Optical Telescope

Written by Tammy Plotner

What is an optical telescope? How does it work? What types of optical telescopes are there? What are some terms I might encounter when I am studying about this type of telescope? What do they mean? What optical telescopes are famous? If you have questions like these, then follow along as we take a closer look at the optical telescope…

What Is An Optical Telescope?

An optical telescope is an instrument used to gather and focus light from a very specific portion of the electromagnetic spectrum. It generally refers to the wavelengths of light which can be perceived by the human eye. These wavelengths are then magnified and studied "optically" – via direct viewing, photographically, or collected on a photo sensor. There are three primary types of optical telescopes: the refractor, the reflector and the catadioptric telescope.

How Does An Optical Telescope Work?

At the heart of all optical telescopes is the major light gathering source. In the optical refractor telescope, this is called the primary objective lens. In the optical reflector telescope, it is known as the primary mirror. In the optical catadioptric design, it is also a primary mirror. These gather the incoming light from a distant object and focus it back along a path called the focal plane. When the light rays re-converge, they form a "real image" or reach a focal point. This image may then be gathered by the optical telescope's cameras or photo sensors – or it may be magnified by an additional series of lenses called an eyepiece and studied by the human eye.

Terms Associated With An Optical Telescope

There are many terms associated with an optical telescope, but there are a few which might might commonly encounter. While these are far from all you'll find, they will help explain the many different terms expressed by astronomers who use optical telescopes and terms you may find in advertisements or instructions with personal optical telescopes.

Aberrations: Aberrations are nothing more than a flaw in the performance of an optical system. Types of aberration include blurring of the image produced by an image-forming optical system. It occurs when light from one point of an object after transmission through the system does not converge into (or does not diverge from) a single point.

Aperture: Aperture is a simple work that expresses the diameter of the primary light gathering source of any optical telescope – not just the size of the opening. For example, a reflector telescope with a 4.5" mirror would have an aperture of 114mm. A refractor telescope with a front lens diameter of 6" would have an aperture of 150mm.

Chromatic: Chromatic aberration of a lens is seen as "fringes" of color around the image, because each color in the optical spectrum cannot be focused at a single common point on the optical axis.

Coma: Another type of aberration is coma, which derives its name from the comet-like appearance of the aberrated image.

Dawe's Limit: Dawes' limit is a formula to express the maximum resolving power of a microscope or telescope. It is so named for its discoverer, W. R. Dawes, although it is also credited to Lord Rayleigh.

Diffraction Limit: The diffraction limit is the minimum angular separation of two sources that can be distinguished by a telescope depends on the wavelength of the light being observed and the diameter of the telescope.

Focal length and f-ratio: The focal length of an optical telescope is the length of the light path to form a real image. These numbers help to determine how wide the visual field can be achieved with a particular eyepiece or photographic equipment. The f-ratio of an optical telescope is the ratio between the focal length of the telescope and the diameter of the primary light gathering source. Low f-ratios, such as f/4 indicate wide fields of view with low practical magnification limits, while high f-ratios, such as f/10 indicate restricted fields with high practical magnification limits.

Meniscus: Convex-concave (meniscus) lenses can be either positive or negative, depending on the relative curvatures of the two surfaces. To obtain exactly zero optical power, a meniscus lens must have slightly unequal curvatures to account for the effect of the lens' thickness.

Optical Coating: An optical coating is a process of placing thin layer of material on an optical component such as a lens or mirror which alters the way in which the optic reflects and transmits light. One type of optical coating is an antireflection coating, which reduces unwanted reflections from surfaces, and is commonly used on refractor telescope lenses. Another type is the high-reflector coating which can be used to produce mirrors which reflect greater than 99% of the light which falls on them – used in reflector telescopes.

Optical Path: The path that light takes in traversing an optical system is often called the optical path. The physical length of an optical device can be reduced to less than the length of the optical path by using folded optics.

Practical Magnification: The magnification, or power, of the telescope depends on two optional characteristics: the focal length of the main telescope and the focal length of the eyepiece used during a particular observation. Both minimum and maximum practical magnifications can apply and are determined by the telescopes focal ratio.

Resolving Power: Resolving power is the ability of the components of an optical instrument to measure the angular separation of the points in an object. The term resolution or minimum resolvable distance is the minimum distance between distinguishable objects in an image.

