The Role of Images in Astronomical Discovery Read online

  of the new large reflector.45 The images also confirmed the importance of locating such

  instruments on dry, high mountain sites such as those of southern California. Indeed, these

  high mountain sites provided not only extensive cloudless periods, but also benefited from

  the stabilizing effects of the cool deep waters of the Pacific Ocean. Long periods of uninter-

  rupted air stability led to finer images, better than anything achievable at low-altitude sites.

  Gone was the “boiling” atmosphere that affected the Birr Castle observers.

  Ritchey’s photographs surpassed, in their depth and definition, anything that had been

  done before. This allowed him to tackle the Gordian knot of the distances to “nebulae.” He

  started searching for variable objects in them. In photographing the “nebulae,” he found and

  followed relatively rare types of explosive stars, novae. Several of these had been observed

  in the Milky Way over the previous decades and their light curves were well established.

  Novae are not regular pulsating stars. Their brightening is triggered by the “dumping” of

  material from a red-giant companion onto a white dwarf. A nova corresponds to the flash

  of light, ignited by catastrophic nuclear detonation at the surface of the receiving star. The

  stellar blast causes a rapid brightening of several magnitudes, a phase that lasts for sev-

  eral days to a few weeks. Hence, novae could be registered on photographic plates taken

  only weeks apart. Although he could then not have known the nova mechanism, Ritchey

  declared correctly that novae were as numerous in Messier 31 as they were in the Milky

  Way, about a dozen events a year. The objects appeared very faint, he explained, because

  of their much greater distances. He followed a nova in Messier 31 over several nights. He

  also found a nova in Messier 81, two in Messier 101 and one in NGC 2403.46 Assuming

  they were analogues of novae in our own Milky Way and scaling for their difference in

  brightness, he derived distances to the host systems to be millions of light-years! Ritchey

  became convinced of the extragalactic nature of most “nebulae.”

  Donald Osterbrock summarized Ritchey’s main abilities as follows: “superb tech-

  nical skill and instrumentation, striking photographs, and very few hard scientific

  44 A. Danjon and A. Couder, Lunettes et télescopes, Paris: Librairie scientifique et technique Albert Blanchard, 1999, p. 694

  (original work published in 1935).

  45 A collection of George Ritchey’s photographs (including glass plates) were auctioned by Bonhams for about half a million US

  dollars in 2014.

  46 “Novae” were not all of the same nature: those in Messier 81, 101 and NGC 2403 were actually supernovae, which are much more powerful and brighter exploding stars than ordinary novae, as demonstrated by Rudolph Minkowski and Fritz Zwicky (1941).

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  Fig. 3.8 Transformational Image: Photographs of “Novae” in Spirals. Supernovae in NGC

  4321 that Curtis thought were novae, significantly less luminous objects. From Curtis (1917), Lick

  Observatory Bulletins.

  results.”47 Indeed, the procrastinating Ritchey did not proceed fast enough with publish-

  ing his results, probably not having realized the significance of his novae finding. Curtis

  at Lick Observatory did otherwise. Rushing to the 36-inch plate archive of spirals, Curtis

  found several more novae. He quickly prepared and published two excellent papers before

  Ritchey managed to do so (Fig. 3.8).48 From these “novae,” he argued spirals to be 20

  million light-years away and 60,000 light-years in diameter.

  Images and the Astronomical Time Domain

  By the end of the first decade of the 1900s, astronomers had available a new genera-

  tion of superb telescopes located at outstanding sites of mountainous California, Mount

  Wilson in particular. The conditions of air stability that prevailed at these high sites helped to

  finally establish the superiority of photography over visual observing and hand drawing. By

  47 D. E. Osterbrock, Pauper & Prince: Ritchey, Hale & Big American Telescopes, Tucson: University of Arizona Press, 1993, p. 52.

  48 H. D. Curtis, Three Novae in Spiral Nebulae, Lick Observatory Bulletin, 1917, Vol. 9, Number 300, pp. 108–110.

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  developing telescope systems for long exposures, astrophotographers finally demonstrated

  that photographs were much more reliable recorders of astronomical features, especially of

  flimsy objects such as “nebulae.” The reliability of the photograph quickly surpassed that

  of drawings, even when supplemented by extensive notes. Photography was especially use-

  ful and faithful at registering elusive features that the eye could never have registered nor

  sketched reliably. The amount of darkening on the emulsion could be related quantitatively

  to the brightness of objects. Photographs did not need much of a commentary; just a short

  caption and a description of the technical set-up sufficed.

  In the light of these new achievements, the excruciating work of Birr Castle observers

  appeared now to be quite obsolete. At the same time, it was realized how meritorious their

  findings had been. This impression was reinforced by the confirmation that what the Earl

  of Rosse’s team had recorded turned out to be relatively faithful images of “nebulae.”

