded him as head of the company. Zeiss was the first manufacturer to come up with and use the scientific method as applied to optical manufacturing. They were able to make the best optical surfaces yet produced, but still had problems with inconsistent glass quality. When Dr. Otto Schott founded his glass company across town dedicated to applying the same scientific principals to the manufacture of glass, Jena quickly became the premier optical manufacturing site in the world.In 1848, Dr. Carl Kellner opened the competing firm of Leitz in nearby Wetzlar, Germany. Kellner introduced what might be called the first modern eyepiece design in 1850. The Kellner has a double convex field lens and an achromat eye lens. The Kellner and its closely related Zeiss designed counterpart, the Modified Achromatic (sometimes referred to as the Achromatic Ramsden), are not as good as later eyepiece designs, but are vastly superior to the Huygens and Ramsden designs. They also have the virtue of being relatively inexpensive. Kellners work very well on telescopes of f/6 or longer and have apparent fields of about 45 degrees. Eye relief is acceptable in eyepiece focal lengths greater than about 12mm. When we refer to the apparent field of an eyepiece, we are describing the size of the circle of light that you see. When we say that an eyepiece has an apparent field of 45 degrees, it does not mean that the actual field of view is 45 degrees. The actual size of the field of view can be calculated from the apparent field and the magnification. Meanwhile, back in Jena, Dr. Abbe came up with the eyepiece design that is the standard to which all other designs are compared to this day. The Abbe Orthoscopic uses a triplet field lens and a double convex eye lens. It has an apparent field of 40 to 45 degrees, good eye relief in all focal lengths, very minor aberrations which are limited to the edges of the field, and works best on telescopes of f/6 or longer. It should be noted that there is considerable variance in quality and price in eyepieces of orthoscopic design that are sold today. Any eyepiece that has four or more lenses in it really needs top-notch anti-reflection coatings on all of the lens surfaces to perform at its best. As Germany became more and more military oriented into the 20th century, Zeiss optical designers had to come up with eyepiece designs that were suitable for artillery, tank, and submarine telescopes. These applications required very wide and well corrected fields of view. Heinrich Erfle and Albert Koenig addressed these problems with designs that used 5 to 7 lenses and, for the first time, lenses that used aspheric surfaces. The invention of antireflective magnesium fluoride coatings in the 1920's made it practical to use this many lenses in an eyepiece. The same qualities that made these eyepieces excellent for hunting targets on the ground also made them great for looking at targets in the sky. They feature long eye relief, an apparent field of 60 to 70 degrees, and work well down to about f/5. Modern EyepiecesToday's most popular eyepiece design was invented by G. S. Plossl in 1860. The Plossl design uses two achromats with the field lenses to the inside of the eyepiece. The Plossl design was first mass produced by the French firm of Clave after World War 2 and today Clave Plossls are still regarded as premium eyepieces. The Plossl design features optical correction almost as good as the orthoscopic design, but has longer eye relief and a larger apparent field. It is also easier to produce in mass quantity than the orthoscopic and because of these features it has gone from being an obscure design made by one company as it was during the 1970's to being the dominant design in today's market.A number of new designs have come onto the market in the last 20 years as a result of the growth of the astronomy market, advanced lens making techniques, and the availability of new types of glass. These eyepieces, with examples being the Televue Nagler and Panoptics, Meade Ultra Wides and Super Wides, the Vixen Lanthanums, the Pentax SMC-XLs, and the Takahashi LE series are all outstanding performers with wide fields, lots of eye relief, and sharp images. The only significant drawbacks to these eyepieces is their cost, and some of them are considerably heavier than the norm for telescope eyepieces. Understanding the specsWhen considering the purchase of telescope eyepieces, there are several specifications that will tell us what kind of performance we can expect from them. These are: focal length, design type, barrel diameter, apparent field, and coatings. You will need several of your telescope design specifications to do the performance calculations: focal length, primary lens or mirror diameter, and focuser Size. The first parameter we will calculate is magnification. Magnification is the ratio of the image size enlargement and is calculated by dividing the focal length of the telescope by the focal length of the eyepiece. Although it is mathematically possible to get any magnification out of any telescope, there are practical upper and lower limits. The exit pupil (the size of the light cone leaving the eyepiece) dictates the lower magnification limit of telescopes. As the human eye's pupil will only dilate to a maximum of about 7mm, exit pupils larger than this will spill light over the sides of the iris wasting some of the light gathering power of the telescope. Exit pupil is calculated by dividing the diameter of the objective mirror or lens by the magnification. In all of these formulas, we must use like units, for example angles all in degrees and lengths all in mm, to get the proper results. For several common telescope sizes, the minimum magnification is listed below.
