< PREV | NEXT > | INDEX | SITEMAP | GOOGLE | LINKS | UPDATES | BLOG | EMAIL | $Donate? | HOME

[2.0] Refining The Telescope

v3.0.0 / chapter 2 of 5 / 01 nov 16 / greg goebel

* The first generation of telescopes was marked by crude optics and limited capabilities. By the end of the 17th century, however, telescope technology had seen what would eventually amount to a major advance in the form in the form of Isaac Newton's "reflector" telescope. It was, in its initial form, not that big of an improvement over the older lens-based refractor telescopes, but it helped set off a race between reflector and refractor telescopes that would lead to continuous advances in both technologies over the next two centuries.

The late 19th century also saw the development of two auxiliary technologies that would have major impact on astronomy: photography, which allowed the recording of imagery obtained through telescopes, and spectroscopy, which permitted the chemical analysis of remote objects.

Rosse's Leviathan


[2.1] NEWTON & THE INVENTION OF THE REFLECTING TELESCOPE
[2.2] PRACTICAL REFLECTORS / ACHROMATIC REFRACTORS
[2.3] HERSCHEL'S REFLECTORS
[2.4] FRAUNHOFER'S REFRACTORS / STELLAR PARALLAX
[2.5] ADVANCES IN REFLECTORS
[2.6] PHOTOGRAPHY & ASTRONOMY
[2.7] THE INVENTION OF SPECTROSCOPY

[2.1] NEWTON & THE INVENTION OF THE REFLECTING TELESCOPE

* While astronomers made discoveries with their long refracting telescopes, work on alternatives was under way. Isaac Newton thought with good reason that the telescopes of his era left something to be desired and decided there had to be a better way of doing things. The lens-based "refracting" telescopes used to that time suffered from the problems of spherical and chromatic aberration. Spherical aberration was seen clearly at the time as a technology problem that could be addressed if someone figured out better ways to grind lenses, but chromatic aberration was a much tougher nut to crack. In fact, Newton's experiments in optics convinced him that it was a inescapable flaw of the refracting telescope and couldn't ever be fixed.

He was wrong, but even in being wrong he came up with something new and ingenious. Newton knew that mirrors could be used to focus light, and that reflection, unlike refraction, didn't cause chromatic aberration: light of all colors reflected at exactly the same angle. Why not, then, build a telescope using mirrors instead of lenses? It would get rid of the chromatic aberration. Of course, it wasn't any easier to grind a mirror than it was to grind a lens in those days and spherical aberration remained a problem, but there was no inherent obstacle to fixing that problem over the long run.

There was a problem with building a telescope using mirrors. It's not possible to see through a mirror, and so a "reflecting" telescope had to have some means of diverting the reflected image to an eyepiece, which complicated its construction. Newton was not actually the first to try to build a reflecting telescope; the Scots astronomer James Gregory (1638:1675) had proposed one in 1663. Gregory's reflecting telescope featured a main or "primary" mirror with a hole in the center at the bottom of the telescope tube, and a small concave "secondary" mirror mounted on struts in the center of the top of the tube. The telescopic image was collected by the primary mirror to be focused on the secondary mirror, with the image passing through its focus on the way to the secondary mirror. The secondary mirror then bounced it back through the hole in the primary and into an eyepiece. However, the optical technology of the time was simply not up to the task, and Gregory couldn't get his idea to work. It was still a perfectly sound idea and effective "Gregorian" telescopes would eventually be built. Newton chose a simpler configuration in hopes of actually getting something to work, and he did.

His first problem was fabricating a primary mirror. Bronze, an alloy of copper and tin, seemed like an appropriate material, since it was cheap, could be polished to a fine shine, and didn't corrode quickly. Its color would have tinted imagery, however, and so Newton added arsenic to turn the alloy white. His "speculum (mirror)" metal was an alloy of copper, tin, and arsenic in the ratio 6:2:1. Newton ground the mirror to a spherical surface, and then mounted in the bottom of a telescope tube. He then mounted a small flat secondary mirror in the center of the top of the tube, with the mirror set at an angle of 45 degrees to the axis of the tube to reflect the image into an eyepiece at the side of the top.

Newton completed his reflecting telescope, the first workable reflecting telescope ever made, in 1668. He was a handy fellow, a competent experimentalist as well as a brilliant theoretician, and built the thing himself. It was only about 15 centimeters long and had a mirror with a diameter of only 2.5 centimeters, but it was much more compact than the contemporary long refractors and could still provide a magnification of 40 times.

