The Dawn of Binocular Astronomy. By Fred Watson. Published in the '2001 Yearbook of Astronomy', with illustrations. At any given moment, how many optical instruments are directed skywards from the dark side of our planet? Thousands? Tens of thousands? Perhaps even hundreds of thousands? The answer clearly depends on many things, such as the amount of cloud cover, the time of year, and which hemisphere is in darkness at the time. It is impossible to be definitive. There is, however, a little that can be said about the sorts of instruments they might be. For example, a handful will be telescopes with household names. These are the giants of the species, the Geminis, William Herschels and Anglo-Australians of this world, whose mirrors range between about 4 and 10 m in diameter. The 2.4-m Hubble Space Telescope will join them for the dark segment of each of its 96-minute orbits. Further down the aperture spectrum will be a much larger group of instruments bigger than, say, 0.3 m that remains within the category of professional equipment. Today, this overlaps with the still larger number of telescopes used by serious amateur astronomers, with mirrors up to 1 m in diameter. At the very bottom of the pecking order are the humblest tools of astronomy, the hosts of instruments whose apertures are measured not in metres but in millimetres. These are nearly always refracting telescopes (ones that use lenses rather than mirrors to form the images), and they include that special category designed for terrestrial use with both eyes?ordinary binoculars. Such is the ubiquity of binoculars, and so popular is their use both in casual and serious stargazing, that it is quite possible they will outnumber every other class of telescope scanning the sky at any given time. And that is something to reflect on?or, if you prefer, to refract on. Binocular astronomy Notwithstanding the sensitivity and convenience of today’s high-tech CCD cameras (charge-coupled devices), binoculars offer the most natural way of all to observe the night sky. Whether it be with a common-or-garden 7?50 night glass (in which the 7? represents the magnification and 50 mm is the diameter of the front, or objective lenses) or with one of those big, stand-mounted binoculars championed by comet-hunters, the use of two eyes stimulates an intimacy with the heavens that is quite special. Of course, it is entirely understandable. We have evolved as two-eyed beings, and to use them both in direct astronomical observation is to bring to bear our most highly-developed perceptive faculty on the sky in a manner with which we are immediately at ease. And for anyone trying to do serious visual observing of, say, variable stars, the use of both eyes brings real advantages. Today, astronomy with binoculars enjoys a level of popularity greater than ever before. Paradoxically so, perhaps, because when today’s amateur astronomer goes shopping for new observing equipment, he or she is confronted with a dazzling array of high-performing telescopes. But binoculars, too, have reached a level of development that places them well above the ordinary and into the serious league of astronomical instruments. Even a modern, compact 8×24 model can be a useful tool in the hands of an accomplished observer. And stand-mounted binoculars with objectives up to 150mm in diameter represent the ultimate for sky-watchers who prefer to do their astronomy with both eyes open. Between these two extremes lies a broad spectrum of binoculars of differing type, size, magnification and quality, all suitable in one way or another for astronomical observation. Because binoculars are such versatile instruments, they are used for sky- watching by people who would never admit to an interest in astronomy. There can’t be many binocular-owners who haven’t tried out their prized possession on the Moon, for example. And you might be surprised to know that at the other end of the scale, professional astronomers are prone to relieving the stress of large-telescope observing by spending a few minutes outside in contemplation of the binocular sky. The instruments they use are often quite small, perhaps no bigger than 8×30, so as to fit conveniently into the briefcase or backpack they carry on their observing trips. But these astronomers enjoy a special privilege denied to most people. The odds are that they will be conducting their research at an observatory on a remote site, far from city lights and pollution. Here, the sky is truly dark, and their binocular sky-watching will benefit enormously as a result. On moonless nights in such locations, stars and nebulae become spectacular when even modest binoculars are used. How long has this been going on? Probably most people imagine that observing the sky with binoculars is a relatively new pursuit, an offshoot of amateur astronomy that has blossomed only recently. Many might attribute its success to contemporary popularisers of astronomy? luminaries such as the editor