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[3.0] Giant Telescopes & New Technologies

v3.0.0 / chapter 3 of 5 / 01 nov 16 / greg goebel

* By the late 19th century, telescope technology had finally matured to the point where effective giant telescopes could be built. Refractors reached their zenith with the big 40-inch Yerkes telescope, but the future for big telescopes was clearly with the reflector, resulting in the first of the real giant telescopes -- the 100-inch Hooker telescope. The push towards giant telescopes was accompanied by the development of improved instrument technologies, as well as specialized telescopes, such as the Schmidt wide-field telescope and its brethren.

Yerkes telescope


[3.1] THE ULTIMATE REFRACTORS
[3.2] DOMINANCE OF THE BIG REFLECTORS
[3.3] MAPPING THE SKY BY CAMERA
[3.4] ADVANCES IN SPECTROSCOPY
[3.5] THE MOUNT WILSON & HOOKER TELESCOPES
[3.6] MICHELSON & THE ORIGINS OF OPTICAL INTERFEROMETRY
[3.7] THE SCHMIDT TELESCOPE / THE CORONAGRAPH

[3.1] THE ULTIMATE REFRACTORS

* The USA had not made much of a mark on astronomy up to the middle of the 19th century, and European astronomers didn't pay much attention to what went on in America. A Massachusetts portrait painter named Alvan Clark (1804:1887) got interested in grinding large telescope lenses and decided to take up the work full time, but though his lenses were of excellent quality, the mainstream of astronomers in Europe took no notice.

Clark decided to build his own telescopes and perform cutting-edge observations with them, then publicize his results in the science press to promote his lenses. The tactic worked: by the early 1850s, Clark lenses were being sought after in Europe. In 1859, Clark went to London and met with prominent astronomers, including Lord Rosse and John Herschel. With his product in demand, Clark set up a factory in Cambridge, Massachusetts. He was assisted by his two sons -- with one of them, Alvan Graham Clark (1832:1897), later acquiring fame in his own right.

In 1860, the Clarks received a major order from the University of Mississippi. "Old Miss" wanted the biggest telescope in the USA, with an objective lens 47 centimeters in diameter. The lenses for the telescope were ready by 1862; to test them the Clarks put them in a rough frame and performed some difficult observations.

During the tests, Alvan Graham Clark took a look at the star Sirius and notice a tiny pinpoint of light there. Doublechecks revealed it really was there and was not an artifact of the telescope. He had discovered the dark companion of Sirius, which turned out to be fairly bright but small star. It was the first "white dwarf" star to be observed. Alvan Graham Clark received a well-deserved award from the French Academy of Sciences, enhancing the reputation of the family firm as well as the reputation of American astronomy. Of course, Mississippi had seceded from the Union in 1861 and by 1862 contacts with the North consisted mostly of exchanges of gunfire, so Old Miss never saw the telescope. It ended up in the Dearborn Observatory of the University of Chicago.

* The Clarks were only getting warmed up. In 1870, the Canadian-American astronomer Simon Newcomb (1835:1909) was casting around for a leading-edge telescope for the US Naval Observatory. The US government had granted $50,000 for the job, equivalent to millions now; Newcomb could afford the best, and the Clarks could provide it. A few years of work resulted in biggest and most sophisticated refracting telescope built to that time, with a length of 13 meters and an objective lens 66 centimeters across with a weight of 50 kilograms.

In August 1877, the American astronomer Asaph Hall (1829:1907) decided to use it to observe Mars during a close approach or "opposition" of the planet. He was trying to see if Mars had any moons; nobody had seen any up to that time, so if there were moons of Mars they were tiny and likely close to the surface of the planet. He strained his eyes for days and was about to give up, but his wife encouraged him to keep on trying. On 16 August he discovered one moon; on the 17th he discovered a second. They were in fact small bodies, orbiting close to the planet. A correspondent from England suggested that they be named "Phobos" and "Deimos", and so they were.

During the same opposition of Mars, the Italian astronomer Giovanni Virginio Schiaparelli (1835:1910) created the best map of Mars to that date. One of its features was a set of fine lines that he called "canali" or "channels". The word was more or less mistranslated as "canals", which led to decades of speculation that there was or had been an intelligent civilization on Mars that had created a global network of artificial waterways to distribute water as the planet dried out. Not everybody could see the canals, and they would never show up in photographs; they would lead to a long-standing controversy.

* The success of the Clark refractors inspired Europeans to build new refractors that increased the objective diameter a few more centimeters, but the Clarks leapfrogged the competition. In 1874, a wealthy American financier named James Lick (1796:1876) announced that his estate would provide $700,000 to build the best telescope in the world. The Clarks set about working on a telescope with an objective lens 91 centimeters in diameter. It took 14 years and $50,000 to fabricate the lens, which used a special high-quality glass obtained from Paris.