Newton was notoriously secretive and reluctant to publicize his research, but word of his reflecting telescope leaked out, and he was eventually forced to "go public". He built a larger reflecting telescope, featuring a mirror with a diameter of 5 centimeters, and demonstrated his "Newtonian reflector" to the British Royal Society on 11 January 1672, accompanying the demonstration with a lecture on his optical studies. Newton's second reflector still survives as a priceless museum piece.

In the meantime, a Frenchman named N. Cassegrain (dates unknown) came up with a third configuration for a reflecting telescope. As it would eventually emerge, the "Cassegrainian reflector" was like a Gregorian reflector, with a secondary mirror bouncing an image back down through a hole in the center of the primary mirror, but instead of a concave secondary mirror as in the Gregorian telescope, Cassegrain used a concave mirror. This allowed the secondary mirror to be placed before the focus of the image and made for a much shorter telescope.

* The reflecting telescope seems in hindsight to have been such a slick idea that it might be thought it immediately pushed refracting telescopes to second place, but early reflectors had significant limitations. First, the reflectivity of the speculum metals available at the time was poor; Newton's primary mirror had a reflectivity of only 16%. This was a major drawback for viewing faint objects in the sky. Second, speculum metal tended to tarnish, meaning that a reflecting telescope had to be broken down periodically so its optics could be polished. Finally, it wasn't easy to incorporate a micrometer into a reflecting telescope. Refracting and reflecting telescopes would remain in competition for two centuries, and at the outset of the race the reflector was seen as not much more than an interesting toy.

Newtonian telescope

Incidentally, modern manufacturers of amateur telescopes often advertise telescopes that look exactly like Newtonian reflectors, but call them "Dobsonians" instead. In fact, a Dobsonian really is a Newtonian reflector, pure and simple, but it uses a specific type of "alt-azimuth" (gun-style) mounting invented by an amateur astronomer named John Dobson in the 1950s. The Dobson mounting is simple, cheap, effective, and popular with amateurs.

BACK_TO_TOP

[2.2] PRACTICAL REFLECTORS / ACHROMATIC REFRACTORS

* The first reflecting telescope to be regarded as a serious instrument was built by an English mathematician named John Hadley (1682:1744), and unveiled to the British Royal Society in 1721. His Gregorian-type reflecting telescope had a mirror made of speculum metal and 15 centimeters in diameter. Members of the society found Hadley's telescope to be almost as good as the society's long refractor, which had lenses ground by Huygens. The long refractor had a bigger aperture and gave brighter images, but the reflector was vastly easier to handle, being only 1.8 meters long. The refractor was 37.5 meters long!

As or more significantly, Hadley's reflector was the first telescope to seriously tackle the problem of spherical aberration. Hadley's breakthrough was to devise an optical test, placing a solid sheet above the mirror with a pinhole at the desired mirror focus. The focus of the mirror was where all parallel rays of light falling on the mirror would be reflected and come together; similarly, light rays emitted by the focus would all be reflected back upward in parallel. Hadley could look at the mirror to find any spots of inconsistent brightness and grind them down accordingly.

Hadley's efforts with reflectors were surpassed by those of a Scots craftsman named James Short (1710:1768). Short established procedures to grind parabolic mirrors conveniently and accurately, and by 1740 he was building them full-time, selling them to both professionals and amateurs. The largest of them had apertures of 45 centimeters, though the biggest were generally bought by the wealthy as toys and were not put to particularly good use.

* These reflectors were vastly superior to the long refractors. Work on fitting micrometers to them also made substantial progress, and it might have seemed at the time that the reflectors would put refractors out of business for good, except for spyglasses and the like. To be sure, improved optical tests and grinding techniques promised to provide parabolic lenses that would both cut the length of a refracting telescope to reasonable lengths and get rid of spherical aberration, but what about chromatic aberration? Newton had believed that refractors would always be cursed by it, and such was his authority that few questioned him.

It is obviously true that the index of refraction of a lens varies with the frequency of the light; if it didn't, prisms wouldn't cast a rainbow and there would be no chromatic aberration. What is not so obvious was that the variation in index of refraction with frequency varies between different types of materials, or in other words some types of glass have much greater chromatic dispersion than others. An English lawyer and mathematician named Chester Moor Hall (1703:1771) got the idea that a combination of lenses using different types of glass might help solve the problem of chromatic aberration.