of the Yearbook you now hold in your hands. Patrick Moore’s Exploring the Night Sky with Binoculars made its début in 1986, prompting several other authors to try out similar explorations of their own. These books have become very popular, and without doubt, they have contributed to the outstanding level of interest in binocular observing today. Remarkably, though, a strikingly similar situation prevailed a hundred years ago. The turn of the twentieth century found binocular astronomy enjoying a wide following, surprisingly so in an era when science tended to be the province of the educated and well-to-do. There seem to be several reasons for this. First, there was an upsurge of popular interest in astronomy as a whole towards the end of the nineteenth century. This is reflected in the number of astronomical societies that appeared around that time. Two good examples are the British Astronomical Association (1890) and the Astronomical Society of South Australia (1892), but there were many others. Secondly, binocular astronomy had a champion then, too, in the shape of a man who was as illustrious in his own sphere as Patrick is on the world scene today. This man lived and worked in Brooklyn, New York, and we shall make his acquaintance in due course. Finally, and most importantly, turn-of-the-century binoculars had just undergone a technological revolution that had dramatically improved their performance. The new prismatic glasses then becoming popular seemed to offer much more to astronomers than the old-fashioned Galilean type. As we shall see, that was only partly true, but the point remains that the popularity of binoculars for astronomy followed the technology of the time. Astronomy did not of itself drive the development of binoculars; their evolution was spurred on by far more earthly pressures. That has been true throughout the entire history of binocular astronomy. Even today, those large binoculars in the 100 to 150 mm class that are manufactured especially for astronomy owe their origin to Japanese ship- mounted binoculars of the Second World War. It turns out that to trace the history of binoculars in astronomy, we must trace the history of binoculars themselves. The long awakening of binocular astronomy is intimately connected with dawning technological development. The earliest binoculars There is still debate and uncertainty over the early history of the refracting telescope. The focus of recorded events is The Hague in the Netherlands, where, in October 1608, the government of the States General was asked to consider a number of patent applications for what we now call the telescope. First among them was that of Hans Lipperhey, a Middelburg spectacle-maker, and the response he elicited was that he should develop a version for both eyes, and then try his luck again. Apparently, this he did, and when he fronted up to the States General in December 1608, he brought with him the world’s first binocular. Whether or not it actually worked terribly well is another matter, but to Lipperhey belongs the credit for the first two-eyed optical instrument?even though his patent for the telescope was denied him on the grounds that several others already knew of the invention. Lipperhey’s patent application (and his binocular) used the simple lens system shown in Figure 1. This combination of convex (thickest in the middle) and concave lenses (thickest at the edge) is now known as the Galilean telescope. Of course, it was Galileo who, in 1609, seized the opportunity missed by the simple spectacle-maker and used the new telescope to begin dismantling contemporary astronomy, thereby rocketing to infamy in the courts of the cardinals. But Lipperhey and his ilk received recognition of sorts?the instrument is still sometimes known as the “Dutch telescope”. While the design of the telescope itself advanced steadily during the seventeenth century, little attention was paid to the construction of binoculars. No doubt this was because the successful manufacture of a binocular demands the construction of two identical telescopes fixed with their axes parallel. These requirements stretched the technology of the time to its very limits, and few opticians made the attempt. One who did was a Capuchin monk by the name of Chérubin d’Orléans, and it may have been one of his extraordinary Galilean binoculars that was first used for astronomy. Published works by Chérubin dating from the 1670s and 1680s describe his instruments as having cardboard tubes of rectangular cross-section up to 4 m long, and a much later engraving depicts three bewigged gentlemen apparently using one of these monsters for astronomy. All this would be frankly rather hard to believe were it not for the existence of just such a binocular, about 1-m long, in the Museum of the History of Science in Florence (Figure 2). This instrument is almost certainly the earliest binocular preserved today. It is most fitting that it was probably used for astronomy