The Role of Images in Astronomical Discovery Read online

  Hirshfeld has presented fine portraits of these inventors and eccentrics who have contributed

  in unusual and innovative ways to the progress of astronomy.34 These outsiders unsettled

  the rather quiet professional astronomers.

  While professional astronomers, working mostly in positional astronomy, pushed for

  the use of refractors that they found more tractable and precise, the grand amateur wanted

  larger instruments and pressed for increased aperture. Further, they improved telescope

  mount technology and added a piggyback viewing telescope for sidereal tracking during

  hour-long exposures. The innovative William Lassell was an inspirational figure for the

  American astronomer George Ellery Hale. Lassell built several reflectors of steadily

  increasing size and installed them on equatorial mounts, first using them at his own private

  observatory. The largest, a 48-inch reflector, he moved, with family and all, to Malta,

  32 D. Klumpke, The Isaac Roberts Atlas of 52 Regions, a Guide to William Herschel’s Fields of Nebulosity. Diffuse nebulosity was detected in only four of the Herschel fields.

  33 S. Shapin and S. Schaffer, Leviathan and the Air Pump, Hobbes, Boyle, and the Experimental Life, Princeton: Princeton University Press, 1985, 2011 with a new introduction, pp. 8–9.

  34 A. Hirshfeld, Starlight Detectives: How Astronomers, Inventors and Eccentrics Discovered the Modern Universe, New York: Bellevue Literary Press, 2014.

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  Part I – Images and the Cosmos

  seeking clearer skies, as Hale would later do by moving from Yerkes Observatory in

  Wisconsin to Southern California.

  One telescope built by grand amateurs has had a particularly astonishing story. Andrew

  Common (1841–1903), a plumbing and heating engineer, built “the first of the line of suc-

  cessful large metal-film-on-glass modern reflectors,” the Common’s 36-inch reflector.35

  Common put this instrument to task for astrophotography at his home in Ealing, a sub-

  urb of London. He was quite successful, and he received the gold medal from the Royal

  Astronomical Society for his photographs of “nebulae.” This was start of an extraordinary

  venture for his 36-inch reflector. As he proceeded to build a 5-ft reflector, Common sold

  his 36-inch to Edward Crossley (1841–1905), head of J. P. Crossley and Sons, a family-run

  textile mill in Halifax, Yorkshire.36 In late 1885, Crossley transported his newly acquired

  telescope to Bermerside where he built a new spherical dome to shelter it better. The very

  busy Crossley employed a full-time astronomer and took part in the observations when-

  ever possible. Disappointed by the poor English climate, Crossley decided to dispose of the

  reflector and dome.

  After convoluted negotiations with the Regents of the University of California, finally

  completed in 1895, Crossley donated the telescope and dome to Lick Observatory, located

  on Mount Hamilton in the Sierra Mountains of California. He had to pay for a fair fraction

  of the moving expenses because the university was short of funds for a project it considered

  highly risky. By June 1896, the Crossley, as it came to be called, was installed at the Lick

  Observatory and made suitable for groundbreaking photographic investigations on “nebu-

  lae,” and this opened an amazing new chapter.

  As demonstrated by Parsons and Lassell, reflectors could be made big, and hence were

  more powerful for detecting and observing faint “nebulae.” They would triumph in the

  twentieth century, while the great refractors became “extinct” soon after the arrival of the

  mammoth Yerkes Observatory 40-inch in 1895. With foresight, Isaac Roberts had predicted

  that great progress would be achieved in probing the sky by photography using big reflect-

  ing telescopes, which would bring astrophotography to full maturity. At the dawn of the

  twentieth century, the essential tools were in place to solve the thousand-year-old mystery

  of “nebulae.”

  B – P H OT O G R A P H I N G M Y R I A D S O F G A L A X I E S : T H E G O L D E N

  AG E O F A S T RO P H OT O G R A P H Y

  How Astrophotography Revealed the Distance and Nature of Galaxies

  The increased use of photography, along with the improved sensitivity of photographic

  emulsions at the turn of the twentieth century, contributed to make astronomy a leading

  image science. As astronomers adapted to and adopted photography, telescopes were now

  35 R. P. S. Stone, The Crossley Reflector: A Centennial Review – I & II, Sky and Telescope, 1979, October issue, pp. 307–311; November issue, pp. 396–400.

  36 In 1933, the 5-ft was refurbished and set up as the 1.5-m Boyden–UFS reflector at Boyden Observatory in South Africa.

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  constructed to enable photography. Astrographs were designed to provide a large opti-

  cally corrected field of view. During the first decades of the twentieth century, many of

  the promises alluded to by George Ritchey for “a broader science” were being realized in

  spectacular ways.