Hall's research showed that the chromatic dispersion of "flint" glass, a sort of glass incorporating lead compounds, was much greater than that of ordinary "crown" glass. He thought to make a biconvex lens out of crown glass and a concave-convex lens out of flint glass, with the two lenses designed to fit together. Light rays would pass through the convex lens towards a focus, suffering chromatic dispersion; the rays would then pass through the concave surface of the concave-convex lens, where they would diverge again slightly, to be bent back to a focus on the convex side of the lens. However, the passage through the flint glass component of the lens eliminated the chromatic dispersion.

Hall built the first "achromatic" refracting telescope in 1733. It had an aperture of 6.5 centimeters and a compact length of 50 centimeters. He contracted out the grinding of the lenses, making sure that the two pieces of his achromatic lens were ground by different firms to keep his idea secret. However, as chance would have it, both companies subcontracted the grinding of these two lenses to the same man, an optician named George Bass. Bass quickly figured out what Hall was up to, and passed the idea around to interested parties.

Hall wasn't linked into the astronomical or optical communities and his achromatic telescope didn't amount to much more than an intellectual exercise, at least as far as he was concerned. However, an optician named John Dolland (1706:1761) picked up the idea from George Bass and ran with it, performing meticulous research on the design of an achromatic lens. He went much farther than Hall, even discovering that with careful design of the two lenses he could effectively eliminate spherical aberration as well without having to grind nonspherical lenses.

Dolland didn't go public until 1757, but he was far more aggressive in pushing the technology than Hall, going so far as to patent his own design for an achromatic lens. Hall might have been the original inventor of the concept, but Dolland did much more research and was the first to put it to serious use, and so Dolland is often credited as the inventor of the achromatic telescope.

a modern amateur refractor

John Dolland became a prominent builder of achromatic telescopes, even founding a dynasty of sorts, with his son Peter Dolland (1730:1820) and his son-in-law Jesse Ramsden (1735:1800) getting into the business as well. Peter Dolland introduced a "triple lens" in 1765, featuring a biconcave lens sandwiched between two biconvex lenses. In any case, by the second half of the 18th century, the refracting telescope had finally emerged into what amounted to its modern form. The long refractor was all but dead, much to the relief of astronomers who had wrestled with the thing.

BACK_TO_TOP

[2.3] HERSCHEL'S REFLECTORS

* The achromatic telescope kept the refractor in business, but building a large objective lens was difficult. It was hard to make the glass in a large lens uniform, so that it had the same refractive properties overall, and hard to grind such a lens. That meant that if somebody wanted to build an unusually big telescope, the reflector was the only game in town for the time being.

The first age of the "big reflector" was more or less begun by Sir William Herschel (1738:1822). He was born in the German state of Hanover and became a composer of music. He then emigrated to Britain to evade the draft for the army, working as a music teacher. Eventually his circumstances improved to the point where he was able to spend more and more of his time pursuing his interest in astronomy.

He began by tinkering with refractors, but he soon wanted more capability. Finding that the reflectors available at the time couldn't give him what he wanted, and wouldn't have been affordable if they had, he decided to build his own, assisted by his sister Caroline. He developed a speculum alloy that had a reflectivity of 60%, a big jump from earlier reflectors, and learned how to accurately grind parabolic surfaces. His first reflector was completed in 1774 and had a mirror 15 centimeters in diameter. He then went on to build reflectors with diameters of 22.5 centimeters and 45 centimeters.

Herschel's telescopes were of unusual design: he mounted the mirror at an angle so he could simply look over the edge of the mount of the telescope tube to see the image. That saved him the trouble of grinding a second mirror, at the expense of making his "Herschelian reflectors" more awkward to use than a Newtonian reflector. He was, however, very skilled in their use, and in fact his observations were as meticulous as his construction techniques.

On 13 March 1781, Herschel was observing the skies with his 15-centimeter telescope and spotted a disk-shaped celestial object. He tracked it and it turned out to be a new planet, well beyond the orbit of Saturn. The planet would eventually be named "Uranus". As it turned out, it had been observed a number of times before and even mistakenly cataloged as a star, since the telescopes of earlier times were incapable of resolving it as a disk and it moved very slowly through the skies. It was the first planet besides the five classical planets -- Mercury, Venus, Mars, Jupiter, and Saturn -- to be discovered.