The maximum magnification that can be practically achieved with a telescope is limited by the quality of the optics and more importantly, the steadiness of the air being observed through. 100x per inch of telescope aperture is commonly listed as the limit, but a value of 40x to 60x per inch is much more realistic. When eyepieces of a sufficiently short focal length are not available to get the magnification desired, a Barlow lens is used to boost magnification. Barlow lenses that increase magnification 2, 3, or even 5 times are available. When purchasing a Barlow lens, pay attention to the coatings it has on it. A Barlow lens without high quality optical coatings on it will significantly dim the image. The actual field of view that will be seen in an eyepiece is calculated by dividing the eyepiece's apparent field by the magnification. For example, a 32mm Erfle eyepiece with an apparent field of 65 degrees used in a telescope giving a magnification of 32x will produce a field of view of 2 degrees. Besides the obvious that a 2-inch barrel eyepiece will not fit in a 1.25-inch focuser, the size of the focuser can have an effect on optical performance and eyepiece selection. Eyepieces of greater than about 30mm focal length will suffer from vignetting if they are in a 1.25-inch barrel. Vignetting is a restriction of the light reaching the focal point. Unless the focuser is of a short profile design, 1.25-inch eyepieces of shorter focal lengths can also suffer from vignetting from the focuser tube. Thus, there is an advantage to using a 2-inch focuser even if you only use 1.25-inch eyepieces. ConclusionAs discussed previously, the type of eyepiece tells you what optical quality performance you can expect. It is important to remember that although price is usually a pretty good indicator of quality where eyepieces are concerned, there may not really be that much improvement between an eyepiece that costs $100 and one that costs $300. Many of the latest designs put an emphasis on wide fields but if your main objects of study are the moon and planets this wide field design is a totally unnecessary expense. A good quality Orthoscopic or Plossl design is less expensive and gives better performance. Also, when observing the moon, planets, or bright stars there is always the problem of internal reflections off the eyepiece lenses. Because of this eyepieces with four or more lenses in them must have good anti-reflection coatings on all lens surfaces to work well. Eyepieces that only have the external surfaces coated should be avoided.The best references for a prospective eyepiece purchaser are articles in Sky & Telescope magazine's April 1996 issue and in Astronomy magazine's June 1998 issue. The book Star Ware by Phillip S. Harrington also has a pretty good section on eyepieces. Unfortunately, most of the standard references for telescope making contain information which is basically obsolete. While it might be interesting to read how to make a Ramsden or Huygenian, nobody today is going to use one of these designs. Even Kellners are getting uncommon these days. References and even magazine articles on the complex 4- to 8-lens systems used in today's finest eyepieces and the exotic glass they use are nearly non-existant. For these eyepieces the only references you will find are the manufacturer's advertisements and perhaps a web site or two discussing performance. Today's astronomers enjoy the use of eyepieces that significantly outperform those available even 20 years ago. With their wide, bright fields and excellent correction at short focal lengths, these marvels of modern optical technology help make astronomy the exciting hobby it is today. Copyright ©1998 Calumet Astronomical Society. |
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