  Commenting on Parson’s drawings of Messier 31 (Andromeda Galaxy) executed with the

  Leviathan, Isaac Roberts wrote: “These make one acquainted with the difficulties encoun-

  tered by the observers in their efforts to make intelligible, by descriptive matter and by

  drawings, an object having a structure so complex that no eye and hand, however well

  trained, could possibly delineate it.”49

  It was not only a matter of a more faithful recording of features. Photographs made it

  easier to compare a new observation of a given object with a previous observation. Time-

  domain observing was put on a solid footing. With novae, Ritchey and Curtis had shown the

  power of astrophotography in discovering and monitoring time-variable phenomena.50 One

  just needed to compare images taken a few nights, weeks or months apart. Furthermore, the

  relatively large field of view afforded by photographic plates provided a number of stars that

  could be used as coordinate references and calibrators of brightness. Calibration ensured

  that quantitative physical information was derived from photographs. The brightness of new

  stars could be established from other reference stars outside the field; the latter became the

  “standard stars,” specific objects photometrically calibrated through a rigorous procedure.

  This was the origin and driver of the massive photometric and spectroscopic surveys of

  Harvard College Observatory.

  An Unruly Perfectionist

  Unfortunately, the eccentric and individualistic Ritchey ran into trouble with his colleagues.

  Ritchey was an absolute perfectionist. “An artist rather than a ‘bench’ research scientist,

  Ritchey took infinite pains to achieve perfection. If he thought a certain piece of equipment

  would help perfect the final product, he made request after request until he
obtained the

  item.”51 Furthermore, in his mind “the goal of a telescope was to produce pretty pictures

  49 I. Roberts, Photograph of the Spiral Nebula M33 Trianguli, Monthly Notices of the Royal Astronomical Society, 1895, Vol. 56, p. 70.

  50 G. Ritchey, Novae in Spiral Nebulae, Publications of the Astronomical Society of the Pacific, 1917, Vol. 29, pp. 210–212.

  51 A. R. Sandage, Centennial History of the Carnegie Institution, Volume 1: The Mount Wilson Observatory, Cambridge: Cambridge University Press, 2004, p. 169.

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  of the highest technical quality, regardless of any other use the images might have for a sci-

  entific project.”52 Therefore, it was no surprise that the arrogant Ritchey became his own

  worst enemy, the victim of his own sense of importance. American astronomer and astron-

  omy historian Donald Osterbrock described the tandem of Ritchey and Hale as “pauper and

  prince.”53 Around 1912, Ritchey fell from grace.

  Ritchey’s egotist personality, his grandiose visions and failures at delivery on promises

  were factors that had made relations with his Mount Wilson colleagues turn sour. He had

  unwisely bypassed Hale in approaching donors, and had misrepresented his role and author-

  ity too many times. In reaction, Hale and Walter S. Adams treated him quite unfairly by for-

  bidding him access to the 100-inch, by blocking his nominations to prestigious prizes and

  by censoring publications that highlighted Ritchey’s achievements. “Ritchey was unsparing

  of himself and of those who worked with him. He believed that he alone knew the secrets of

  making large telescopes; he was contemptuous of rivals like John A. Brashear and Francis

  Pease. Ritchey was tremendously self-centered, and in the end his failure to accommo-

  date himself to the vision of this director cost him his job at Mount Wilson.”54 In 1919,

  following years of insubordination and disagreements with Hale and with the succeeding

  director Walter S. Adam, as well as with other staff, he was dismissed from Mount Wilson

  Observatory.

  Ritchey’s Wide-Field Imaging Legacy

  After leaving Mount Wilson Observatory, Ritchey went to France where he worked with

  French optical designer Henri Chrétien (1879–1956). Both understood the power of wide-

  field photography, as they pushed for bigger fields of view. Being the perfectionist he was,

  Ritchey worked to correct optical aberrations. Mirror systems with parabolic mirrors led

  to coma, a simple optical aberration that produces elongated images of point sources away

  from the optical axis. Working together, Ritchey and Chrétien developed a radically new

  concept of telescope optics, with new curving surfaces that eliminated coma (Fig. 3.9; com-

  pare with Fig. 3.6). They designed primary and secondary mirrors with hyperbolic shapes

  instead of parabolic ones. A result was that the shape of images remained perfect over a

  large field view in the focal plane, empowering even more astrophotography in catching a

  greater patch of the sky in each exposure. Ritchey’s later work was devoted to designing

  future very large telescopes with primary mirrors up to 8 m in diameter. Chrétien is also

  known as the inventor of cinemascope and was awarded a Hollywood “Oscar” from the

  American Academy of Motion Pictures Arts and Science in 1954.55

  52 A. R. Sandage, Centennial History of the Carnegie Institution, Volume 1: The Mount Wilson Observatory, Cambridge: Cambridge University Press, 2004, p. 169.

  53 D. E. Osterbrock, Pauper & Prince: Ritchey, Hale & Big American Telescopes, Tucson: University of Arizona Press, 1993.

  54 D. E. Osterbrock, Pauper & Prince: Ritchey, Hale & Big American Telescopes, Tucson: University of Arizona Press, 1993, pp. 284–285.