by its first owner, the Grand Duke Cosimo III de Medici. Though binoculars themselves were few and far between, the foundations of the modern instrument were laid in a series of developments in optics during the seventeenth and eighteenth centuries. Briefly, they were: --The invention, in 1611, of the inverting telescope by Johann Kepler. This has a wider and more uniformly-illuminated field of view than the Galilean, but presents an upside-down image to the observer (Figure 3). --The improvement of Kepler’s design in about 1645 by the use of additional lenses to re-invert the image (Figure 4). Usually known as the terrestrial telescope, this arrangement was due to Anton Maria Schyrle de Rheita, who was also an early binocular-maker. --The use of compound eyepieces in Keplerian and terrestrial telescopes from about the middle of the seventeenth century. The single convex lens was replaced by two spaced components (in the most common design, due to Christian Huyghens) to improve definition and field of view. --The solution, in 1729, of the problem of chromatic aberration (coloured fringing) in telescope objectives by combining component lenses of different materials. The discovery was made by a London barrister, Chester Moor Hall, but the achromatic (colour-free) lens was eventually perfected and patented by the optician John Dollond. --The use of a simple hinge arrangement to allow the two halves of a binocular to be adjusted to match the separation of the user’s eyes (the inter-pupillary distance). The Venetian optician, Lorenzo Selva, described such an instrument in 1787. Binoculars come of age Despite these various advances, it was a reversion to the simplest optical system of all that brought the first commercially-successful binocular. The instrument was a rudimentary opera glass, consisting of two small Galilean spy-glasses mounted side-by-side, their extending eye-tubes moving independently for focusing. It was introduced in 1823 by the Viennese optician, Johann Friedrich Voigtländer, founder of an optical dynasty that shortly afterwards moved to Brunswick in Germany where it became famous for the manufacture of cameras. Voigtländer’s basic design was quickly adopted by enterprising Parisian opticians, who added a bridge to the eye-tubes and a central focusing wheel to produce the instrument everyone recognises today as an opera glass (Figure 5). By the middle of the nineteenth century, outdoor versions of the little opera glasses were also being manufactured throughout Europe. Though they used the same Galilean optical system, they tended to be larger, more robust and less ornate than their theatre-going counterparts. Their magnification was usually greater, too; up to 5? or 6? in the best glasses, compared with 2? or 3? in an opera glass. Properly termed “binocular field glasses” (but commonly known simply as “binoculars”), they appeared in different guises as manufacturers coined fanciful names for them: race glasses, marine glasses, hunting glasses, pilot glasses and so on. As time went on, various improvements appeared?usually in response to military demands. Refinements in higher-quality instruments included extending ray-shades to keep out direct sunlight and rain, adjustable bridges to suit differing inter-pupillary distances, and sling-loops for the attachment of neck-straps?once the message had got through that a binocular dropped on the floor was frequently rendered useless by the loss of parallelism of its two halves. Optically, the most significant refinement was the incorporation of achromatic objectives and eyepieces. The individual components of each lens were cemented together, so that to all intents and purposes, the instrument retained the simplicity of having only two lenses in each half. This was a very important aspect of the Galilean binocular, particularly when used at the faint levels of illumination encountered in astronomy. Why was this the case? The principal light-loss in a lens is caused by partial reflection as the light enters and leaves it, crossing its two polished “air-to-glass” surfaces. In a typical Galilean instrument, the total wastage of light resulting from its passage through the two lenses is about 17 percent?a modest amount compared with the instruments that were to follow. Complex optical systems with many elements are costly in terms of light losses, a defect that was not properly remedied until the development of anti-reflection coatings in the 1930s. Until then, a well- designed Galiliean offered the brightest image of any binocular type available. It also boasted the greatest freedom from ghosting and flaring?effects produced when the unwanted reflected light finds its way into the eye. Today, of course, optical instruments have sophisticated anti-reflection coatings on all their optical surfaces, reducing light-losses almost to zero. Poor transmission in binoculars is simply no longer an issue. The dawn of binocular astronomy The mass-production