Wavefront Error: Wavefront is an imaginary surface joining all points in space that are reached at the same instant by a wave propagating through a medium – such as optics. Wavefront error is expressed in fractions, such as 1/25, which reveals the optics abilities to accurately focus the light waves.

Just A Few Famous Optical Telescopes

Are you interested in some of the most famous optical telescope of all? While some of these telescopes also equipped to use other portions of the electromagnetic spectrum, like all humans, we enjoy what we can see! How many of these do you recognize?

Hubble Space Telescope: The Hubble Space Telescope is a 2.4 meter aperture telescope in space. Images are not limited by atmospheric seeing. They are therefore diffraction limited. The seeing of typical HST images is resolved at about 0.1 arcsec. Observations can be made at wavelengths that are blocked by the atmosphere from the ground, particularly in the ultraviolet.

Kitt Peak: Kitt Peak National Observatory (KPNO), part of the National Optical Astronomy Observatory (NOAO), supports the most diverse collection of astronomical observatories on Earth for nighttime optical and infrared astronomy and daytime study of the Sun. Founded in 1958, KPNO operates three major nighttime telescopes, shares site responsibilities with the National Solar Observatory and hosts the facilities of consortia which operate 19 optical telescopes and two radio telescopes. (See the Tenant Observatories list.) Kitt Peak is located 56 miles southwest of Tucson, AZ, in the Schuk Toak District on the Tohono O'odham Nation and has a Visitor Center open daily to the public.

Gemini Observatory: The Gemini Observatory consists of twin 8-meter optical/infrared telescopes located on two of the best sites on our planet for observing the universe. Together these telescopes can access the entire sky. The Gemini South telescope is located at almost 9,000' elevation on a mountain in the Chilean Andes called Cerro Pachon. Cerro Pachon shares resources with the adjacent SOAR Telescope and the nearby telescopes of the Cerro Tololo Inter-American Observatory. The Frederick C. Gillett Gemini North Telescope is located on Hawaii's Mauna Kea as part of the international community of observatories that have been built to take advantage of the superb atmospheric conditions on this long dormant volcano that rises almost 14,000' into the dry, stable air of the Pacific. The Gemini Observatory's international headquarters is located in Hilo, Hawaii at the University of Hawaii at Hilo's University Park.

Palomar Observatory: The Palomar Observatory is located in northern San Diego county, about 70 miles northeast of the San Diego airport. It has been in operation since 1948. The 200-inch (5 meter) Hale telescope is jointly operated by Cornell University, the California Institute of Technology and the Jet Propulsion Laboratory. Cornell astronomers have access to one quarter of the observing time, and use the 5m to conduct observations in the optical and infrared wavelength regimes.

Keck Observatory: From a remote outpost on the summit of Hawaii's dormant Mauna Kea volcano, astronomers at the W. M. Keck Observatory probe the deepest regions of the Universe with unprecedented power and precision. Their instruments are the twin Keck Telescopes, the world's largest optical and infrared telescopes. Each stands eight stories tall and weighs 300 tons, yet operates with nanometer precision. At the heart of each Keck Telescope is a revolutionary primary mirror. Ten meters in diameter, the mirror is composed of 36 hexagonal segments that work in concert as a single piece of reflective glass.

Lick Observatory: The University of California's Lick Observatory, located in the Diablo Range east of San Jose, California, has a long and fascinating history. The legacy of the eccentric California millionaire James Lick, the Observatory was founded in 1888 and has been part of the University of California ever since. Lick Observatory has grown to keep pace with the changing demands of astronomy, and, after more than a century of operation, remains among the most productive research observatories in the world. Lick Observatory is open to daytime visitors nearly every day of the year. The Observatory is closed to the public on Thanksgiving Day, Christmas Eve and Christmas Day, and at night after 5:00pm.

Yerkes Observatory: By chance the new Professor of Astrophysics at the University of Chicago, George Hale, discovers that two optically perfect disks are available. These 42-inch "blanks" could be ground to create a 40-inch refracting telescope, the largest in the world. He and the dynamic president of the university set off to find a donor willing to purchase these disks; build the telescope; and pay for a "suitable observatory" to house the World's Largest Telescope. Yerkes Observatory occupies a unique niche for the education and the scientific community. It bridges several important perspectives in formal and informal education. The history of astronomy and astrophysics of the observatory is a solid foundation for introducing all the important topics in current research as well as the practice of observational astronomy.