The new telescope finally went online in early 1888, not long after the death of the elder Clark. It was set up in an observatory at 1,400 meters on top of Mount Hamilton, east of the San Francisco Bay Area in California. The site was named "Lick Observatory", and Lick was laid to rest in the telescope's pier.

Attempts to use it to discover more details of the "canals" of Mars proved futile, in hindsight for the simple reason that they didn't exist, and there was some grumbling that the Lick telescope was nothing but a big and expensive white elephant. However, in 1892 the astronomer Edward Emerson Barnard (1857:1923) used it to discover the fifth moon of Jupiter, later named "Amalthea". He is also said to have actually seen craters on Mars, but he was skeptical enough not to publish his findings, and it wouldn't be until robot spacecraft visited the planet much later that it was found they honestly did exist. In any case, the Lick telescope had proven its worth, and would make other significant discoveries.

* The University of Southern California was impressed enough by the Lick telescope to want to upstage it and contracted with Clark to build a refracting telescope with a 101-centimeter -- 40-inch -- objective lens. Work was well underway when the finances for the project collapsed, leaving Clark with a big, expensive, and for the moment useless hunk of glass.

However, an assistant professor at the University of Chicago named George Ellery Hale (1868:1938) saw the potential in that hunk of glass, and began to aggressively lobby potential backers to get the telescope built. His patron ended up being Charles Tyson Yerkes (1837:1905), who ran the trolley system in Chicago. The fact that Yerkes had a popular reputation as less than completely scrupulous in his business dealings was not any obstacle to Hale, and he managed to pry $349,000 out of Yerkes to set up an observatory at Lake Geneva, Wisconsin, about 130 kilometers (80 miles) northwest of Chicago. The site was at low elevation, but Hale regarded mountaintops as too stormy, and besides the winters in the region were cold enough to make a higher site seem distinctly unattractive.

The objective lens was completed in late 1895 and had a weight of 225 kilograms. It was fabricated into a telescope with a length of 18 meters and a weight of 18 tonnes, but the telescope was so well mounted that its position could be shifted very easily. Yerkes Observatory went online in the spring of 1897, only weeks before the death of the younger Clark, who at had the satisfaction of completing the two biggest refracting telescopes ever built.

They would remain the largest. As the size of a lens increases, uniformity of the glass used to fabricate it becomes increasingly important and more difficult to achieve, and the lens also tends to deform under its own weight, distorting the optics. Clark believed that he could build larger lenses and had been planning a 152-centimeter lens at the time of his death. Even if it had been possible to fabricate such a huge lens, there was no longer any good reason to do so. Reflecting telescopes had advanced to the state where they would dominate astronomy from that time on. The Yerkes telescope, however, remains in use in the 21st century, though it is now fitted with electronic instrument systems that Clark would have been hard-pressed to imagine.

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[3.2] DOMINANCE OF THE BIG REFLECTORS

* The problem with reflecting telescopes well into the 19th century was the fact that they had to rely on metal mirrors, which were heavy, expensive, and prone to tarnishing.

Techniques for fabricating mirrors made of glass backed by a metal layer had been around for centuries. Since the metal backing was sealed from the air it did not tarnish, and glass was lighter and cheaper than metal. Newton was aware of metal-backed glass mirrors when he invented the reflecting telescope, but he did not use them in his design. That was because the metal layer was in back of the glass, and light would have to go through the glass to hit the metal, eliminating much of the usefulness of a reflecting configuration.

James Short tried to build a glass mirror with the metal in front, but the glass mirror technology of the time had a thick metal layer and the backside of the metal didn't follow the surface of the glass well enough to provide a useful optical surface without grinding and polishing. A metal mirror would be simpler to fabricate. In 1856, however, a German chemist named Justus von Liebig (1803:1873) figured out how to deposit a very thin layer of silver on glass, using a solution of silver ammonium nitrate. The layer was so thin that it conformed to the curvature of the glass surface underneath, and it also did not require large amounts of expensive silver. Silver did not tarnish quickly, it could be easily polished to clean it up, and the mirror could be resurfaced when necessary by chemically stripping off the silver and laying down a new surface.

Astronomers saw the potential of the idea for their work and quickly built reflecting telescopes based on silvered glass mirrors. Carl August von Steinheil built one in 1856 and Focault built one in 1857. Focault also introduced an improved optical test for checking the geometry of a mirror. Halley's original test featured a pinhole at the focus shining light on the mirror; Focault modified this scheme by moving the pinhole to one side, and then observing the reflection from the mirror from the same displacement on the other side. He also added a scheme where the viewing pinhole was shut off gradually by a knife edge: due to the operation of the eye, which is not very sensitive to variations in brightness across the field of view but is sensitive to changes in brightness in time, any non-uniformities would show up as variations in brightness during the time the pinhole was shut off. Focault's test was very precise and resulted in a highly accurate mirror.