  The Giant Steps of Astrophotography

  A new generation of young astronomers took over Draper’s, Common’s and Roberts’ pio-

  neering work in nebular astrophotography in the nascent astronomy community of North

  America. James Keeler and Heber Doust Curtis (1872–1942) at the Lick Observatory on

  Mount Hamilton, central California employed the refurbished Crossley 36-inch reflector in

  most skillful ways.37

  Having been shipped over and received at Lick, the 36-inch was in such a poor shape

  that it was declared by not just a few of the observatory staff as a “piece of junk,” a “mon-

  strosity.” After a difficult transition period involving many repairs, the leading astronomer at

  the observatory, James Edward Keeler (1857–1900), successfully put it to work (Fig. 3.3).38

  However, it was American astronomer Charles Dillon Perrine (1867–1951) who completely

  redesigned and rebuilt the telescope, giving it its familiar look. Due to these diligent efforts,

  the telescope mount was rebuilt and the telescope transformed into a powerful astrograph

  that started operation in 1898.

  The high mountain site also offered fine observing conditions. Thus equipped with

  an optically fast, very fine, glass mirror, Keeler and Curtis conducted a systematic pho-

  tographic survey of the sky with a new wide-field photographic camera. Following the

  untimely death of Keeler at the age of 43 in 1900, Curtis vigorously carried out the Lick

  Observatory “nebulae” astrophotography program. The Crossley camera covered 0.9 degree

  at any one time, a field with an area almost ten times that of the apparent size of the Moon.

  With a survey covering only a few areas of the sky, the deep photographs revealed thousands

  of galaxies. Extrapolating from these counts, Curtis estimated in 1918 that one million or

  more spirals would be detectable with the Crossley, if the whole sky were photographed to

  the same depth.39

  In 1918, Curtis published extensive papers accompanied by detailed descriptions of pho-

  tographs of “nebulae” and star clusters made with the Crossley from 1898 until 1 February

  1918 (Fig. 3.4). The lists contained 762 entries, of
which 513 were classified as spirals. Cur-

  tis was quite clear about the nature of the latter objects: “It is my belief that all the many

  thousands of nebulae not definitely to be classed as diffuse or planetary are true spirals. . . .

  37 Built by Andrew A. Common in Great Britain in 1879, the Crossley was the first large reflector to be equipped with a concave, silver-coated glass mirror. Having been refurbished several times, the telescope is still in use today. For a fine historical review, see Remington P. S. Stone, The Crossley Reflector: A Centennial Review – I & II, Sky and Telescope, 1979, October issue, pp. 307–311; November issue, pp. 396–400.

  38 J. Keeler, The Crossley Reflector of the Lick Observatory, Publications of the Astronomical Society of the Pacific, 1900, Vol. 12, pp. 146–167.

  39 H. D. Curtis, The Number of the Spiral Nebulae, Publications of the Astronomical Society of the Pacific, 1918, Vol. 30, pp. 159–161.

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  Fig. 3.3 The Lick Observatory 36-inch Crossley reflector, still with the original structure from

  Crossley. From Keeler (1900). The Astrophysical Journal. C

  AAS. Reproduced with permission.

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  Fig. 3.4 Heber Curtis. Credit: Bentley Historical Library, University of Michigan.

  Were the Great Nebula in Andromeda itself situated five hundred times as far away as at

  present, it would appear as a structureless oval about 0.’2 long, with very bright center, not

  to be distinguished from the thousands of very small, round or oval nebulae found wherever

  the spirals are found.”40 (Note that 0.’2 means 0.2 arcminute of angular size.)

  Curtis’ article provides interesting reading as it followed very much the tabular listing

  of objects of the reports of the Birr Castle works published by the Third and Fourth Earls

  of Rosse decades earlier. Similarly, Curtis employed short telegraphic descriptors to depict

  the various objects: for example NGC 7723 is “A nearly round, rather faint, open spiral of

  the -type, 1’.5 long. Bright stellar nucleus. 6 s.n.” (Note that meant that it was classi-

  fied as a barred spiral.) However, there was a stark difference with the earlier publications:

  Curtis’ article was accompanied by a fine set of photographic plates illustrating the differ-

  ent forms of barred spirals. Despite Curtis’ achievements, observing with the refurbished

  36-inch remained challenging. American astronomer William Hammond Wright (1871–

  1959), director of the Lick Observatory from 1935 to 1942, wrote sarcastically “the only

  40 H. D. Curtis, Descriptions of 762 Nebulae and Clusters Photographed with the Crossley Reflector, Publications of the Lick Observatory, 1918, Vol. XIII, Part I, p. 12.

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  Part I – Images and the Cosmos

  Fig. 3.5 George Ellery Hale. Credit: University of Chicago Photographic Archive, [apf6–00263],

  Special Collections Research Center, University of Chicago Library.

  comfortable and safe way to work with the Crossley would be to flood the dome with water

  and observe from a boat.”41 But another team of American astronomers soon came on the

  scene with new revolutionary telescopes and cameras at a fine mountain site.

  Trailblazer of Modern Astrophotography

  During the first decade of the twentieth century, a gigantic effort in creating a completely

  new astronomical facility was taking place at Mount Wilson Observatory, Southern Cali-

  fornia. American astronomer George Ellery Hale (1868–1938), who understood the impor-

  tance of good sites for astronomical work, was leading the colossal venture (Fig. 3.5).