of Galilean opera and field glasses in huge quantities during the second half of the nineteenth century led to an awakening of their possibilities for astronomy. Their use for observing stars and star-clusters in particular rated a mention in some popular astronomy books of the time, though a telescope was always regarded as the more desirable instrument. But then, onto the scene (in the USA, at least) strode binocular astronomy’s first great advocate. This man was not a scientist but a journalist with a burning enthusiasm for the night sky and, by virtue of his profession, a particular eloquence in communicating its beauties. His name was Garrett P. Serviss, and he lived from 1851 until 1929, spending virtually his whole life in the state of New York. By his writing and lecturing, he did more to popularise astronomy than any American before him; truly, he was the Carl Sagan of his age. Astronomy with an Opera Glass is but one of Serviss’s books. It was, in fact, his first, appearing in 1888 as a digest of articles he had written for a popular science magazine the previous year. It is a book of great charm, and its clear exposition and sound practical advice made it a best- seller that ran to at least eight editions. Serviss’s title was well- chosen, for he believed the relatively wide field of view of opera glasses made them more suitable for astronomy that their higher-magnification outdoor counterparts?though he did include advice for users of field glasses with magnifications up to 7?. The flavour of the book can be gleaned from a few excerpts?in which some allowance has to be made for the florid writing style of the period. Here, for example, is Serviss waxing lyrical as an apologist for the ancient legends of the constellations: “While we may smile at these stories, we cannot entirely disregard them, for they are intermingled with some of the richest literary treasures of the world, and they come to us, like some old keepsake, perfumed with the memory of a past age.” And again, exhibiting a dry humour as he discusses certain deficiencies of the constellation Taurus: “The constellation-makers did not trouble themselves to make a complete Bull, and only the head and fore-quarters of the animal are represented. If Taurus had been completed on the scale on which he was begun, there would have been no room in the sky for Aries; one of the Fishes would have had to abandon his celestial swimming-place, and even the fair Andromeda would have found herself uncomfortably situated. But, as if to make amends for neglecting to furnish their heavenly Bull with hind-quarters, the ancients gave him a most prodigious and beautiful pair of horns, which make the beholder feel alarm for the safety of Orion.” Finally, he had a real flair for understatement: “In attempting to view the planets with an opera-glass, too much must not be expected?” That did not, however, prevent Serviss from detailing just what might be expected from opera-glass views of the planets?with as much enthusiasm and insight as his descriptions of other celestial vistas better suited to the instrument. Astronomy with an Opera Glass certainly advanced the cause of binocular sky-watching on Serviss’ side of the Atlantic. Its influence outside the USA is more difficult to judge, but at the very least it reflected the spirit of the time in Britain and Europe, and probably made some contribution to it. The message was clear: binoculars were worthwhile instruments both for casual star-gazing and for more serious astronomical work such as variable-star observing and comet-hunting. Signor Porro and Professor Abbe Meanwhile, the technology of binoculars was moving on. At the time Astronomy with an Opera Glass was being written, it was possible to go out and buy new types of binoculars that overcame the main defect of the Galilean: namely, its very restricted field of view at magnifications greater than about 6?. Both Kepler’s inverting telescope (Figure 3) and de Rheita’s terrestrial telescope (Figure 4) had by then appeared in binocular form. Inverting binoculars were much the rarer of the two, being produced mainly for astronomical use at observatories. The Munich firm of Steinheil, for example, manufactured an instrument that closely resembled a Galilean binocular in appearance but was actually fitted with convex eyepieces. Having few optical components, it was frugal with faint light, but of course, the upside-down images rendered its application very limited. Comet-seeking was its main purpose. (Some 30 years later, the exigencies of war resulted in an inverting binocular being manufactured by the London firm of Dollond to allow naval signals to be read in poor conditions. No doubt a few of these unusual instruments eventually found their way into the hands of astronomers.) More practical than the inverting binocular was the so-called binocular telescope. Such instruments were often known in Britain as “deer- stalker’s binoculars”, but Serviss would