Mount Graham International Observatory: The Mt. Graham International Observatory is located on Mt. Graham in the Pinaleno Mountains near Safford , Arizona. The observatory site is operated by the University of Arizona. Mt. Graham is part of the Coronado National Forest . The construction of the Observatory was approved by Congress in November of 1988. Two telescopes are now in operation, the Vatican Observatory/Arizona 1.8m Lennon optical telescope (VATT) and the 10m diameter Heinrich Hertz Submillimeter Telescope (SMT), a joint project of Arizona and the Max-Planck-Institut für Radioastronomie, Germany. The preliminary indications are that both the site and the telescopes will reach highest expectations. The carbon fiber Hertz telescope is proving very stable, and the surface adjustment has surpassed its goal of 15 microns rms. The Lennon telescope with its f/1 primary figured to 17 nm rms is now in regular astronomical use. The third and largest telescope for Mt Graham is the 2 x 8.4 Large Binocular Telescope (LBT); it is a partnership between Arizona, Ohio, Italy (Arcetri), Germany and the Research Corporation.

South African Astronomical Observatory: The South African Astronomical Observatory (SAAO) is the national centre for optical and infrared astronomy in South Africa. It is a facility of the National Research Foundation under the Department of Science and Technology. Its prime function is to conduct fundamental research in astronomy and astrophysics by providing a world-class facility and by promoting astronomy and astrophysics in Southern Africa. SAAO headquarters are in the suburb of Observatory in Cape Town. The main telescopes used for research are located at the SAAO observing station near Sutherland in the Northern Cape, a 4 hour drive from Cape Town. The Southern African Large Telescope (SALT) is the largest single optical telescope in the southern hemisphere, with a hexagonal mirror array 11 metres across. Although very similar to the Hobby-Eberly Telescope (HET) in Texas, SALT has a redesigned optical system using more of the mirror array. It will be able to record distant stars, galaxies and quasars a billion times too faint to be seen with the unaided eye – as faint as a candle flame at the distance of the moon.

Galileo's Telescope

From Antiquity to the 16 th century, instruments for astronomical observation underwent minimal changes. Apart from some perfecting touches devised by individual astronomers, they consisted of revolving frames fitted with graduate scales and appropriate sights. These instruments, which equipped the major Islamic and European observatories, allowed the positions of the heavenly bodies to be precisely determined. Using quadrants, sextants, rings and rules of various kinds and sizes, the astronomer carried out his work, which consisted of updating the numerical parameters that governed the motion of the stars.

In this scientific environment, apart from the general structure of the Universe in which an astronomer could believe, the heavens were considered to have been fully explored. At the start of the 17 th century it was thought that all of the existing planets were known and all of the fixed stars had been identified and catalogued. Not even the most attentive and systematic observation of the sky would ever have revealed anything new, apart from some sporadic comet or nova. No one could ever have imagined what wondrous new things were about to be revealed by an instrument created by inserting two eyeglass lenses into the ends of a tube.

Galileo and the Telescope

• Galileo's Telescopes
• Galileo's Observations
• Further Information
• Questions

The science of astronomy took a huge leap forward in the first decade of the 1600s with the invention of the optical telescope and its use to study the night sky. Galileo Galilei did not invent the telescope but was the first to use it systematically to observe celestial objects and record his discoveries. His book, Sidereus nuncius or The Starry Messenger was first published in 1610 and made him famous. In it he reported on his observations of the Moon, Jupiter and the Milky Way. These and subsequent observations and his interpretations of them eventually led to the demise of the geocentric Ptolemaic model of the universe and the adoption of a heliocentric model as proposed in 1543 by Copernicus.

Galileo's drawings of phases of the Moon.
Question: What features are visible here that cannot be seen with the unaided eye?

Galileo's Telescopes

The basic tool that Galileo used was a crude refracting telescope. His initial version only magnified 8x but was soon refined to the 20x magnification he used for his observations for Sidereus nuncius. It had a convex objective lens and a concave eyepiece in a long tube. The main problem with his telescopes was their very narrow field of view, typically about half the width of the Moon.

Galileo's drawing of the optical path of his telescope

The earliest known sketch of a telescope, August 1609.