The Australians wanted to build a reflector with a mirror 122 centimeters in diameter and set it up in Melbourne. They didn't trust the newfangled glass mirror and specified a metal mirror instead, but when the telescope went into operation in 1862, the metal mirror tarnished so quickly as to make the telescope useless. That expensive fiasco was the effective end of the metal mirror.

The American astronomer Henry Draper (1837:1882) had been trying to build a reflector with a metal mirror and going nowhere, but on learning about the new glass mirrors from John Herschel, Draper switched gears and acquired considerable expertise in building the mirrors, even inventing an improved silver deposition technique. In 1862, he put a reflecting telescope into operation with a silvered glass mirror 39 centimeters in diameter, following it up in 1872 with one with a mirror 71 centimeters in diameter. A reflecting telescope with a glass mirror 120 centimeters in diameter was put into service in France in 1877. The age of the modern big reflector had arrived.

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[3.3] MAPPING THE SKY BY CAMERA

* In the middle of the 19th century, a German astronomer working at the Bonn Observatory named Friedrich Wilhelm August Argelander (1799:1875) constructed the best star catalog produced to that time, the four-volume BONNER DURCHMUSTERUNG (BONN SURVEY), published from 1859 to 1862. It listed over 450,000 stars and would be extended in later editions.

The BONNER DURCHMUSTERUNG was a landmark, not merely because of its quality but because it was the last major star catalog to be constructed using direct visual measurements of stellar positions. With the invention of photography and its introduction to astronomy, astronomers had quickly realized that it would be much easier to photograph a field of stars and then determine their positions relative to the bright stars in the photograph.

The problem was that wet-plate photography couldn't do the job. Photographing stars took long exposures, and a wet plate would dry out well before the exposure could be completed. However, in 1871 a British chemist named Richard Leach Maddox developed photographic plates that used gelatin instead of collodion. The gelatin was more or less solid and didn't dry out in any hurry, permitting exposures as long as necessary. Further work to the end of the decade produced dry plates that were far more sensitive than the best wet plates. In addition, the "dry plates" could be bought prepackaged and ready to use, and once exposed they could be set aside and developed later.

All that was well and good for portrait photography, but there was another problem as far as photographing stars was concerned: photographic plates were most sensitive in the blue-violet end of the visible light spectrum and were not very sensitive in the red-orange end, and so photographs of stars, which varied in color, would not reflect their actual visual magnitudes. However, work was being performed in parallel with the development of the dry plate in which various dyes were mixed with the photographic emulsion, resulting in a much more uniform spectral response.

* In 1882, a Scotsman named David Gill (1843:1914) at the Cape of Good Hope observatory in South Africa decided to use the new dry-plate technology to photograph a comet. The results were excellent, and in fact Gill was so taken by the crispness of the background stars in the image that he decided to begin work on a photographic survey of the southern skies, which had not been covered by the BONNER DURCHMUSTERUNG. A Dutch astronomer named Jacobus Cornelius Kapteyn (1851:1922) performed all the analysis and documenting of Gill's plates, resulting in the 1904 CAPE PHOTOGRAPHIC DURCHMUSTERUNG, which listed over 450,000 stars around the south celestial pole. In France, the brothers Paul Pierre Henry (1848:1905) and Prosper Mathieu Henry (1849:1903) conducted a similar photographic survey of the northern skies.

Such were the beginnings of modern photographic sky surveys, which not only proved directly useful for constructing star catalogs, but also provided a relatively unambiguous archival record of the skies that could be inspected later for clues about objects whose significance wasn't appreciated at the time the photographs were taken.

The fact that the new dry plates could be used for long exposures led to another benefit. Very long exposures of celestial objects might well reveal details that were too faint to be seen by eye. The logical target for such long exposures were the diverse cloudlike nebulas. A British astronomer named Isaac Roberts (1829:1904) became interested in photographing nebulas, modifying his 51-centimeter reflector to obtain the rocklike steadiness needed for very long exposures. In 1886, his photograph of the bright Pleiades star cluster seemed to show that its stars were embedded in a cloud of gas and dust. In the same year, he took a photograph of the Andromeda nebula, or what we now call the Andromeda galaxy, with the long exposure showing the previously unseen outer regions of the object The photograph revealed that the Andromeda nebula had a spiral form, which had been seen previously in a few other nebulas.