no doubt have been more familiar with the American “long-John”?an eloquently descriptive name. Though they were much more expensive than Galilean binoculars, they appeared in quite large numbers. Ordinary draw-tube telescopes based on de Rheita’s arrangement of lenses had been commonplace for well over a century. To make a binocular version, you simply took two of these, mounted them side-by-side, and?voila!?you had your deer-stalker?or long-John (Figure 6). Certainly, the models produced during the late nineteenth century offered magnifications up to 15? or 20? with an apparent field of view (the angular diameter of the illuminated image presented to the eye) of around 35 degrees. While that is pitifully small by today’s standards, it was two or three times that of a large Galilean. Of course, the drawback with these instruments was their inordinate length. This made them unwieldy (especially for astronomy) and prone to accidental misalignment. What was needed was a way of re-inverting the image in a Keplerian telescope that did not require all those additional lenses. In fact, though very few opticians were aware of it, the solution had already been provided as long ago as 1854 by an obscure Italian artillery officer named Ignazio Porro. This inventive soldier was familiar with optics through his experience of using surveying instruments, and he had devised and patented two different combinations of right-angled glass prisms that would neatly turn an image upside-down. Introduced into the light-path of an inverting telescope, they turned it into a terrestrial one; moreover, they had the effect of folding up the light-path to shorten the instrument. Porro’s invention was successfully incorporated into hand-held telescopes manufactured in small numbers in France in the 1850s. Today we would call them prismatic monoculars. The obvious next step of making a binocular version was attempted by a number of optical firms. Success, however, eluded them. The problem was reminiscent of the tribulations of seventeenth-century binocular makers: manufacturing methods were simply not up to the fine tolerances required. Even the one design that got as far as the optical instrument trade (a product of the Parisian firm of Luquin et L’Hermite) quickly faded into obscurity because, basically, it didn’t work. The spark of genius that eventually turned prismatic binoculars into everyday objects belonged to one of the greatest scientists of his day, the German physicist Ernst Abbe (Figure 7). Abbe taught at the University of Jena, and he was also a partner in the optical instrument manufacturing firm of Carl Zeiss. In 1873, he independently re-invented Porro’s prisms, and subsequently designed a binocular that incorporated them. Faced with the problem of obtaining the right optical glasses for his project, he simply enlisted the help of a chemist, Otto Schott, and with him founded a glass factory to overcome the difficulties. He had a similar “can-do” approach in his collaboration with Zeiss on the manufacture of fine components. Abbe tried to patent his prismatic binocular in 1893, but was astonished to discover that a patent already existed. He had been completely unaware of Porro’s work and the subsequent attempts to manufacture binoculars. Undismayed, he resorted to a clever aspect of his own design that was entirely original. He had placed the prisms in such a way as to increase the separation of the objectives over that of the eyes, thereby enhancing the observer’s stereoscopic perception (Figure 8). By basing his application on this striking three-dimensional effect, Abbe won his patent, and the new binoculars began emerging from Zeiss’s factory the following year. Produced in 4?, 6? and 8? versions, the new prismatic glasses were a vast improvement over most of what had gone before. Of course, they were more expensive than Galilean binoculars, but not outrageously so. To our eyes, they look remarkably modern?their inventor had chosen the optimum arrangement of the optical components, and it had no need of improvement. Abbe’s persistence in the development of prismatic binoculars was soon rewarded. With their compact, finely-engineered construction and pleasing stereoscopic enhancement, the new Zeiss glasses were an immediate success. Astronomy and the new prismatic binoculars Because Abbe’s patent was awarded on the grounds of increased objective separation, other manufacturers found themselves free to develop prismatic binoculars?as long as the objective separation was the same as (or less than) the eyepiece separation. It did not require any great genius to work out that the way to do this was to arrange the prisms so they stepped the optical axis vertically, rather than horizontally. Thus were the floodgates of prismatic binocular manufacture opened during the closing years of the nineteenth century. First to compete with Zeiss were continental firms. Voigtländer launched the assault, describing themselves in