Herschel went on to build bigger reflectors, one with a mirror 61 centimeters across, and monster with a mirror 1.22 meters across, with the big instrument seeing "first light" in 1789. The 1.22-meter instrument was by far the most powerful telescope built to that time, but it was too clumsy to be very useful. The 61-centimeter telescope was much more practical. In 1787, Herschel used it to discover two moons of Uranus, which would later be named "Titania" and "Oberon". Herschel did use the 1.22-meter instrument to discover two new moons of Saturn, eventually named "Mimas" and "Enceladus", shortly after bringing it into service in 1789.

Herschel was just getting started on a monumental exercise in mapping the heavens. He would go on to discover binary star systems, in which two stars were in orbit around each other; observe distant star clusters; create a detailed catalog of nebulas; perform the first, not entirely successful, attempt to map the distribution of stars in our Milky Way Galaxy; and determine that the Sun had a "proper motion" of its own through the skies, which showed not only that the Earth wasn't the center of the Universe, the Sun wasn't either.

Herschel remains best known for his astronomical discoveries, but his reflecting telescopes were significant advances in the art. The major problem with his telescopes were the mirrors made of speculum metal, which required repeated repolishing to remain in service. A French astronomer named Alexis Marie de Rochon (1741:1817) came up with the idea of using platinum as a base for speculum metal alloy instead of copper to eliminate tarnishing. He did obtain 45 kilograms of platinum for such a telescope in 1793, but the metal ended up being used instead to fabricate reference weights for the new metric system. Platinum was really too expensive and too heavy for building telescope mirrors anyway, and nobody would ever build a serious platinum telescope.

Herschel's 1.22-meter reflector had fallen into disrepair by the time of his death in 1822, but his son John Herschel (1792:1871), a well-known astronomer in his own right, made good use of his father's 61-centimeter reflector. John Herschel took it to the Cape of Good Hope at the southern tip of Africa in 1833 and mapped the southern skies with it, publishing his star catalog in 1847.

John Herschel's trip to the southern hemisphere led to one of the odd side stories in the history of astronomy, when in 1835 an imaginative American reporter named Richard Locke ran a series of newspaper articles that claimed Herschel had built a "super telescope" to observe the Moon, finding strange plants, animals, and a "man-bat" creature. Of course, Locke's stories were quickly revealed as a hoax.

BACK_TO_TOP

[2.4] FRAUNHOFER'S REFRACTORS / STELLAR PARALLAX

* The large reflectors soon had competition from improved refractors. In 1798, a Swiss artisan named Pierre Louis Guinand (1748:1824) began experimenting with improved means of fabricating large lenses. He developed methods of stirring molten glass that gave large castings highly uniform properties and helped eliminate air bubbles. In 1807, Guinand signed on to a German optical firm, where he worked with an optician named Joseph von Fraunhofer (1787:1826).

Fraunhofer found Guinand's techniques very interesting, using them to build a refractor that was 4.3 meters long and had an objective lens 24 centimeters in diameter. It also featured a superbly designed mount that made it far easier to handle than a big reflector, with the mount including a clockwork system to allow the telescope to track the sky once it had been targeted and locked in position. It was the best refracting telescope built to that time; it was originally placed in a observatory in Dorpat, then in Russia but now in Estonia, and later put in the observatory at Pulkovo, just south of Saint Petersburg.

Fraunhofer began to build other high-class refractors, fitting them with appropriately high-class micrometers. With such tools, astronomers could perform angular measurements far more precise than any made before. The German astronomer Friedrich Wilhelm Bessel (1784:1846) of the Konigsburg observatory in East Prussia used a Fraunhofer telescope to build a new atlas of the heavens. It was published in 1818 and listed 50,000 stars to an unprecedented level of accuracy.

Bessel then moved on to bigger game: determination of stellar parallax. Although he had Fraunhofer's superior tools at the outset, he needed even better tools to take on this difficult task. A French mathematician named Pierre Bougher (1698:1758) had built an instrument that he named the "heliometer" to measure the angular diameter of the Sun. This device obtained two images of the Sun through lenses. Tweaking a knob would bring the two images together and give the angular diameter. Jesse Ramsden improved on the design by using two half-lenses. Fraunhofer began work on an extremely precise heliometer; he didn't live to finish it, but it was completed by others, and in 1837 Bessel began his observations of stellar parallax.