An American astronomer named James Edward Keeler (1857:1900) of the Lick Observatory in California began a two-year photographic survey of nebulas in 1898. He used a 91-centimeter reflector and obtained outstanding results. The plates suggested that the number of nebulas visible over the entire sky to the technology had ran to 100,000, an order of magnitude more than had been assumed previously. Even more interestingly, many of the nebulas had spiral forms -- but nobody was sure what to make of them at the time.

Yet another application of effective astronomical photography was to hunt asteroids. A German astronomer named Maximilian Franz Joseph Cornelius Wolf (1863:1932) found that if he took long exposures of stars, with the telescope tracking the stars to keep them in focus, asteroids moving through the field of view would appear as a streak. The number of asteroids discovered every year had declined greatly in the late 19th century, but using long-exposure photography Max Wolf and others discovered hundreds more. Photography had become one of the foundation stones of astronomy, rivaling the telescope itself in importance.

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[3.4] ADVANCES IN SPECTROSCOPY

* Once photography had been refined for imaging the skies, it was a short step to using it in spectroscopic studies. William Huggins had tried to take photographs of the spectra of the bright stars Capella and Sirius in 1863, but wet-plate technology simply wasn't up to the job, and the exercise went nowhere. Henry Draper gave it a shot in 1872, taking photographs of the spectrum of the bright star Vega. After some false steps, he managed to get a photograph with a few spectral lines of the star.

It was an unimpressive feat, but it was a first step. Over the next ten years, up to his death in 1882, Draper obtained increasingly detailed spectrographs of dozens of stars. A German astronomer named Hermann Karl Vogel (1841:1907) focused on photographic spectroscopy in parallel with Draper's efforts, and was able to use the Doppler shifting of spectral lines to give the first reasonably accurate measurements of the radial velocities of a number of stars.

The director of the Harvard College Observatory, Edward Charles Pickering (1846:1919), sorted through Draper's spectrographs to see if he could find common patterns among groups of stars, showing for example that the stars Arcturus and Capella had similar spectra to that of our own Sun. The Draper sampling was a good start, but to do a good job of stellar spectral classification Pickering needed a much bigger sample. He came up with a prismatic plate that could be fitted to the front of a telescope, allowing the spectra of all the stars in the field of view to be photographed at one time, and by 1889 he had completed the first photographic spectral survey of the northern skies. The work was extended to the southern skies by an observatory in Peru, with the HENRY DRAPER CATALOGUE finally published in 1924. It listed a quarter of a million stars.

Spectrographic sky surveys permitted the construction of a useful scheme of stellar classification. The modern scheme includes the classes "O B A F G K M R N S", and was pioneered by the American astronomer Annie Jump Cannon (1863:1941). Another benefit of photographic spectroscopy was that it led to the discovery of "spectroscopic binaries", stars that were so close together that they couldn't be seen as separate. They were revealed by their spectra, which contained merged sets of lines from both stars, with the lines moving together and then apart as the two stars orbited around each other.

* Other technical advances followed the introduction of photography to astronomical spectroscopy. In 1881, a American physicist named Henry Augustus Rowland (1848:1901) came up with a diffraction grating scribed onto a concave metal disk that would not only break the light of stars into spectra, but focus them on the photographic plate without need for lenses, reducing light losses. Rowland also produced the best gratings built to that time, with up to 8,000 lines per centimeter.

In 1891 George Ellery Hale, the father of the big Yerkes refractor, came up with an ingenious scheme to photograph the Sun at a specific spectral line, obtaining an image at a specific wavelength, without using a color filter system. His "spectroheliograph" mechanically scanned the solar disk through a moving slit, with the light from the slit passed through a prism. The spectrum obtained from the prism fell on a second slit whose position could be adjusted to match the position of any desired line in the spectrum, and the light from this second plate was scanned across a photographic plate.

In this way, the scan built up a image in the desired spectral line only, allowing the mapping of specific elements across the face of the Sun. The spectropheliographic was the ancestor of modern spectral scanning systems, often used by Earth resources spacecraft. A French astronomer named Henri Alexandre Deslandres (1853:1948) came up with the same idea in parallel.

To Hale, inventing such a gadget was a means to an end, not an end in itself, and he went on to tackle the integration of the spectroheliograph with telescopic astronomy. He tried to fit a spectroheliograph to the big Yerkes telescope, but the device was too big and clumsy to be workable. Hale decided that, given the bulk and weight of the spectroheliograph, it might be wiser to fix it and the telescope in place and simply divert the image of the Sun into the assembly with a pivoting mirror, or "coelostat".

Hale's efforts to build such an optimized solar telescope proved troublesome. The first was lost in a fire, the second was completed but gave worthless results. The slow scanning process made the images sensitive to vibration or other sorts of mechanical interference, and also meant that the air needed to be relatively undisturbed to guarantee good "seeing".