their advertising as the “original makers of ? binocular field glasses”. They did not trouble to add that this was their opera glass of 1823 rather than a modern prismatic instrument. No doubt they hoped no-one would notice. The Berlin firm of Goerz followed with their Trieder binocular (Figure 9), of which more in a moment. Well-known British manufacturers, such as Ross, Watson, Dallmeyer and Aitchison, soon joined them. The scientific world took great interest in these developments. After all, binoculars were regarded as novel scientific instruments rather than merely accessories for tourism, sport and so on, as they are today. Thus, astronomers were made aware of the benefits of the new glasses at an early stage. Most astute in this respect was the firm of C. P. Goerz of Friedenau in Berlin. They were quick to realize that astronomers were the only users of binoculars who would gain no benefit from enhanced stereoscopic perception. While the Zeiss glasses extended the range of three- dimensional vision very significantly (to more than 10 km in the 8? model, for example), the horizons of astronomers are infinitely more distant, and stereoscopic enhancement is irrelevant. Convinced that they could therefore compete with Zeiss on an equal footing in astronomy, Goerz launched a major advertising campaign in April, 1899. A lengthy supplement in the scientific journal Nature on the 6th of that month tells the story: “Goerz’s Trieder-Binocular [is] particularly suitable for observations of Variable Stars on account of its extraordinarily wide field of view (= 40 degrees apparent) and the perfectly uniform distribution of light over the whole field.” An account of the advantages of the Trieder over Galilean binoculars follows, after which the supplement continues: “The Trieder-Binocular is made with either of these 4 magnifications [3?, 6?, 9? or 12?], but we beg to point out that 12 fold magnifying power is rather high for portable glasses, for hand use, so that it is not easy to keep the stars in steady position in the field of view. We, therefore, recommend 3 to 9 fold magnifying powers. On clear, moonless nights, the Trieder-Binoculars will show stars of the following magnitudes with sufficient distinctness to enable the observer to make determinations of the intensity of their light on Argelander’s method, viz.: Trieder-Binocular [3?]: Stars up to the 7.5th magnitude “ “ [6?]: “ “ “ “ 8 “ “ “ [9?]: “ “ “ “ 8.5 “ “ “ [12?]: “ “ “ “ 9 “ We also make the Trieder-Glasses in the form of Monoculars, that is to say for one eye? The Trieder-Binoculars are also excellently suitable for use as Terrestrial Telescopes for theatre, hunting, field, travelling, races, the army, navy, &c.” While the prose hardly compares in elegance with Serviss’s, some interesting issues are raised. The point concerning the uniformity of illumination of the field of view, for example, is well made. And the warning about the use of the 12? model will strike a chord with anyone who has ever attempted to observe the sky with hand-held high-powered binoculars. But when it comes to the measurement of star magnitudes by Argelander’s method, Goerz’s advertising copy seems at first sight to be rather optimistic. It does get the technique right. Friedrich Argelander was a most capable astronomer and former director of the Bonn Observatory, who had died only 24 years earlier. His Stufenschätzungsmethode, or step-estimation method, was still seen as state-of-the-art in the visual measurement of star magnitudes. Indeed, it is used today by many observers of variable stars. It requires the careful estimation of star brightnesses within a sequence of comparison stars, and it assumes that the field of view of the telescope is completely uniform. However, it is not the uniformity of illumination that is at issue here, so much as the capabilities of the glasses in revealing faint stars. It is questionable whether stars of the quoted magnitudes would even be visible in the binoculars, let alone measurable. If the effects of night-sky background glow are ignored, the limiting magnitude of a telescope depends only on the diameter of its objective and the efficiency with which its optical system transmits light. The same is true of binoculars. Nowhere in Goerz’s supplement is the diameter of the binoculars’ objectives specified, but that is not surprising, since it was considered of little importance at the time. The “7?50”-type specification we use today was not adopted for another 20 years. In fact, the objective diameters were 15 mm for the 3? and 6? models and 20 mm for the 9? and 12? versions, sizes typical of their day. Taking into account the 37 percent light-loss caused by the ten air-to-glass surfaces in the optical path, we can roughly estimate the limiting magnitudes for these glasses as about 7.1 for the 3? and 6? models, and 7.8 for the 9? and 12?. These figures are based on a naked-eye limit of 6.0, which is attainable by most observers. They