One of Galileo's telescopes. The focal length is 1330 mm with a 26 mm aperture, it magnifies 14x. It has an objective bi-convex lens and a plano-concave eyepiece.

Galileo's Observations

Galileo made several key discoveries through his systematic use and refinement of the telescope.

The Moon

According to Aristotelian principles the Moon was above the sub-lunary sphere and in the heavens, hence should be perfect. He found the "surface of the moon to be not smooth, even and perfectly spherical,...,but on the contrary, to be uneven, rough, and crowded with depressions and bulges. And it is like the face of the earth itself, which is marked here and there with chains of mountains and depths of valleys." He calculated the heights of the mountains by measuring the lengths of their shadows and applying geometry.

One of Galileo's lunar drawings.
Note the craters, mountains and mare or "seas". The terminator between lunar day and night is clearly seen down the centre.

Moons of Jupiter

Observations of the planet Jupiter over successive night revealed four star-like objects in a line with it. The objects moved from night to night, sometimes disappearing behind or in front of the planet. Galileo correctly inferred that these objects were moons of Jupiter and orbited it just as our Moon orbits Earth. For the first time, objects had been observed orbiting another planet, thus weakening the hold of the Ptolemaic model. Today these four moons are known as the Galilean satellites; Io, Europa, Ganymede and Callisto.

Galileo's drawings of the moons of Jupiter of successive nights

The Phases of Venus

Venus was observed to go through a sequence of phases similar to the Moon. This could not be explained in the Ptolemaic model but could be accounted for by either the Sun-centered Copernican model or the Earth-centered Tychonic model that had the other planets orbiting the Sun as it orbited the Earth. Galileo rejected Tycho's model as an unnecessary hybrid and used the discovery to consolidate his support of the Copernican model.

Sunspots

Along with contemporaries such as Thomas Harriot, David Frabicius and Christoph Scheiner, Galileo observed dark regions that appeared to move across the surface of the Sun. Debate centered on whether these were satellites of the Sun or actual spots on its surface. Galileo, in his Letters on Sunspots supported the sunspot interpretation and used it to show that the Sun was rotating. Its blemishes and imperfections again undermined the Aristotelian ideal of a perfect cosmos.

"Appendages" on Saturn

Galileo noted two appendages from the sides of Saturn. These disappeared then later reappeared. It was not until 1656 that the Dutch scientist, Christiaan Huygens correctly described them as rings.

Stars in the Milky Way

Even through a telescope the stars still appeared as points of light. Galileo suggested that this was due to their immense distance from Earth. This then eased the problem posed by the failure of astronomers to detect stellar parallax that was a consequence of Copernicus' model. On turning his telescope to the band of the Milky Way Galileo saw it resolved into thousands of hitherto unseen stars. This posed the question as to why there were invisible objects in the night sky?

More stars are resolved in this drawing by Galileo of the Pleiades than are visible to the unaided eye.

Further Information

1. An excellent online source for all things related to Galileo is: The Galileo Project. It is hosted by Rice University and includes his writings, details on his experiments and observations and links. It is also the source for the image of Galileo at the top of this page.
2. Another worthwhile site is The Art of Renaissance Science: Galileo and Perspective. It has a wealth of diagrams matched with clear. concise text. There are some animations of his experiments.
3. The Institute and Museum of the History of Science in Florence has a wealth of detail on the history of astronomy including Galileo's work. Their new website is worth exploring. Much of it is in English although some sections, including an excellent simulation of early telescopes, is only available in Italian at present. They also have an excellent new site: Galileo's Telescope, the Instrument that Changed the World.
4. Afocal CCD Images Through a Galilean Telescope is an excellent resource that provides CCD images that approximate what the human eye would see through a Galilean telescope. It has images of the Sun, Moon, Venus, stars and nebulae. The site provides historical background and technical details.
5. Galileo's Sidereus nuncius or The Sidereal messenger is available in translation by A. van Helden from University of Chicago Press, 1989.
6. The Cambridge Illustrated History of Astronomy, ed Michael Hoskin, Cambridge University Press, 1997 provides a wealth of information and diagrams and is an authoritative yet readable guide to the topic.
7. The Sleepwalkers by Arthur Koestler is a classic book dealing with the development of astronomical thought up to the time of Newton.