A few years before, Hale had been told by the astronomer William Henry Pickering (1858:1938) about Mount Wilson, 45 kilometers northeast of Pasadena in southern California. The mountain was 1,800 meters tall and the weather was generally clear; Pickering thought it would make a good site for an astronomical observatory, and Hale decided to follow up on the suggestion.

In 1904, Hale set up shop on top of Mount Wilson. It was troublesome to get the equipment up there and proved troublesome to get it working properly once it was, but Hale was nothing if not persistent, solving each problem methodically. Within a few years, he was taking spectacular spectral images of sunspots, and in 1908 observed that spectral lines in the spots were sometimes doubled. This was clearly due to the "Zeeman effect", the splitting of spectral lines in a strong magnetic field, discovered by the Dutch physicist Pieter Zeeman (1865:1943) in 1896. What the finding meant was that sunspots were focal points of the solar magnetic field. Sunspots come and go on a regular 11 year cycle; in 1913, the current cycle ended and a new one began, and observations showed that the magnetic field of the sunspots reversed their orientation, with north flipped to south and the reverse.

In 1926, Hale introduced an improved version of the spectroheliograph, called the "spectrohelioscope". In the spectrohelioscope, the scanning mechanism moved back and forth to register successive images on a film strip. This allowed astronomers to view solar events in a moving-picture format, which was particularly valuable for observing the evolution of transient events such as solar flares.

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[3.5] THE MOUNT WILSON & HOOKER TELESCOPES

* Despite Hale's difficulties in getting his observatory on top of Mount Wilson online, he was enthusiastic about the site and worked to expand it. He already had a 153-centimeter -- 60-inch -- mirror blank on hand, his father having bought it in Paris for $25,000 a few years earlier. It had originally been intended for a new telescope at the Yerkes observatory, but the funding fell through. Hale, undiscouraged, lobbied the Carnegie Foundation for new funding and was awarded a grant in 1903, giving him the money to build the biggest telescope in the world and put it on Mount Wilson. Technically speaking, Rosse's 184-centimeter Leviathan had been bigger, but it was history by that time and, more to the point, given the advances in large-telescope technology since Rosse's pioneering work, the new 153-centimeter instrument would be far superior.

The new telescope was built under the supervision of George Willis Ritchey (1864:1945), an associate of Hale's. The work was performed in what would now be called a "clean room", in a temperature-controlled environment to ensure the best optical quality. Ritchey was also one of the first telescope-makers to use carborundum (silicon carbide), which had been discovered in 1891, to grind the mirror. Carborundum wasn't as hard as diamond powder, but it was much cheaper, and it was six times more effective as a grinding agent than the emery (aluminum oxide) powder typically used up to that time.

Mount Wilson 60-inch telescope

The 153-centimeter telescope went online on top of Mount Wilson in December 1908. Not only was it built to exacting standards, with a superb mount and excellent observatory site that Rosse could have only dreamed about, it was optimized for use with cameras, spectroscopes, and spectroheliographs. In 1894, a French-Austrian astronomer named Maurice Loewy (1833:1907) had come up with the "coude" (French for "bent-elbow") focus system, which was something like a periscope, with two flat mirrors that could in effect "pipe" the image from the focus of a telescope on a moving mount into a camera or other device on a stationary mount. It made use of such auxiliary instruments much simpler.

The Mount Wilson telescope was quickly put to use. In 1915, the American astronomer Walter Sydney Adams (1876:1956) used it to photograph the spectrum of Sirius B, the dim companion of the bright star Sirius A. It turned out, somewhat to his shock, to be very hot. That implied it should be bright, except if it was very small. The diameter of Sirius B could be estimated from its temperature and apparent brightness, and its mass was known from its orbit around Sirius. The end result of the analysis was that Sirius B had the mass of a full-blown star but the diameter of a large planet, meaning it was very dense. Theoreticians puzzled over the matter and suggested that the gravity field of the star was so strong that it had crushed atoms, which by that time were known to be mostly empty space, into nuclei swimming in a sea of electrons, resulting in densities unattainable by normal matter. Sirius B was the first "collapsed matter" star to be recognized as such.

Of course, the Mount Wilson telescope was used for planetary observations as well. The controversy of the Martian canals was in full swing at that time, being energetically stoked by Percival Lowell (1855:1916), a son of a prominent and wealthy Boston family who had become interested in astronomy. Although he was technically an amateur, he had the money to buy excellent gear, setting up an observatory in Flagstaff, Arizona, and was methodical in his work. He was convinced the Martian canals were real and wrote books suggesting that Mars was inhabited by an ancient civilization.