  Originally at Yerkes Observatory in Wisconsin, Hale was a visionary astronomer with an

  unusual ability for promoting science and an incredible instinct for successful fundraising.

  He applied these talents to the creation of scientific institutions and the building of big

  41 Cited by R. P. S. Stone, The Crossley Reflector: A Centennial Review – I & II, Sky and Telescope, 1979, November issue, p. 400.

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  telescopes.42 Hale also understood well that the size of telescope was not everything. Good

  instruments and a creative staff were needed as well. Having raised ample amounts of

  money, he swiftly used these funds, hiring researchers at the forefront and constructing

  superb astronomical facilities equipped with the best cameras and spectrographs.

  Among the newly hired staff was George Willis Ritchey (1864–1945). Ritchey was a

  first-class engineer and inventor who saw the enormous advantages of the arrival of large

  blank disks of glass that could be configured and polished to an extremely smooth and pre-

  cise optical surface. Following the successful techniques developed by French astronomer

  Léon Foucault (1819–1868) at Marseille Observatory, he knew that the glassy surface could

  also be coated with a thin layer of silver for optimal reflection – as had been done with the

  Crossley. Because of their superiority in several physical and technical aspects, glass mir-

  rors supplanted metal ones (and glass lenses) as soon as they could be cast at meter size

  and larger. In addition, Ritchey redesigned mechanical structures for telescope mounts and

  drives appropriate for large reflectors. These innovations resulted in spectacular improve-

  ments in the optical quality of telescopes and in the precision of their mechanical movement.

  Ritchey had understood the requirements and potential of photography. He spearheaded

  the new technology of photography in unique ways. While at Yerkes Observatory, the insti-

  tution that then hosted the largest refractor in the world, he designed and constructed a fast

  (f/4) 24-inch reflector in 1901 (Fig. 3.6). He conducted illustrative demonstrations high-

  lighting his design. Until then, only grand amateurs favored reflectors, if we exclude Léon

  Foucault in France. “Ritchey took a spectacular series of photographs of nebulae with it

  [the 24-inch], demonstrating to skeptical astronomers just how useful reflecting telescopes

  could be.”43 Having moved to Mount Wilson, Ritchey became lead designer for the new

  telescopes being built at the new California site. He designed two scientific monuments

  optimized for deep photographic work: the 60-inch and in part the 100-inch reflecting tele-

  scopes of the Mount Wilson Observatory (Figs. 3.7a and 3.7b). Francis Gladheim Pease

  (1881–1938), who had been Ritchey’s assistant, replaced him as project leader on the 100-

  inch, for reasons we will see later. The 60-inch was the first great modern reflector. Many

  considered it an engineering marvel.

  Together, Hale and Ritchey made a powerful scientific tandem that lasted for a few

  decades. They left a huge and rich legacy. The fruits of the Hale–Ritchey alliance were

  an extraordinary set of new astronomical facilities capable of exploiting the exquisite con-

  ditions of the mountain sites of southern California with the added power of astrophoto-

  graphy. Due to Hale’s excellent management and diligent work, the 60-inch reflector was

  in operation
before the end of 1908. Combining passion with a fanatic sense of preci-

  sion, Ritchey came with a background that made him an outstanding astrophotographer.

  He used the 60-inch to obtain deep photographs of “nebulae,” spirals in particular. Many

  of Ritchey’s photographs were obtained through very long exposures: to reach as deeply

  42 J. R. Goodstein, Millikan’s School, New York: W. W. Norton and Company, 1991, pp. 64–87.

  43 D. E. Osterbrock, Yerkes Observatory 1892–1950: The Birth, Near Death, and Resurrection of a Scientific Research Institution, Chicago: University of Chicago Press, 1997, pp. 36–37.

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  Fig. 3.6 George Ritchey’s 24-inch reflector (1933). Credit: University of Chicago Photographic

  Archive, [apf6–01377], Special Collections Research Center, University of Chicago Library.

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  (a)

  Fig. 3.7 (a) Century-old modernized 60-inch reflector at Mount Wilson Observatory. Credit: Creative

  Commons Attribution/Heven729.

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  (b)

  Fig. 3.7 (b) The Mount Wilson 100-inch reflector as seen today. Credit: Creative Commons

  Attribution/Ken Spencer.

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  as possible, exposures on the same plate were spread over several nights, in one case total-

  ing 60 hours. In their 1935 classic work, Lunettes et télescopes, French astronomers André

  Danjon and André Couder wrote that the photographs obtained by Ritchey with the Mount

  Wilson Observatory 60-inch reflector 25 years earlier were still the best available.44

  Astrophotographer Guru

  To highlight the excellence of the 60-inch optical design and its mechanical accuracy,

  Ritchey repeated what he had done with the Yerkes 40-inch refractor and his 24-inch reflec-

  tor, producing spectacular photographs of the Moon to demonstrate the quality and power