There are numerous other books and web sites covering Galileo's work and the history of astronomy.

Questions

1. Why was the telescope an advance over naked-eye astronomy?
2. What did Galileo's observations of the Moon reveal?
3. What was the significance of Galileo observing phases of Venus?
4. How did Galileo's observations help undermine the existing paradigm of the Ptolemaic model of the Universe and Aristotle's physics?

The Galileo Project > Telescope

The Telescope

The telescope was one of the central instruments of what has been called the Scientific Revolution of the seventeenth century. It revealed hitherto unsuspected phenomena in the heavens and had a profound influence on the controversy between followers of the traditional geocentric astronomy and cosmology and those who favored the heliocentric system of Copernicus. It was the first extension of one of man's senses, and demonstrated that ordinary observers could see things that the great Aristotle had not dreamed of. It therefore helped shift authority in the observation of nature from men to instruments. In short, it was the prototype of modern scientific instruments. But the telescope was not the invention of scientists; rather, it was the product of craftsmen. For that reason, much of its origin is inaccessible to us since craftsmen were by and large illiterate and therefore historically often invisible.

Although the magnifying and diminishing properties of convex and concave transparent objects was known in Antiquity, lenses as we know them were introduced in the West [1] at the end of the thirteenth century. Glass of reasonable quality had become relatively cheap and in the major glass-making centers of Venice and Florence techniques for grinding and polishing glass had reached a high state of development. Now one of the perennial problems faced by aging scholars could be solved. With age, the eye progressively loses its power to accommodate, that is to change its focus from faraway objects to nearby ones. This condition, known as presbyopia, becomes noticeable for most people in their forties, when they can no longer focus on letters held at a comfortable distance from the eye. Magnifying glasses became common in the thirteenth century, but these are cumbersome, especially when one is writing. Craftsmen in Venice began making small disks of glass, convex on both sides, that could be worn in a frame--spectacles. Because these little disks were shaped like lentils, they became known as "lentils of glass," or (from the Latin) lenses. The earliest illustrations of spectacles date from about 1350, and spectacles soon came to be symbols of learning.

The Spectacle Vendor by Johannes Stradanus, engraved by Johannes Collaert, 1582 [click for larger image]

These spectacles were, then, reading glasses. When one had trouble reading, one went to a spectacle-maker's shop or a peddler of spectacles (see figs. 2 and 3) and found a suitable pair by trial and error. They were, by and large, glasses for the old. spectacles for the young, concave lenses[2] that correct the refractive error known as myopia, were first made (again in Italy) in the middle of the fifteenth century. So by about 1450 the ingredients for making a telescope were there. The telescopic effect can be achieved by several combinations of concave and convex mirrors and lenses. Why was the telescope not invented in the fifteenth century? There is no good answer to this question, except perhaps that lenses and mirrors of the appropriate strengths were not available until later.

In the literature of white magic, so popular in the sixteenth century, there are several tantalizing references to devices that would allow one to see one's enemies or count coins from a great distance. But these allusions were cast in obscure language and were accompanied by fantastic claims; the telescope, when it came, was a very humble and simple device. It is possible that in the 1570s Leonard and Thomas Digges in England actually made an instrument consisting of a convex lens and a mirror, but if this proves to be the case, it was an experimental setup that was never translated into a mass-produced device.[3]

The earliest known illlustration of a telescope. Giovanpattista della Porta included this sketch in a letter written in August 1609 [click for larger image]

The telescope was unveiled in the Netherlands. In October 1608, the States General (the national government) in The Hague discussed the patent applications first of Hans Lipperhey of Middelburg, and then of Jacob Metius of Alkmaar, on a device for "seeing faraway things as though nearby." It consisted of a convex and concave lens in a tube, and the combination magnified three or four times.[4] The gentlemen found the device too easy to copy to award the patent, but it voted a small award to Metius and employed Lipperhey to make several binocular versions, for which he was paid handsomely. It appears that another citizen of Middelburg, Sacharias Janssen had a telescope at about the same time but was at the Frankfurt Fair where he tried to sell it.