Many astronomers insisted they couldn't see the canals at all, some claiming they were just a figment of Lowell's imagination -- a suggestion that of course infuriated him. Photographs didn't show the canals, but at that time photography still didn't provide the same resolution possible with the human eye. In 1911, during an opposition of Mars, Edward Barnard, noted for his observing skills, tried to hunt for the canals but found nothing. He was fair enough to observe that the images of Mars he saw through the Mount Wilson telescope weren't of much higher quality than he had seen before, and so left open the possibility that they existed.

The fact that the Mount Wilson telescope didn't give better images of the planets than had been seen before was no real criticism of the quality of the telescope as such -- it simply made it clear that as telescopes got bigger, they were beginning to run into fundamental obstacles that might prove difficult to overcome. The big Mount Wilson telescope was able to pick up dimmer and fainter objects, but the nearby planets like Mars were very bright, and so the bigger aperture was wasted for that purpose. In principle, the bigger aperture also meant more detail and resolution, but in practice the wavering interference of the atmosphere on an extended object like a planet kept the image from being much more detailed than it would be in a smaller telescope. It would be over 50 years before the canal controversy would be settled, and it would be by more or less first-hand inspection of the planet.

* Despite this limitation, the Mount Wilson telescope remained an outstanding and extremely useful instrument. Astronomers wanted more. An American astronomer and telescope maker named John Alfred Brashear (1840:1920) began working on a 183-centimeter reflector for the Dominion Observatory on Vancouver Island, in British Columbia, Canada. It was finally brought online in 1918 and was an excellent instrument, one of the first to feature an aluminum coating, which is substantially more reflective than silver. The site was not the best, unfortunately, since Vancouver Island tends toward the damp.

Brashear's telescope didn't stay the world's biggest for very long. Hale had also been working on a new telescope, much bigger than any ever built, with a 254-centimeter -- 100-inch -- mirror. He managed to obtain funding from a Los Angeles businessman named John D. Hooker, but getting the money was the relatively easy part. Nobody had ever manufactured a glass blank that big, and only the company in Paris that had built the blank for the 153-centimeter telescope was willing to even take the job.

The first blank was delivered to Pasadena in 1908, but it was partly crystallized and full of tiny air bubbles, and it was rejected as unsatisfactory. Further efforts didn't work out any better, and in 1914 World War I broke out, ending any likelihood of getting an acceptable blank until the conflict was over. The fighting stretched out agonizingly; Hale, for want of anything better to do, decided to use the first blank he had received. The air bubbles weren't close to the surface and might not affect the grinding and polishing. They didn't, and the crystallization didn't prove a show-stopper either. The Hooker telescope went online at the end of 1918. It was a marvel of precision engineering, able to move easily despite its weight of 90 tonnes.

Mount Wilson 100-inch telescope

With unarguably the most powerful telescope ever built at their disposal, astronomers were able to focus on the big questions of the day. One of the big questions to obtain a reliable indicator for cosmic distances. To be sure, parallax measurement could give distances to the nearer stars, but it was worthless for objects more than about 100 light-years away. A tool for determining much greater distances had been discovered by Henrietta Swan Leavitt in 1912. She inspected photographic plates of the Small Magellanic Cloud, one of the two little satellite galaxies of the Milky Way that can be seen from the southern hemisphere, the other of course being the Large Magellanic Cloud. She had a particular focus on the pulsating stars that are known as "Cepheid variables", after the star Delta Cephei, a relatively nearby variable.

Since the Small Magellanic Cloud was clearly a long way away, the percentage difference in the distances of the Cepheids in the cloud was small, and so they could be assumed to be at the same distance. She discovered that the brightness of a Cepheid was proportional to its period of pulsation. That meant that "period-luminosity relationship" of the Cepheids might be used as "cosmic yardstick".

The problem was calibrating the yardstick. There were no Cepheids close enough to the Sun to allow their parallax to be determined, which would have established their absolute brightness. However, in 1913, the Danish astronomer Ejnar Hertzsprung (1873:1967) came up with a clever and subtle means of determining the distance to star clusters by examining the paths of the stars in the cluster and using the laws of geometric perspective to obtain an estimate of the range. It was an approximate method, but it was workable, and yielded estimates of the distance to a number of Cepheids.

With the Cepheid yardstick in hand, the American astronomer Harlow Shapley (1885:1972) used them to unravel the mystery of the globular star clusters. These fuzzball star clusters had been known since the early days of telescopic astronomy; one of their odd features was that they were concentrated in only part of the sky, in the general direction of the constellation Sagittarius. Shapley suspected that they were really concentrated around the center of the Milky Way galaxy. However, the conventional wisdom was that the Sun was actually near the center of the Milky Way, since star counts seemed to be the same on the average in every direction along the galactic plane.