He chose the star 61 Cygni as a target. It isn't a bright star, but it has a large proper motion, which suggested correctly to Bessel that it is relatively nearby. He used two dim stars that had no apparent proper motion and were presumably much more distant as references for his measurements. In 1838, after a year's observation and the travel of the Earth from the ends of its orbit, Bessel announced that the parallax of 61 Cygni was 0.31 arc-seconds. In modern terms, it was 11 light-years away. That was another big conceptual jump in cosmic scales for astronomers.

Other astronomers were also chasing after stellar parallax at the time. The German-Russian astronomer Georg Wilhelm von Struve (1793:1864) used the Fraunhofer refractor at Dorpat to obtain the parallax of the bright star Vega. In 1840, Struve announced the parallax was 0.29 arc-seconds, which would have placed it slightly farther away than 61 Cygni. However, Struve's measurements were off by about a factor of two, and Vega is actually about 27 light-years away.

At about the same time, a Scots astronomer named Thomas Henderson (1798:1844) was working at the Cape of Good Hope to measure the parallax of the star Alpha Centauri, which is not visible from Europe. It is bright and has a large proper motion, and Henderson thought it likely to be nearby. In 1839, Henderson announced that it had a parallax of 0.91 arc-seconds. This is not too far off the actual value of 0.76 arc-seconds, which places Alpha Centauri at a distance of 4.3 light-years.

Astronomers continued to hunt down and verify the parallax of other stars. Of course it could only be done for relatively nearby stars, less than about 100 light-years away. Bessel continued his studies of stellar parallax, attempting in 1844 to try to obtain the distance to the bright star Sirius. What he found instead was that it moved through the skies along the line of its proper motion in a spiral fashion. Bessel determined that Sirius was accompanied by some large "dark companion" star, with the two stars in mutual orbit. He then also determined that the bright star Procyon had a dark companion.

* Along with more exacting measurements of position, astronomers were also learning to obtain better measures of stellar brightness, a study now known as "photometry". John Herschel came up with an optical system that compared the brightness of the star with that of the full Moon. He could adjust the brightness of the Moon's image until it matched the brightness of the star, and used his adjustments to gauge the brightness of the star. A German astronomer named Carl August von Steinheil (1801:1870) came up with a similar scheme, but compared the brightness of a star with a reference star.

Steinheil's scheme worked better than Herschel's, but both approaches suffered from a fundamental problem: they were "eyeball" methods and the results were necessarily subjective to a degree. A German medical researcher named Ernst Heinrich Weber (1795:1878) did manage to provide useful data on how the human eye responded to light, which allowed astronomers to determine that a star of one magnitude was about 2.5 times brighter than a star of the next higher magnitude. However, really accurate photometric measurements required new technologies and lay in the future.

BACK_TO_TOP

[2.5] ADVANCES IN REFLECTORS

* Although Fraunhofer's excellent refracting telescopes were a challenge to reflectors, reflectors builders fought back. William Herschel's 1.22-meter reflector had been a great step forward in terms of sheer size, if not usability. An Irish nobleman named William Parsons, 3rd Earl of Rosse (1800:1867) got into telescope-making, coming up with an interesting scheme of building large metal mirrors in pieces, assembling them with rivets and solder, and then covering the assembly with molten tin before polishing. Over a period of 17 years, he refined his techniques to build 38-centimeter, 61-centimeter, and 91-centimeter reflectors.

In 1842, Rosse decided to build the biggest telescope made to that time, a Newtonian with a mirror 1.84 meters wide. The mirror had a weight of 3,600 kilograms and the telescope tube was 17 meters long. The telescope took three years to complete. Rosse then used it to perform observations of nebulas. Unfortunately, the "Leviathan", as it was known, was more or less a white elephant. It was a transit instrument, hoisted between two high brick walls, capable of being raised and lowered but not turned. Worse, it was sited on Rosse's estate in Ireland, a land famous for its emerald green landscapes -- a feature closely related to the fact that the place is more than ordinarily cloudy and damp. Rosse's Leviathan got little use and accomplished far less than might have been otherwise expected of it.

Technically, however, Rosse had addressed at least some of the problems of building a very large telescope, and he had conscientiously published his work. Those interested in his efforts saw that his ideas were on the right track, though much a better mounting scheme needed to be developed, and of course more consideration had to be given to siting such instruments.