Galileo's telescopes [click here for larger image]

The news of this new invention spread rapidly through Europe, and the device itself quickly followed. By April 1609 three-powered spyglasses could be bought in spectacle-maker's shops on the Pont Neuf in Paris, and four months later there were several in Italy. (fig. 4) We know that Thomas Harriot observed the Moon with a six-powered instrument early in August 1609. But it was Galileo who made the instrument famous. He constructed his first three-powered spyglass in June or July 1609, presented an eight-powered instrument to the Venetian Senate in August, and turned a twenty-powered instrument to the heavens in October or November. With this instrument (fig. 5) he observed the Moon, discovered four satellites of Jupiter, and resolved nebular patches into stars. He published Sidereus Nuncius in March 1610.

Verifying Galileo's discoveries was initially difficult. In the spring of 1610 no one had telescopes of sufficient quality and power to see the satellites of Jupiter, although many had weaker instruments with which they could see some of the lunar detail Galileo had described in Sidereus Nuncius. Galileo's lead was one of practice, not theory, and it took about six months before others could make or obtain instruments good enough to see Jupiter's moons. With the verification of the phases of Venus by others, in the first half of 1611, Galileo's lead in telescope-making had more or less evaporated. The next discovery, that of sunspots, was made by several observers, including Galileo, independently.

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A typical Galilean telescope with which Jupiter's moons could be observed was configured as follows. It had a plano-convex objective (the lens toward the object) with a focal length of about 30-40 inches., and a plano-concave ocular with a focal length of about 2 inches. The ocular was in a little tube that could be adjusted for focusing. The objective lens was stopped down to an aperture of 0.5 to 1 inch. , and the field of view was about 15 arc-minutes (about 15 inches in 100 yards). The instrument's magnification was 15-20. The glass was full of little bubbles and had a greenish tinge (caused by the iron content of the glass); the shape of the lenses was reasonable good near their centers but poor near the periphery (hence the restricted aperture); the polish was rather poor. The limiting factor of this type of instrument was its small field of view--about 15 arc-minutes--which meant that only a quarter of the full Moon could be accommodated in the field. Over the next several decades, lens-grinding and polishing techniques improved gradually, as a specialized craft of telescope makers slowly developed. But although Galilean telescopes of higher magnifications were certainly made, they were almost useless because of the concomitant shrinking of the field.

As mentioned above, a the telescopic effect can be achieved with different combinations of lenses and mirrors. As early as 1611, in his Dioptrice, Johannes Kepler had shown that a telescope could also be made by combining a convex objective and a convex ocular. He pointed out that such a combination would produce an inverted image but showed that the addition of yet a third convex lens would make the image erect again. This suggestion was not immediately taken up by astronomers, however, and it was not until Christoph Scheiner published his Rosa Ursina in 1630 that this form of telescope began to spread. In his study of sunspots, Scheiner had experimented with telescopes with convex oculars in order to make the image of the Sun projected through the telescope erect.[5] But when he happened to view an object directly through such an instrument, he found that, although the image was inverted, it was much brighter and the field of view much larger than in a Galilean telescope. Since for astronomical observations an inverted image is no problem, the advantages of what became known as the astronomical telescope led to its general acceptance in the astronomical community by the middle of the century.

The Galilean telescope could be used for terrestrial and celestial purposes interchangeably. This was not true for the astronomical telescope with its inverted image. Astronomers eschewed the third convex lens (the erector lens) necessary for re-inverting the image because the more lenses the more optical defects multiplied. In the second half of the seventeenth century, therefore, the Galilean telescope was replaced for terrestrial purposes by the "terrestrial telescope," which had four convex lenses: objective, ocular, erector lens, and a field lens (which enlarged the field of view even further).

Hevelius's 60- and 140-foot telescopes (Machina Coelestis, 1673) [click for larger image]

With the acceptance of the astronomical telescope, the limit on magnification caused by the small field of view of the Galilean telescope was temporarily lifted, and a "telescope race" developed. Because of optical defects, the curvature of lenses had to be minimized, and therefore (since the magnification of a simple telescope is given roughly by the ratio of the focal lengths of the objective and ocular) increased magnification had to be achieved by increasing the focal length of the objective. Beginning in the 1640s, the length of telescopes began to increase. From the typical Galilean telescope of 5 or 6 feet in length, astronomical telescopes rose to lengths of 15 or 20 feet by the middle of the century. A typical astronomical telescope is the one made by Christiaan Huygens, in 1656. It was 23 feet long; its objective had an aperture of several inches, it magnified about 100 times, and its field of view was 17 arc-minutes.