Shapley used the Hooker telescope to obtain photographs of the globular clusters, and managed to pick out Cepheids in them. What his observations suggested was that the globular clusters were in orbit around a center of mass about 27,000 light-years away, in the direction of Sagittarius, and of course this center of mass was the actual center of the Milky Way. The misleading star counts were due to the fact that there were large dark clouds of dust in the plane of the Milky Way that blocked the light from the central regions. The Milky Way was about 100,000 light-years across, much bigger than had been previously thought.

Shapley also used the Hooker telescope and the Cepheid yardstick to determine the distances to the Magellanic Clouds. To be sure, the Cepheid yardstick would turn out to be trickier than thought at first, but it was the first useful step in determining the distances of deep-space objects, and it gave astronomers a much better idea of the scales they were actually working with.

* That was merely the first step towards a massive expansion of the size of the Universe. Over the previous few decades, there had been an intense debate over the nature of the "spiral nebulas" and some of the other nebular objects. Where they part of our Galaxy, or distant galaxies in their own right? Observations of novas in the spiral nebulas seemed, given what was known about the brightness of novas at the time, to show the spiral nebulas were part of our Galaxy.

That didn't completely settle the matter, since it was based on the assumption that the novas seen in the spiral nebulas actually were as bright as novas that were seen in our Galactic neighborhood when it was possible they were much brighter. In 1919, the American astronomer Edwin Powell Hubble (1889:1953) fired a major shot in favor of the extragalactic hypothesis, using the Hooker telescope to take photographs that showed the big Andromeda nebula was made up of vast numbers of stars.

In 1923, Hubble began to find Cepheid variables in the Andromeda nebula, and gave an estimate for its distance of about 800,000 light-years. The Andromeda nebula became the Andromeda galaxy, and other nebulas were also determined to be external galaxies. However, something seemed to be wrong, because our Milky Way Galaxy seemed to be enormous compared to the other known galaxies. As it turned out, Hubble and others had assumed that the variable stars observed in the Andromeda galaxy and other nearby galaxies had the same period-luminosity relationship. In the 1950s, after several decades of observations, it turned out that the stars that had been observed had a different period-luminosity relationship. The Andromeda galaxy was actually much bigger and about 2,300,000 light-years away.

In any case, the Hooker telescope had shown the Universe was far bigger than most had previously assumed. Observations with the telescope by Hubble and others through the 1920s not only demonstrated the vast numbers of galaxies in the observable Universe, but also showed that they were moving away from each other. The Universe was expanding, as if it was the product of a massive explosion in the distant past.

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[3.6] MICHELSON & THE ORIGINS OF OPTICAL INTERFEROMETRY

* The Hooker telescope was not only a marvel of contemporary technology in itself, it provided a basis for trials of other advanced technologies. In 1920, the great German-American experimental physicist Albert Abraham Michelson (1852:1931) decided to use the Hooker telescope to conduct experiments in "optical interferometery".

Michelson had developed optical interferometry a few decades earlier in experiments on measuring the velocity of light. In his original interferometer scheme, he took a beam of light, split it into two beams using a half-silvered mirror at a 45-degree angle to the beam, reflected the beams back after they had traveled over a short distance, and then combined them again. Light being a wave phenomenon, at least as far as Michelson's instrument was concerned, the two beams would interfere with each other if they were out of phase, producing an interference pattern that could be observed to determine the exact phase difference. Since the wavelengths of light are very short, that made Michelson's optical interferometer able to measure very small differences in the velocities of the two waves.

Michelson had been trying to determine how the motion of the Earth affected the velocity of light. As it turned out, it didn't affect it at all, which would lead, somewhat indirectly, to Albert Einstein's theory of special relativity. The "null result" of this experiment did not discredit his apparatus in the least, and conceptually similar interferometers are now used as exquisitely precise measurement systems, usually by passing one of the split beams through a "transducer" of some sort that slows down the light beam in response to the input to be measured.

Michelson also came up with the idea that the same sort of principles could be used for astronomical observations. Suppose two telescopes observe the same point source in the sky. If the source is at cosmic distances, the electromagnetic waves will arrive effectively in parallel. If the path length from the source to each of the two telescopes is exactly the same, the waves will arrive exactly in phase. If the path length isn't exactly the same, the two beams will produce an interference pattern that, once again, can be observed to determine the phase difference. The interference pattern will be different depending on the apparent diameter of the source, permitting measurement of that apparent diameter.

In 1920 Michelson, working with F.G. Pease, performed astronomical interferometry experiments using two mirrors mounted at the ends of a 6.1-meter-long girder suspended across the opening of the Hooker telescope. Other mirrors mounted at the center of the girder reflected the starlight collected by the outboard mirrors into the telescope, where it was combined at the focus. They were able to resolve the angular separation of close binary stars, and most significantly to resolve the diameter of a number of red stars. The red giant Betelgeuse in the constellation of Orion turned out to have 350 times the diameter of the Sun.

