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The Role of Images in Astronomical Discovery

Page 11

by Rene Roy


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  properly classed among the ‘spiral nebulae’, under the definition given by their first discov-

  erer, William Parsons, the Third Earl of Rosse: including in the term all objects in which a

  curvilinear arrangement, not consisting of regular re-entering curves, may be detected.”7

  In Europe, the controversy was raging. English astronomer Richard A. Proctor (1837–

  1888) said that the Birr Castle drawings were the fanciful product of imagination gone

  astray or alluring artistic trends. German astronomer Wilhelm Tempel (1821–1889), our

  most vocal doubter, wrote that the spiral form did not exist among nebulae and that it

  was a mere creature of fantasy.8 Were the many features of “nebulae” discovered at Birr

  Castle, especially the spirality that appeared to be shared by hundreds of systems, real?

  The skeptic Tempel was not entirely wrong in criticizing overconfident apostles of spi-

  rality, as they tend to generalize the pattern and draw more continuous structures than

  there really were. Photography would either confirm the reality or show these features as

  illusions.

  Photographs versus Drawings

  Photography brought an epistemological revolution. Drawing, even by a most experienced

  and careful observer like Worthington, could be unreliable. Worthington was extremely

  honest and candid in his assessment. His 1908 book summarized his research and included

  197 photographs to accompany the short text.9 While sketching, the observer consciously

  or unconsciously singles out features in a certain way. The photograph does not select. Con-

  sequently it records the irregularities and accidental features that the mind might consider

  irrelevant or distracting. This is what Worthington had experienced: the role of the self as

  a partial observer. The switch from recording the same experiment by visual drawing to

  using photography had unveiled this epistemological bias in a brutal way.

  Still, despite its inherent selective mode of construction and production, most schol-

  ars agree that drawing serves an important learning function. Sketches introduce various

  degrees of interpretation of the object under study. With the understanding and acceptance

  that drawing highlights the subjective elements, the process of viewing and selecting can

  be seen as a teaching and learning asset. This can be employed as a pedagogical tool. As

  I will show later, photography did not instantaneously or completely displace drawing and

  sketching.

  Indeed, there are cases where the science “artist” intentionally adapts the representation

  for educational or esthetic purposes. Of his painting Portrait of a Crater (Second Effort) of

  1993, astronaut Alan Bean writes: “The Moon is mostly neutral gray rocks and dust with

  no atmosphere to soften the brilliant Sun or lighten dark shadows. To reproduce this world

  in paint is more science than art, so I ‘key’ the lunar surface neutral off-gray. That is, I paint

  7 G. P. Bond, On the Spiral Structure of the Great Nebula in Orion, Monthly Notices of the Royal Astronomical Society, 1861, Vol. 21, p. 205.

  8 O. W. Nasim, Observing by Hand: Sketching the Nebulae in the Nineteenth Century, Chicago: University of Chicago Press, 2013.

  9 A. M. Worthington, A Study of Splashes, London: Longmans, Green, and Co., 1908. The book was “dedicated to the Natural History Society of Rugby School and its former president Arthur Sidgwick in remembrance of the encouragement given to the early observations made in boyhood by my old school-friend H. F. Newal from which this study sprang.”

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  Part I – Images and the Cosmos

  the lunar soil slightly greenish gray, or orangish gray, and so forth. This technique opens up

  possibilities for hues that do no exist if a neutral gray is selected.”10 Sometimes, drawing

  and sketching may lead the mind the right way, possibly more effectively than photography.

  Baby Steps for Astrophotography

  The first successful astronomical photograph known was probably a daguerreotype of the

  Sun obtained by French physicist and artist Louis Daguerre (1787–1851) and astronomer

  François Arago (1786–1853), taken during the solar eclipse of 15 March 1839.11 The

  daguerreotype was one of the first photographic processes: a latent image is recorded

  on a light-sensitive compound (silver bromide) and revealed by a chemical process.

  Early daguerreotypes required very long exposure times – several seconds for brightly lit

  objects. Subsequently, many researchers improved the process. A more effective process of

  negative-on-glass was invented in the late part of the nineteenth century and was quickly

  applied to astronomy. Among the first applications of astrophotography were the monitor-

  ing programs of the brighter celestial objects. For example, French astronomer Jules Janssen

  initiated a photographic program of the Sun at Observatoire de Meudon from 1876 to 1903.