Aerial telescope (Christiaan Huygensm Astroscopium Compendiaria,1684) [click for larger image]

Telescopes had now again reached the point where further increases in magnification would restrict the field of view of the instrument too much. This time another optical device, the field lens came to the rescue. Adding a third convex lens--of appropriate focal length, and in the right place--increased the field significantly, thus allowing higher magnifications. The telescope race therefore continued unabated and lengths increased exponentially. By the early 1670s, Johannes Hevelius had built a 140-foot telescope.

But such long telescopes were useless for observation: it was almost impossible to keep the lenses aligned and any wind would make the instrument flutter. After about 1675, therefore, astronomers did away with the telescope tube. The objective was mounted on a building or pole by means of a ball-joint and aimed by means of a string; the image was found by trial and error; and the compound eyepiece (field lens and ocular), on a little stand, was then positioned to receive the image cast by the objective. Such instruments were called "aerial telescopes."

Although some discoveries were made with these very long instruments, this form of telescope had reached its limits. By the beginning of the eighteenth century very long telescopes were rarely mounted any more, and further increases of power came, beginning in the 1730s, from a new form of telescope, the reflecting telescope.

Since it was known that the telescopic effect could be achieved using a variety of combinations of lenses and mirrors, a number of scientists speculated on combinations involving mirrors. Much of this speculation was fueled by the increasingly refined theoretical study of the telescope. In his Dioptrique, appended to his Discourse on Method of 1637, René Descartes addressed the problem of spherical aberration, already pointed out by others. In a thin spherical lens, not all rays from infinity--incident parallel to the optical axis--are united at one point. Those farther from the optical axis come to a focus closer to the back of the lens than those nearer the optical axis. Descartes had either learned the sine law of refraction from Willebrord Snell (Snell's Law)[6] or had discovered it independently, and this allowed him to quantify spherical aberration. In order to eliminate it, he showed, lens curvature had to be either plano-hyperboloidal or spherico-ellipsoidal. His demonstration led many to attempt to make plano-hyperboloidal objectives,[7] an effort which was doomed to failure by the state of the art of lens-grinding. Others began considering the virtues of a concave paraboloidal mirror as primary receptor: it had been known since Antiquity that such a mirror would bring parallel incident rays to a focus at one point.

Newton's reflecting telescope (1671) [click for larger image]

A second theoretical development came in 1672, when Isaac Newton published his celebrated paper on light and colors. Newton showed that white light is a mixture of colored light of different refrangibility: every color had its own degree of refraction. The result was that any curved lens would decompose white light into the colors of the spectrum, each of which comes to a focus at a different point on the optical axis. This effect, which became known as chromatic aberration, resulted in a central image of, e.g., a planet, being surrounded by circles of different colors. Newton had developed his theory of light several years before publishing his paper, when he had turned his mind to the improvement of the telescope, and he had despaired of ever ridding the objective of this defect. He therefore decided to try a mirror, but unlike his predecessors he was able to put his idea into practice. He cast a two-inch mirror blank of speculum metal (basically copper with some tin) and ground it into spherical curvature. He placed it in the bottom of a tube and caught the reflected rays on a 45° secondary mirror which reflected the image into a convex ocular lens outside the tube (see fig. 12). He sent this little instrument to the Royal Society, where it caused a sensation; it was the first working reflecting telescope. But the effort ended there. Others were unable to grind mirrors of regular curvature, and to add to the problem, the mirror tarnished and had to be repolished every few months, with the attending danger of damage to the curvature.

Hevelius's rooftop observatory, (Machina Coelestis, 1673) [click for larger image]

The reflecting telescope therefore remained a curiosity for decades. In second and third decades of the eighteenth century, however, the reflecting telescope became a reality in the hands of first James Hadley and then others. By the middle of the century, reflecting telescopes with primary mirrors up to six inches in diameter had been made. It was found that for large aperture ratios (the ratio of focal length of the primary to its aperture, as the f-ratio in modern cameras for instance), f/10 or more, the difference between spherical and paraboloidal mirrors was negligible in the performance of the telescope. In the second half of the eighteenth century, in the hands of James Short and then William Herschel, the reflecting telescope with parabolically ground mirrors came into its own.