As far as the telescope itself went, there was something of a happy ending. After the death of the Fourth Earl of Rosse in 1908, it fell to ruin by installments; the mirror was taken to the Science Museum in London in 1914, the metal elements of the structure were melted down for scrap during World War I, and the wooden structures were dismantled for safety reasons in 1925. The Seventh Earl of Rosse managed to organize the restoration of the Leviathan, with work beginning in 1996 and the completed in 1999. The revitalized Leviathan is fully functional; in fact, it has features that would have astounded the Third Earl, such as a digitally-controlled electro-hydraulic control system and a new mirror made of aluminum alloy, weighing a third as much as the original mirror and highly resistant to tarnishing. The site is a tourist attraction and includes a science museum.

* Rosse's ideas were followed up by another ambitious amateur, a prosperous English brewer named William Lassell (1799:1880). Lassell built a 23-centimeter Newtonian using a mount system based on Fraunhofer's concepts. Lassell visited Rosse's estate in 1844 and got ideas for building a larger reflector, resulting in a 61-centimeter instrument.

In 1843, an English astronomer named John Couch Adams (1819:1892) calculated from irregularities in the orbit of Uranus that there was another planet beyond its orbit, with the calculations suggesting where it might be found in the sky. Due to a number of mixups and a lack of drive on Adam's part, his work was ignored, and in 1845 a French astronomer named Urbain Jean Joseph Leverrier (1811:1877) independently duplicated Adams' work. Leverrier was quick to contact a German astronomer named Johann Gottfried Galle (1812:1910), who found the new planet the very first night he checked, on 23 September 1846. The world was named "Neptune".

On 10 October 1846, only a few weeks after the discovery of Neptune, Lassell focused his new 61-centimeter reflector on the planet and discovered that it had a moon, which would be named "Triton". In 1848, he also discovered an eighth moon of Saturn, which would be named "Hyperion", and in 1851 he discovered the third and fourth moons of Uranus, which would be named "Ariel" and "Umbriel".

By this time, Lassell felt like he was running into diminishing returns, and he decided that one way to make better use of his gear was to take it to someplace where the "seeing" was better than Old Blighty. In 1852, he moved his 61-centimeter reflector to Malta, a little island in the sunny Mediterranean south of Sicily that was a British possession, and in 1861 he set up a similar telescope with a mirror 1.22 meters in diameter, as big as William Herschel's giant but with a much more manageable mounting system. Despite all his effort, Lassell made no more significant astronomical discoveries. However, he had done much to advance the state of the art in telescope technology.

BACK_TO_TOP

[2.6] PHOTOGRAPHY & ASTRONOMY

* In 1822, a French artist named Joseph Nicephore Niepce (1765:1833) used a glass plate covered with a type of light-sensitive asphalt named "bitumen" to record the image of an engraving. By 1826, he had built a primitive camera using plates covered the bitumen plus silver salts. Exposures took hours and the results were crude, but they were the first photographs ever made. An associate of Niepce named Louis Jacques Mande Daguerre (1789:1851) carried on the work after Niepce's death and came up with an improved scheme that used plates covered with silver iodide, which required exposures of only about a half hour or so.

The image tended to darken over time, but John Herschel came up with a scheme for "fixing" the image using sodium thiosulfate to wash away unexposed silver salts so that the image would not "fade" rapidly. Daguerre discovered the same technique independently, and his 1839 announcement of his "daguerrotype" scheme led to a public sensation. An Englishman named William Henry Fox Talbot (1800:1877) had been working on much the same technology and was "scooped" by Daguerre, but Talbot came up with an important innovation. He used sheets of paper covered with sodium chloride -- table salt -- as his photographic medium. The paper was exposed in a camera to produce a "negative" image, with black and white reversed, and this was then used to expose another sheet of paper to generate a "positive" image. Talbot was the first to publish a book illustrated with photographs, in 1844.

In 1839, an English-American chemist named John William Draper was the first to perform a photographic portrait, and the next year, 1840, Draper was the first to take a picture of a celestial object, photographing the Moon through a small single lens that tracked the target using a clockwork mechanism. In 1845, two French physicists, Jean B.L. Foucault (1819:1868) and Armand F.L. Fizeau (1819:1896) took pictures of the Sun.

The first serious attempt to use photography in astronomy didn't take place until 1849. In that year, William Cranch Bond (1789:1859), an artisan whose excellent work in amateur astronomy had led Harvard University to appoint him director of their observatory, took a picture of the Moon through the observatory's 38-centimeter refractor. The exposure took 20 minutes, with the telescope kept fixed on the target with a clockwork steering mechanism. The result was so impressive that it was displayed as an exhibit at the Great Exhibition in London that year. In 1850, Bond and a full-time photographer named John A. Whipple took a picture of the bright star Vega, but the long exposure times of the photographic technology available to them at the time meant that these exercises were still really not much more than stunts.