are very different from those given by Goerz, which step upwards neatly with magnification. In fact, magnification does influence detectability considerably, because at higher magnifications, the sky-background glow is diluted and the contrast of faint objects is enhanced. It is therefore possible that the real limits for these binoculars are closer to the figures quoted by Goerz, even though they do look a bit too neat and tidy to be true. It would be interesting to discover the identity of the astronomer who provided these figures, and whether they were arrived at by observation or educated guess. Unfortunately, the chances of doing that are virtually non-existent. The Goerz company ceased to exist in 1926, when it was subsumed into a new venture called Zeiss-Ikon. The astronomical advertising seems to have had no ill-effect on the company’s turn-of-the-century fortunes. But the final sentence of the supplement was probably the most pertinent one. The Trieder binoculars were produced in large numbers and became well-regarded for terrestrial use?perhaps nowhere more potently than in South Africa, where Boer farmers used them to great effect in taking on the might of the British Army. A premature twilight With the turn of the century came a boom in the optical instrument trade as people sought to replace their outmoded binoculars with the new prismatic instruments. That included astronomers, influenced no doubt by the seductive advertising of the manufacturers. The new binoculars exhibited a wide variety of designs as makers jostled to find particular selling points within the restrictions imposed by Abbe’s patent for the Zeiss product. Though no-one said so (except Zeiss), it was tacitly assumed that Abbe’s design was the best. While the new binoculars certainly brought higher magnification than the Galilean type, together with a wider and more uniform field of view, they were not necessarily everything that an astronomer’s heart could desire. As we have seen, the ten air-to-glass surfaces in most binoculars produced severe light-loss, with increased ghosting and flaring. There were other drawbacks. Having more optical components, the new glasses were more easily damaged than the Galilean type. They were also peculiarly susceptible to fungus-growth on their internal optics, especially in moist climates. But most unsatisfactory of all for astronomers was the small size of their objectives, which severely limited their light-grasp. At the turn of the twentieth century, only one manufacturer produced binoculars with objectives more than 20 mm in diameter, and that was Zeiss, who had introduced a range of “large” instruments with 25 mm objectives in 1896. Even these were confined in their astronomical usefulness to little more than the moon and brighter planets?they were effectively only one-inch telescopes. It was a British firm that began the move to larger apertures. In August, 1903, Aitchison introduced a range of binoculars with 35 mm objectives. So revolutionary was this considered that the binoculars were fitted with variable iris diaphragms to allow the light-input to be reduced during the day! Nevertheless, for observing star-clusters and nebulae, they were still eclipsed by a large, light-efficient Galilean. Finally, it was Zeiss who, in 1910, introduced the binocular that matched the diameter of the light-beam emerging from the eyepiece to the 7 mm of a fully-dilated eye-pupil, thereby producing optimum illumination. Its effectiveness at low light levels?even with 30 to 40 percent losses?was quickly demonstrated, and its 7?50 specification soon became the standard for night glasses. These instruments were very expensive when they were first introduced, but no doubt a few became the pride and joy of some well-heeled astronomers. The end of the Edwardian period brought to a close the intense competition between binocular manufacturers that had spawned such a rich variety of designs. Abbe’s patent expired in 1908, and manufacturers quickly adopted the Zeiss “stereo-prism” pattern for their own instruments. Binoculars took on a general uniformity of appearance that lasted for almost half a century. In the same year, German military authorities approved the design of the Fernglas 08 (1908-model binocular), a highly-specified 6? Galilean instrument which, by the standards of the day, was very suitable for astronomical use. It was robustly engineered, and had a high degree of optical correction. But when it eventually emerged from the production- lines of German optical manufacturers, astronomy was the last thing on the minds of its recipients. Tensions had been growing throughout Europe, and armaments had grown alongside them. Just 20 years after the appearance of the first Zeiss binoculars, political brinkmanship cascaded into the slaughter of the Great War. Binoculars?prismatic and Galilean alike?suddenly became optical munitions, and in great demand. It was not long before they faced one another