  “Cinematograph” photographs of sunspots, to track their changes, were first taken around

  1900. However, astronomers in general were hesitant to use the new imaging technique.

  There were some practical reasons why it took some time for photography to be accepted

  by professionals in astronomy. Apart from lack of sensitivity, one important issue was the

  spreading of starry images as a blob of light on photographs, making precise astrometry

  much more difficult than measurements made with a micrometer based on visual observa-

  tion. Furthermore, the changing image quality due to atmospheric turbulence blurred pho-

  tographic images that required longer exposure times, as starlight was spread over a larger

  area of the photographic plates. Contrary to continuous photographic exposure, the eye can

  select the moments of the sharpest images. It is not surprising that several astronomers con-

  sidered photographs a gimmick. For some time, most hated or ignored photography.12 This

  was particularly true for the observations of the Moon and the Solar System planets. Until

  the invention of quick video cameras well into the twentieth century, visual observations

  and sketching of planet features were regarded as the best ways to capture and illustrate fine

  details on planets. Another damning problem with photographic emulsion was the fading

  of images over time due to the perishable nature of early glass negatives. Drawings could

  last for centuries.

  Although not as bright an object as the Sun, the Moon turned out to be less challeng-

  ing for early photographic work. In the winter of 1839–1840, American chemist John

  William Draper (1811–1892), father of astronomer Henry Draper (1837–1882), obtained

  10 A. Bean, Painting Apollo – First Artist on Another World, Washington DC: Smithsonian Books, 2007, p. 211.

  11 J. Lequeux, François Arago: un savant généreux – Physique et astronomie au XIXe siècle, Les Ulis: EDP Sciences/Paris: Observatoire de Paris, 2008, p. 424.

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

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  a daguerreotype of th
e Moon through a telescope. Later, around 1850, William Bond

  (1789–1859), using the 15-inch refractor at Harvard College, obtained successful images

  of the Moon. His son, George Bond, whom we have seen observing the “great nebula” in

  Andromeda, pursued the improvement of photography for astronomical use. He recorded

  images of stars; still, his photographic plates could not reach stars of brightness fainter than

  those seen with the naked eye. He might have been the first astronomer to make quantitative

  photometric measurements on photographic plates, a truly revolutionary step.13,14 George

  Bond invited American inventor and photographer John Adams Whipple to use the Har-

  vard 15-inch and produce daguerrotypes of the Moon. The “high-tech” images, View of the

  Moon, became prize winners at the 1851 Great Exhibition in London.

  Henry Draper was a pioneer of astrophotography; he photographed the Orion Nebula

  in 1880.15 It showed a few fuzzy blobs, nothing to impress and stop astronomers from

  drawing. It was on the old continent that faster progress in astronomical photography was

  being made. Using his 3-ft telescope on 29 February 1883, English amateur astronomer

  Andrew Ainslie Common (1841–1903) also obtained a photograph of the Orion Nebula

  “revealing stars never seen by visual observers.” He won the Royal Astronomical Society’s

  Gold Medal for this image (Fig. 0.3). Others were catching on and progress was swift. In a

  major parallel effort, English astronomer and spectroscopist William Huggins (1824–1910)

  obtained photographic spectra (instead of images) of all bright stars between 1876 and 1886

  (see Plate 5.1 for examples of modern spectra). Interestingly, the push forward was driven

  more by (rich) amateur astronomers, as professionals remained for a long time stubborn

  skeptics of the usefulness of photography for astrometry.

  Dutch Prisoners Construct Maps of the Sky

  Photographic techniques steadily improved in several crucial aspects, which contributed to

  improving their usefulness and reliability in astronomy: the production of durable photo-

  graphic emulsions on dry plates; the optimization of chemical processing; better recipes

  for increased sensitivity; the establishment of appropriate exposure times for the various

  types of astronomical objects, and not least, developing robust techniques for accurately

  measuring the position and brightness of objects on the plates. Star catalogues were the

  immediate beneficiaries of the introduction of photography in astronomical work. The end

  result was unassailable: the positions and magnitudes of stars could be measured in much

  greater number and more accurately than by traditional visual astrometry.

  By the late nineteenth century, the sensitivity of emulsions had improved significantly.

  Much fainter objects than could be recorded visually at the telescope eyepiece were being

  photographed reliably. The number of objects that could be measured exploded and there

  13 G. P. Bond, Stellar Photography, Astronomische Nachrichten, 1858, Vol. 49, pp. 81–100.

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

  15 A. Hirshfeld (op. cit.) gives a fine overview of the development of astrophotography in the United States during the nineteenth century.

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  Part I – Images and the Cosmos

  was an expressed need for a more organized effort in handling astronomical photographs.