Notes: [1]They may have developed independently in China.
[2]Note that the word lens was used only to denote convex lenses until the end of the seventeenth century.
[3]The claim for an "Elizabethan telescope" has recently been made by Colin Ronin, who has demonstrated an instrument based on the writings of Thomas Digges and William Bourne.
[4]Their optical system and magnification was the same as our traditional opera glasses
[5]The Galilean telescope produces an erect image of an object viewed directly but an inverted image of a projected object; by substituting a convex for the concave ocular, this situation is reversed.
[6]The ratio of the sines of the angles of incidence and refraction is constant.
[7]The effect is most apparent for the objective; spherical aberration in the ocular affects the image much less.

Sources: For the invention of spectacles, see Edward Rosen, "The Invention of Eyeglasses," Journal for the History of Medicine and Allied Sciences, 11(1956):13-46, 183-218. The appearance of spectacles with concave lenses is discussed in Vincent Ilardi, "Eyeglasses and Concave Lenses in Fifteenth-Century Florence and Milan: New Documents," Renaissance Quarterly 29(1976):341-360. The entire problem of the invention of the telescope is discussed in Albert van Helden, The Invention of the Telescope, in Transactions of the American Philosophical Society, 67, no. 4 (1977). See also Van Helden, "The `Astronomical Telescope,' 1611-1650," Annali dell'Istituto e Museo di Storia della Scienza di Firenze, 1, no. 2 (1976):13-36; and "The Development of Compound Eyepieces, 1640-1670," Journal for the History of Astronomy, 8(1977):26-37. The most convenient source for information on the general development of the telescope is Henry King, The History of the Telescope (London: Griffin, 1955).

The Telescope

The diagram of the optical principles of the telescope from Sidereus Nuncius.

Image by kind permission of the Master and Fellows of Trinity College Cambridge.

The story of Galileo's telescope is well known, as he recounted it himself in the Starry Messenger. In July 1609, Galileo was in Venice, when he heard of an invention that allowed distant objects to be seen as distinctly as if they were nearby. In October 1608, a Flemish spectacle-maker by the name of Hans Lipperhey had already applied for a patent (which was refused), and news of the gadget was widespread in Europe by the time Galileo had heard of it. Around the same time, a foreigner turned up in Padua with the instrument; Galileo rushed back to Padua, only to learn that the foreigner had gone to Venice to sell his instrument. Galileo's friend, Paolo Sarpi, had advised the Venetian government against purchasing the instrument from the foreigner, since Galileo could at least match such an invention. By then, Galileo had worked out the principle of the telescope and returned to Venice himself with an eight-power telescope. The Venetian government doubled his salary, though Galileo felt that the original conditions were not honoured.

Galileo gradually improved the power of his telescope, grinding lenses himself, and began observing the heavens. In the first two months of 1610, he was writing The Starry Messenger, and by 12 March, the book was already printed at Venice, dedicated to Cosimo de' Medici. Galileo continued his observations with his telescope, some of which he conveyed in ciphers to Johannes Kepler, who had already responded enthusiastically with the Conversation with Galileo's Sidereal Messenger. Galileo's discovery of the 'handles' of Saturn was encoded in 'Smaismrmilmepoetaleumibunenugttaviras', which could be unscrambled as 'Altissimum planetam tergeminum observavi': I have observed the highest of the planets three-formed. Kepler deciphered this within one letter as 'Salve umbistineum geminatum Martia proles': 'Be greeted, double knob, children of Mars.' For the discovery of the phases of Venus, the code 'Haec immatura a me jam frustra leguntur oy' (this was already tried by me in vain too early) hid the message, 'Cynthiae figurae aemulatur mater amorum' (The mother of lovers [Venus] imitates the shapes of Cynthia [the moon]). Despite this exchange, Galileo never accepted Kepler's elliptical orbits.

From 1616, Galileo tried to apply his knowledge of the satellites of Jupiter to the determination of longitude at sea. In order to ensure observation at sea, the Tuscan arsenal made for Galileo a headgear which had a telescope attached. Around this time, he also designed a brass 'Jovilabe', a computing device for prediction positions of the satellites. He hoped to gain support from the Spanish crown for this project, but failed.

Recommended Reading

Galileo Galilei, The Starry Messenger, translated by A. van Helden, Chicago 1989