However, in 1851 an English photographer named Frederick Scott Archer (1813:1857) invented the "wet plate" process, which used a glass plate covered with a runny gel made of cellulose nitrate, alcohol, and ether known as "colloidion" that was infused with silver iodide. Although it was a messy process, it was the first really workable photographic technology. Pictures could be taken with relatively short exposures, and the negative glass plate could be used to produce prints far superior to the negative paper sheets obtained in the Talbot process.

The wet plate process was quickly put to use in astronomy. In 1852, a British astronomer named Warren de la Rue (1815:1889) took pictures of the Moon using his 33-centimeter reflector, with exposures of only ten to twenty seconds. The photographs were very sharp and clear, and by 1865 he was producing photographs of the Moon that were equivalent in detail and clarity to good maps of the lunar surface. He also took pictures of Jupiter and Saturn.

In 1857, John Whipple took a picture of the binary star Mizar, the star in the middle of the Big Dipper, with the exposure lasting 80 seconds and revealing Alcor, Mizar's faint companion star. This exercise suggested some of the benefits of using photography in astronomy. Not only could the angular separation of the two stars be determined by examining the photographic plate instead of tinkering with micrometer readings, the relative brightness of the two stars could be compared. Photography opened the door to photometric measurements more reliable than those obtained by eyeball methods.

The first major discovery obtained with astronomical photography was closer to home than the stars. In 1843, a German amateur astronomer named Heinrich Samuel Schwabe (1789:1875) who had been observing sunspots for 17 years announced that the number of sunspots rose and fell in a ten-year cycle, close to the actual value of eleven years. Other scientists had observed that the Earth's magnetic field fluctuated on a similar cycle, and that the two cycles might be related.

John Herschel suggested that sunspots be photographed on a regular basis to validate and extend Schwabe's discovery. In 1858 de la Rue built a special-purpose refracting telescope for photographing the Sun, featuring an objective lens 8.9 centimeters in diameter and a fast shutter system. In 1860, he took his "photoheliograph" to Spain for a trial involving photographs of a total eclipse of the Sun in July of that year, with the exercise assisted by the Italian astronomer Angelo Secchi (1818:1878). The photographs revealed "solar prominences", great arcs of fire rising above the surface of the Sun. They had been observed by eyeball in earlier eclipses, but with no way to record the fleeting experience nobody had been quite sure they really existed. Photography had now provided clear proof.

BACK_TO_TOP

[2.7] THE INVENTION OF SPECTROSCOPY

* While astronomers were investigating the use of photography in their field, they were also pursuing another new technology that would have a major influence. In the course of his work on improved refracting telescopes early in the century, Josef Fraunhofer had studied the refraction of light through prisms made of different types of glass, using light passed through a slit from lamps. As it turned out, the refracted light passed through the prism did not generate a uniform rainbow spectrum, but had certain bright lines of light in it. The position of the lines seemed to be very uniform and Fraunhofer used them as references for his tests on the refracting properties of different prisms.

Of course, the lines were interesting in themselves and Fraunhofer became more curious about them. He invented a device, which would become known as a "spectroscope", that consisted of a small telescope feeding light to a slit, which in turn passed light through a prism to fall on a graded scale so the position of the lines of light could be recorded. In 1814, he turned his spectroscope on the Sun and found that sunlight cast a continuous spectrum, but one that was broken by distinctive patterns of dark lines, not bright lines as he had expected. The dark lines in the solar spectrum had actually been observed by a prominent British chemist named William Hyde Wollaston (1766:1828) in 1802, but Fraunhofer did a much better job of describing and documenting them.

Fraunhofer observed stars with his spectroscope and found that some had line patterns like the Sun, while others were very much different. He also discovered that he could obtain a spectrum by passing a telescopic image through a set of fine parallel wires, and then went on to use a glass plate scored with fine ruling marks. This was the very first astronomical "diffraction grating". For the moment, the meaning of the line patterns remained obscure -- obviously they had some significance, but for the moment it wasn't exactly clear what it was.