  Several initiatives at photographing the sky led to an ambitious international project, the

  “Carte du ciel,” which began in 1887. This project was to involve 20 observatories; it

  aimed at photographing the whole sky to a magnitude as faint as 11.5, or a couple of

  hundred times fainter than could be perceived by the naked eye. Well ahead of its time,

  the project encountered several technical difficulties and managerial problems. Soon, its

  promoters had to descope the venture. “When ninety-six of the Paris charts had been

  issued, it was estimated that the charts covering the entire sky would make a pile 120 feet

  high!”16

  The Astrographic Catalogue became the more modest, but feasible, version of the

  project. Twenty observatories from around the world collaborated over decades, producing

  and measuring about 22,000 glass plates. The first such plate was taken in August 1891, at

  the Vatican Observatory, and the last plate at the Brussels Uccle Observatory in December

  1951. The Astrographic Catalogue was largely ignored until the arrival of the Hipparcos

  Catalogue in 1997. Hipparcos was the European astronomical satellite that photographed

  2.5 million stars for accurate star positions. One of its purposes was the accurate determina-

  tion of proper motions and parallaxes. This required the use of the historical plate material

  for comparing changing star positions over time; the Astrographic Catalogue provided this

  time reference.

  But let us go back to the end of the nineteenth century. Instead of aiming for the whole

  sky, fields of different galactic latitudes and longitudes were selected and distributed among

  institutions for study. A Dutch astronomer whose intent was to establish the structure of the

  Milky Way, the old dream of William Herschel, led a most successful initiative. Jacobus

  Cornelius Kapteyn (1851–1922) of Groningen University in the Netherlands managed to

  complete a fair fraction of a complete photographic sky survey. Between 1896 and 1900,

  Kapteyn measured thousands of stars recorded on photographic plates; these had been

  obtained by the Scottish astronomer David Gill (1843–1914) who worked at the Cape

  Observatory in South Africa between 1885 and 1890. Kapteyn produced a huge database

  of stellar positions and derived important statistical results on the distribution of stars in

  the Milky Way. The “Kapteyn Universe” described a lens-shaped star distribution 40,000

  light-years in size, decreasing in stellar density towards its edges, and with the Sun located

  about 2,000 light-years from the center.

  The monumental project resulted in augmenting the famous Cape Photographic Durch-

  musterung; this listed the positions and magnitudes of 454,875 stars between 18 degrees

  south in declination and the celestial north pole. There was an interesting aside to this

  research conducted under Kapteyn at Groningen. To accomplish the gigantic task of mea-

  suring thousands of star positions, Kapteyn requested and got the permission from the gov-

  ernor of the town to enroll selected male state prisoners to conduct the measurements.17

  16 R. G. Aitken, Dorothea Klumpke Roberts – An Appreciation, Publications of the Astronomical Society of the Pacific, 1942, Vol. 54, p. 219

  17 J. North, Cosmos: An Illustrated History of Astronomy and Cosmology, Chicago: University of Chicago Press, 2008, p. 520.

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  Judging from the quality of the work published, this was a successful venture of early

  “citizen science.” One hopes that the prisoners also learned something from their unusual

  task.

  At the same time that Kapteyn and Gill were constructing their map of the heavens,

  another importa
nt effort was being conducted, this time on the new continent. Harvard

  College Observatory entered the photographic era by initiating a systematic photographic

  survey of the sky (as well as spectroscopic surveys), carried out with imaging telescopes

  in Massachusetts, Peru and South Africa. More than 500,000 glass photographic plates

  were exposed between 1885 and 1993. These plates are currently being digitized and made

  available for reference in a modern astrometry database. At Harvard, instead of prisoners, an

  extraordinary female team of “human computers” were analyzing, calculating, tabulating

  and publishing the data extracted from the photographic plates.18

  The Carte du ciel had gone through a near-death episode, and was only partly executed

  as the Astrographic Catalogue. The equivalent of the Carte du ciel was successfully res-

  urrected half a century later, under the sponsorship of the National Geographic Society,

  and the Palomar Sky Survey for the northern hemisphere was thus completed in 1958.

  In coordination, the Science and Engineering Research Council of the United Kingdom

  and the European Southern Observatory sponsored the Southern Sky Atlas for the southern

  skies. These surveys provided an important imaging basis for identifying galaxies and clus-

  ters of galaxies. They became the reference image sources for several atlases of galaxies

  (Chapter 10).

  The photographic plates of the two all-sky surveys were later to undergo a major revival.

  The scientific operation of the Hubble Space Telescope (launched in 1990) required accu-

  rate positions for the celestial objects it would be looking at. So the Space Telescope Science

  Institute led an enormous project: the full digitization of the thousands of survey photo-

  graphic plates. The regenerated survey was released as the Digitized Sky Survey in 1994.

  The second-generation Guide Star Catalog was published in 2008 and it contains close

  to a billion objects.19 The initial goals of the Carte du ciel were more than fulfilled 100

  years later: the whole sky has been photographed and the final product is beyond what

  early nineteenth-century astrophotographers could have dreamt of. Yet the drive to image

  the whole sky has continued unabated.20 A new generation of telescopes is being built with

 

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