That was as much as they were able to do. Optical interferometry is a continuous struggle against noise introduced by atmospheric variations or the slightest vibration in the measurement apparatus. When Pease tried to expand the interferometer to a baseline of 15 meters, he was completely unable to obtain useful results. Optical interferometry would go no further for half a century.

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[3.7] THE SCHMIDT TELESCOPE / THE CORONAGRAPH

* The Hooker telescope provided a massive jump in capability, but it had its limitations. One of the big ones was that its parabolic mirror only provided a sharp focus in the center of its field of view. Stars and other celestial objects towards the edges of the field of view were elongated, a distortion known as "coma". Coma was not due to any defect in the mirror as such; it was an inherent feature of the design of all reflecting telescopes built to that time. In any case, although the Hooker telescope had an unprecedented capability to inspect the distant reaches of space, it was a poor instrument for performing wide-area sky surveys.

However, during the 1920s a Russian-German telescope maker named Bernhard Voldomar Schmidt (1879:1935) came up with a sophisticated solution, developing a hybrid refractor-reflector telescope that would become known, of course, as a "Schmidt telescope". It used a mirror with a spherical surface, which shouldn't normally be able to bring an image to a reasonable focus, but he added a lens at the objective to compensate. The lens was very unusual, thick at the center and edge and thin at mid-radius, and hard to grind properly. Schmidt was literally handicapped in grinding his lenses, having blown off most of his right arm as a lad during some careless experiments with homemade black-powder bombs, but he was skillful and got the scheme to work, with the first Schmidt telescope going online in Hamburg in 1931.

By that time, sky surveys were of course conducted photographically, and so Schmidt telescopes were initially made as "Schmidt cameras", with a photographic plate inserted at the focus. The plate had to be built with a spherical surface as well. However, the wide field of view made the Schmidt telescope attractive to amateur astronomers, since it not only made for a more impressive view but also made it easier to find targets, and in modern times some of the more expensive amateur telescopes are built as Schmidt telescopes with a Cassegrain eyepiece system.

* There are variations on the Schmidt scheme. There is a "solid Schmidt", with all the optics built as one solid hunk of glass, curved and silvered at the rear, ground to a correcting plate curve and bored with a shaft to contain a photographic plate at the front. It appears to be a rugged and relatively economical design, but the weight suggests it is only useful for relatively small telescopes.

In 1944, a Soviet researcher named D.D. Maksutov introduced a wide-field camera with a spherical mirror like that of a Schmidt, but using a "meniscus" lens with a spherical surface, like a shallow bowl with the bottom mounted towards the mirror. A Dutch telescope maker named Bouers came up with much the same idea in parallel. The scheme could be adapted to a Gregorian configuration, with the correcting plate flipped around and silvered in the center to focus the image through a hole in the mirror into an eyepiece. Maksutovs are also often used as modern amateur telescopes; externally, they look little different from amateur Schmidt-Cassegrains.

Maksutov telescope

George Ritchey had a falling out with George Ellery Hale during work on the 100-inch telescope, but after a unsuccessful shot at farming he went back into telescope design, collaborating with a French optician named Jean Chretien to develop a different scheme for a wide-field photographic telescope. The "Ritchey-Chretien" telescope is very similar to a Cassegrain reflecting telescope, but the main mirror has a hyperbolic curvature, and the photographic secondary plate is slightly convex. Ritchey built a series of such telescopes himself, the ultimate item being a 1-meter instrument that went online at the US Naval Observatory in Washington DC in 1934. It was later relocated to Flagstaff, Arizona. Ritchey-Chretien telescopes are so broadly similar to Cassegrainian telescopes that they are often described as Cassegrains. The Ritchey-Chretien is a common configuration for modern large telescopes.

* Another specialized telescope was introduced in the 1930s for observations of the Sun, or more precisely the Sun's "corona", which could be more or less described as its atmosphere. The corona was an interesting place; it could be observed and photographed during solar eclipses, but of course that didn't permit regular observations. Attempts to build an "artificial eclipse" into a telescope didn't work out very well, with scattered light leaking through and ruining the neat image of the corona.

A French astronomer named Bernard Ferdinand Lyot (1897:1952) realized that he might be able to get around the problem by ensuring that the optics were of the highest quality, with no flaws that might scatter light, and by siting the instrument on top of a high mountain where the air was clear and free of dust. By 1930, he had sited his instrument, which he called a "coronagraph", on a mountaintop in the Pyrenees and was obtaining excellent results. Larger coronagraphs were built later, some of them with evacuated tubes to reduce the diffusion of light by dust in the air to an absolute minimum.

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