in their scores across the muddy wastes of no-man’s land. For the moment at least, binocular astronomy was but a distant memory. Acknowledgment Some years before Serviss wrote Astronomy with an Opera Glass in America, a certain young woman in England was given a high-quality opera glass of unusual and delicate proportions. It is not known whether she ever turned the instrument on the night sky, but there must have been some predisposition towards astronomy in her family. It eventually manifested itself in the capable and energetic form of her great-nephew?a fellow by the name of Patrick Moore. When Patrick kindly offered to donate his Great-Aunt Alice’s opera glass to my collection of early binoculars, I wondered if there was something I could do in return. The only thing I could think of was this article. So thank you, Patrick! Further Reading Ashbrook, Joseph, “Garrett P. Serviss and Some Brooklyn Amateurs”, Sky and Telescope, 49(1), pp 17-18, 1975. Hearnshaw, J.B., The Measurement of Starlight: Two centuries of Astronomical Photometry, Cambridge, 1996. Moore, Patrick, Exploring the Night Sky with Binoculars, Cambridge, 1986 (and later editions). Serviss, Garrett P., Astronomy with an Opera-Glass, Appleton, New York, 1888 (and later editions). Watson, Fred, Binoculars, Opera Glasses and Field Glasses, Shire Publications, Princes Risborough, 1995. The Dawn of Binocular Astronomy. Figure Captions (see 2001 Yearbook of Astronomy) Figure 1. Light path through a Galilean telescope. The field of view is limited by the diameter of the objective, a drawback unique to the Galilean. However, the simple two-lens construction offers low light-loss, resulting in a bright image. Figure 2. A large Galilean binocular made by Chérubin d’Orléans in the 1670s for the Grand Duke Cosimo III de Medici. It was probably used for astronomy. (© Museum of the History of Science, Florence.) Figure 3. Basic optical system of the Keplerian, or inverting telescope. The objective forms an inverted image, which is magnified by the convex eyepiece. Light from the full area of the objective passes through the exit pupil at the location of the eye. The so-called eye relief is its clearance from the back of the eyepiece. Field of view depends not on the objective diameter, but on the characteristics of the eyepiece, which, in practice, is made with multiple components for better imaging. Figure 4. The terrestrial telescope devised by de Rheita in 1645 is simply a Keplerian telescope with additional lenses to render the image upright. They give the instrument its characteristic length. The ordinary draw-tube telescope works on this principle. Figure 5. An ivory and gilt opera glass dating from the 1870s. Instruments like this used the simple Galilean optical system, and were produced in very large numbers during the second half of the nineteenth century. Towards the end of this period, astronomy with opera glasses became a popular pastime. Figure 6. A late nineteenth-century binocular telescope, or “deer- stalker’s binocular”. Instruments of this type consisted simply of two terrestrial telescopes mounted side-by-side, offering much higher magnification than the Galilean type. However, they were more expensive. They were also rather unwieldy (particularly for astronomy), and prone to misalignment. Figure 7. Ernst Abbe, the German optical physicist who designed the first successful prismatic binocular. He also made significant contributions to microscopy and optical theory. Though he was a professor in the University of Jena, his partnership with the optical instrument maker Carl Zeiss gave his work an immediate practical application. This photograph dates from 1893, the year Abbe applied for his binocular patent. (© Optical Museum, Jena.) Figure 8. Manufacturer’s sectioned view of a Zeiss binocular, showing Abbe’s arrangement of the Porro prisms to increase the separation of the objectives over that of the eyes and enhance the stereoscopic effect. The engraving depicts one of Zeiss’s 1907 models, which had detail improvements over the original 1894 series. It established the pattern of binocular design for decades to come, and countless thousands of people have had their first telescopic views of the night sky through an instrument like this. Figure 9. Goerz binocular of 1899 with an original instruction booklet. It was typical of its day, with an optical arrangement that avoided infringement of Abbe’s patent. The instrument also had an unusual sliding adjustment for the eyepiece separation. Goerz promoted these binoculars for astronomical use, especially for observing variable stars. Biographical Note Dr Fred Watson is Astronomer-in-Charge of the Anglo-Australian Observatory near Coonabarabran, New South Wales. When he observes with the Anglo-Australian Telescope or the UK Schmidt Telescope, binoculars are seldom far out of reach. ================================== 21 August 2002 home page: http://home.europa.com/~telscope/binotele.htm 12