Fraunhofer did observe that two bright lines produced by burning sodium seemed to match the spectral positions of two dark lines in the spectrum of the Sun. In 1849, Foucault followed up this observation by passing sunlight through a hot vapor containing sodium compounds. He thought that would cancel out the dark lines, but it only made them darker. Foucault made a critical observation that under certain conditions sodium would emit light at those two frequencies, and under other conditions it would absorb light at those two frequencies. An American physicist named David Alter (1807:1881) performed comparable experiments with a number of materials and got similar results.

It took the work of two German scientists, the chemist Robert Wilhelm Bunsen (1811:1899) and the physicist Gustav Robert Kirchoff (1824:1887), to weld all these discoveries together. Bunsen developed a type of torch that burned hot and clear and which could be used to heat various elements; his "Bunsen burner" would become familiar to countless future chemistry students. The two scientists used the burner and a spectroscope to determine that different elements produced different spectral patterns. They even discovered two new elements using this technique. Kirchoff established that the bright lines were produced by a glowing gas, and that the dark lines were produced when a bright source with a continuous spectrum shined through a cooler gas, with the gas absorbing the light. By 1860, they had established the science of "spectroscopy".

* Coupled with astronomical photography, which allowed line spectra to be conveniently recorded, spectroscopy promised a revolution in astronomy. Up until that time, nobody could think of any way to determine the chemical composition of cosmic bodies such as the distant stars, and in fact such a feat had been judged completely impossible. Now it turned out the light from the stars actually contained "fingerprints" that could reveal details of their composition.

A Swedish astronomer named Anders Jonas Angstrom (1814:1874) fabricated diffraction gratings out of glass plates with fine scorings across the face. The best of his gratings had 2,000 lines per centimeter. He used his gratings to observe the spectrum of the Sun, announcing in 1862 that he had discovered the lines of hydrogen in the solar spectrum, and publishing his full results in 1868. His spectral map included 800 lines associated with known elements. Angstrom provided the spacings between specific spectral lines using a unit of 10E-10 meter, which would become the "Angstrom unit" in his honor.

An American astronomer named Lewis Morris Rutherfurd (1816:1892), a handy sort who had introduced the first telescope specifically designed for photography in 1860, came up with schemes for fabricating improved diffraction gratings that used reflective principles, eliminating the light loss suffered by a transparent glass-plate diffraction grating. He ultimately was able to build gratings with 6,700 lines per centimeter, and used them to study the spectra of stars. He noticed that there seemed to be a distinct number of "spectral classes" of stars, an observation that was followed up by Secchi in 1867, when he announced that stars could be divided up into four such classes. Secchi's classification scheme would be extended in the future.

An English astronomer named William Huggins (1824:1910) also studied the spectra of cosmic objects, not only of stars but of nebulas, planets, and comets. Comets, as his observations showed, seemed to include hydrocarbon compounds. His most important contribution was to show how broadly spectroscopy could be applied to astronomy.

In 1842, an Austrian physicist named Christian Johann Doppler (1803:1853) discovered that the frequency of sounds was affected by motions of the sound source: an approaching source would raise the frequency, a receding source would lower it. In 1848, Fizeau suggested that the "Doppler effect" applied to light as well. That meant the light from a star moving away from the Earth would be lowered in frequency, or "redshifted", and the light from a star moving towards the Earth would be increased in frequency, or "blueshifted". In 1868, Huggins used the Doppler shift to determine the radial velocity of the star Sirius, obtaining a value close to the modern estimate.

The most impressive early discovery obtained through spectroscopy was the result of spectroscopic observations of a total eclipse of the Sun observed by the French astronomer Pierre Jules Cesar Janssen (1824:1907) in 1868. His observations of solar prominences suggested they were made of hydrogen, but there were also spectral lines he couldn't place. Janssen sent his observations to an English astronomer Joseph Norman Lockyer (1836:1920), who was regarded as highly knowledgeable about the solar spectrum. Lockyer suggested the lines from the element were due to an unknown element, which he named "helium".

Nobody took the idea very seriously, since a large number of other "elements" were discovered in the solar spectrum that turned out to be bogus, usually proving to be the spectra of highly ionized, already known elements. However, in 1895, a Scots chemist named William Ramsay (1852:1916) discovered an inert trace gas on Earth that produced exactly those spectral lines. It was Lockyer's helium, and for the first time a new element had been discovered on a cosmic body.

BACK_TO_TOP
< PREV | NEXT > | INDEX | SITEMAP | GOOGLE | LINKS | UPDATES | BLOG | EMAIL | $Donate? | HOME