  55 Henri Chrétien received the Academy Award of Merit along with Earl Sponable, Sol Halperin, Lorin Grignon, Herbert Gragg, and Carlton W. Faulkner: “For creating, developing and engineering the equipment, processes and techniques known as [20th Century Fox’s] CinemaScope.” The award was also shared with Fred Waller, “ For designing and developing the multiple photographic and projection systems which culminated in Cinerama .”

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  Fig. 3.9 George Ritchey and Henri Chrétien with a 20-inch telescope of their design in 1927.

  C

  Françoise Le Guet-Tully, Fonds Henri Chrétien, with permission.

  Throughout the twentieth century, larger photographic fields and better images became

  a never-ending challenge and goal for telescope and astronomical camera designers and

  optical engineers. Most telescopes nowadays, including the Hubble Space Telescope, use

  the Ritchey–Chrétien design. Notwithstanding the unhappy ending at Mount Wilson, it is

  impossible to overstate the technical mastery and the innovative concepts of the great engi-

  neer and telescope designer that Ritchey was. “Ritchey was an outstanding telescope maker

  and the archetype of all telescope makers.”56 In several ways, his obsession for beautiful,

  large-scale and finely resolved telescopic images paid off.

  In the 1920s, the 60-inch and 100-inch telescopes at the Mount Wilson Observatory

  became the fundamental tools to confront the foremost problems of the nature of galaxies

  and of the structure of the universe. The mapping of our own Milky Way received a boost.

  The plates obtained with the 60-inch helped to establish the numbers of stars to fainter

  56 D. E. Osterbrock, Pauper & Prince: Ritchey, Hale & Big American Telescopes, Tucson: University of Arizona Press, 1993, p. 283.

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  magnitudes and to reconstruct the distribution of stars in our galaxy. William Herschel’s

  dream was achieved. “It was the culmination of the Mount Wilson program to provide

  basic data on faint stars for the Kapteyn program.”57

  The new large reflectors and cameras also represented technological and operational

  breakthroughs that paved the way a decade later for the groundbreaking work of Edwin

  Hubble, Milton Humason, Fritz Zwicky and Walter Baade. Using the 60-inch and the 100-

  inch, Edwin Hubble was able to confirm the extragalactic nature of the majority of the

  “nebulae.”. In Chapter 6, I show how the great telescopes enabled the ultimate breakthrough

  in the exploration of galaxies.

  As companies, especially Eastman Kodak, developed new emulsions for the specific

  needs of astronomy, astrophotography underwent several major improvements throughout

  the twentieth century, which provided both finer resolution and increased sensitivity of the

  photographic plate. Creative processing techniques also helped greatly. British–Australian

  astronomer David Malin, a chemist and microscopist in his early career, was among the

  few who pushed forward with innovative techniques: the stacking of exposures from sev-

  eral plates to increase the signal-to-noise ratio; unsharp masking to reveal faint structures

  barely above the level of the sky background; and the production of three-colour wide

  field images.58 In the late 1980s, electronic digital detectors largely superseded silver-based

  photographic emulsions. The main technology employed ch
arge-coupled devices (CCDs),

  which were several times more sensitive. In addition, their response is linear (the signal

  is proportional to the amount of light hitting the detector) and they can now be assembled

  in mosaics larger than the largest photographic plates. Nevertheless, several of the original

  photographic processing techniques were successfully adapted, improved and optimized

  for use with electronic images.

  Scientific Drawing in the Modern Age

  One may well then ask: “With the advent and universal use of photography, has astro-

  nomical drawing become obsolete?” Certainly not, as various forms of sketching have

  remained useful for scientific illustration, astronomy being no exception. Swiss astronomer

  Fritz Zwicky (1898–1974) gave fine examples of how drawing could be used effectively

  to complement photography. Working at the California Institute of Technology, Zwicky

  remains well known today for surveys of clusters and groups of galaxies and for several

  other brilliant ideas. A Swiss astronomer working at the California Institute of Technology

  and a frequent user of the Mount Wilson Observatory, and later of the Palomar Observatory,

  Zwicky was a very creative and innovative astrophysicist. He made important contributions

  to twentieth-century astrophysics.59 He is credited with several revolutionary concepts, such

  57 A. R. Sandage, Centennial History of the Carnegie Institution, Volume 1: The Mount Wilson Observatory, Cambridge: Cambridge University Press, 2004, p. 225.

  58 D. Malin and P. Murdin, Colors of the Stars, Cambridge: Cambridge University Press, 1984.

  59 A. Stöckli and R. Müller, Fritz Zwicky: An Extraordinary Astrophysicist, Cambridge: Cambridge Scientific Publishers, 2011.

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  Fig. 3.10 Fritz Zwicky at the Palomar 18-inch Schmidt Telescope in 1936. Credit: Courtesy of the

  Archives, California Institute of Technology.

  as neutron stars, gravitational lenses, interacting galaxies and for the first unassailable evi-

  dence of the prevalence of dark matter in the universe (Chapter 8).

  Zwicky had been using the light-tight Palomar Mountain 18-inch and 48-inch Schmidt