The Role of Images in Astronomical Discovery Page 15
telescopes for photographic surveys of galaxies (Fig. 3.10). The name of the telescope optics
design recognizes German optician Bernhard W. Schmidt (1879–1935), who invented the
concept. The Schmidt design combines a Cassegrain reflector configuration and a large
corrector plate, which results in a compact telescope that gives a super-wide field of view.
Schmidt telescopes, although of small aperture, provide significantly larger fields of view
than even the Ritchey–Chrétien design. Fields of view several times the angular size of the
Moon can be imaged in a single shot. Schmidt telescopes were most suitable for Zwicky’s
work because galaxy groupings were of larger angular sizes than the fields of view of large
telescopes, such as the Mount Wilson 60-inch and 100-inch.
Zwicky had found a “surprisingly large number of rather widely separate galaxies which
appeared connected by luminous intergalactic formations.”60 These structures were very
60 F. Zwicky, Multiple Galaxies, Ergebnisse der exakten Naturwissenschaften, 1956, Vol. 29, pp. 344–385.
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Part I – Images and the Cosmos
faint, showing barely above the brightness level of the sky background. A very tight light
bucket was needed to detect them. In terms of light baffling, the powerful Mount Wilson
100-inch was like an open door; it could not record the faint intergalactic features until all
sources of stray light were eliminated. Hence, it was very challenging to bring out faint
features at the limit of the photographic emulsion. Moreover, it was extremely difficult to
reproduce the elusive features on a positive print and even more challenging to get them
to come out in the printed journals. These limitations led Zwicky to draw the features by
hand. He used simple sketches to highlight “multiple galaxies” and the faint filamentary
structures that connected some of the galaxies, structures that he interpreted correctly as
the result of inelastic collisions.61
Zwicky also employed a sequence of drawings to illustrate how interacting galaxies
could produce bridges and tails of stars as material was torn apart from the parent galaxies.
In a seminal drawing, he described the transfer of momentum and the formation of tidal
tails as two galaxies pass by each other and interact (Fig. 3.11; see also Fig. 6.8). These
examples from Zwicky’s work represent a fine example of the use of sketching to illustrate
a new phenomenon, based on the detection of features then at the very limit of instrument
capabilities. The sketches were somewhat a reversal from the Worthington problem: While
Worthington used photography to show that his earlier drawings had idealized the shapes
of splashing drops, Zwicky employed old-style drawing to highlight real features barely
visible on the original photographic plates.
It is compelling that for his 1953 article in Physics Today, Zwicky chose to show a
sketch of the double galaxy Messier 51.62 For the Swiss–American Carnegie Observatories
astronomer François Schweizer, Zwicky’s “sketch shows faint details that became visible to
most folks only later when the new Kodak IIIa-J plates showed significantly fainter details
than the old 103a-0 or IIa-O plates did.”63
Today’s astronomers, professional and amateur, have pushed the art and science of
astrophotography to new heights.64 Researchers using images obtained with the Hubble
Space Telescope and modern ground-based telescopes are now producing superb colour
images by combining images obtained in filters of different wavelength passes. These
images are used to derive important scientific measurements. The images are also a pow-
erful means to convey the beauty of astronomical objects and to share the excitement of
discovery with a larger public. Additionally, “when processed correctly, an attractive and
evocative picture brings out the scientific content within.”65 Amateur astronomers have
also caught up spectacularly with their efficient equipment and advanced image processing
with computers. They now produce images of a quality that were not even dreamt of by
professional astronomers of a few decades ago.66
61 F. Zwicky, Multiple Galaxies, Ergebnisse der exakten Naturwissenschaften, 1956, Vol. 29, pp. 366–370.
62 F. Zwicky, Luminous and Dark Formations of Intergalactic Matter, Physics Today, 1953, Vol. 6, pp. 7–11.
63 Private e-mail note to the author (Sept. 12, 2014).
64 T. A. Rector, et al., Image-Processing Techniques for the Creation of Presentation-Quality Astronomical Images, Astronomical Journal, 2007, Vol. 133, pp. 598–611.
65 R. Villard and Z. Levay, Creating Hubble’s Technicolor Universe, Sky & Telescope, 2002, September issue, pp. 28–34.
66 R. Gendler, Forays into Astronomical Imaging: One Person’s Experience and Perspective, Astronomy Beat, Astronomical Society of the Pacific, 2011, No. 79, August 30, pp. 1–6.
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3. From Celestial Snapshots to Photographing the Realm of Galaxies
89
Fig. 3.11 Drawing sequence by Zwicky highlighting the dynamical phases of an interacting pair of
galaxies and the formation of tidal tails. From Zwicky (1956), Ergebnisse der exakten Naturwis-
senschaften.
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Part I – Images and the Cosmos
Fig. 3.12 Each of the four PanSTARRS cameras is equipped with a mosaic of 64 x 64 CCDs, spread-
ing over an area of about 40 cm and providing a total of 1.4 gigapixels. Credit: Institute for Astronomy
University of Hawai’i.
Fig. 3.13 Schematic of the LSST camera. Credit: LSST Corp./National Science Foundation.
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3. From Celestial Snapshots to Photographing the Realm of Galaxies
91
A new phase of systematic photography of the sky is beginning. The 1.8-m Panoramic
Survey Telescope and Rapid Response System (PanSTARRS) and the 8-m Large Synoptic
Survey Telescope (LSST) will transform our view of the universe by fully exploiting the
time domain (Fig. 3.12). Located in the northern hemisphere on Haleakala, Maui Hawai’i,
the PanSTARRS is a 1.8-m wide-field telescope, which can observe the entire available sky
several times each month. Built by the US Department of Energy for the southern hemi-
sphere (site of Cerro Pachón, Chile), the 2.8-ton camera of the 8-m LSST blows the mind.
Providing a 3.5-degree field of view with 189 sixteen megapixel CCDs, a single image has
a total of 32 gigapixels (Fig. 3.13). The LSST science plan is to image most of the sky
visible from the southern hemisphere through several filters a few times a month.
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4
Portraying “Nebulae” for the Mind
Often the most effective way to describe, explore and summarize a set of
numbers – even a very large set – is to look at pictures of these numbers.
Edward R. Tufte 1
Photographs and naked-eye drawings of the Milky Way, however, must
picture somewhat different portions of the stellar world.
Cornelis Easton 2
When one becomes more familiar with the ordering of the hundred-odd
chemical substances wit
hin the table, the symmetries seem so obvious,
the sequences so natural, that most people find hard to imagine a time
when this object did not exist . . .
Michael D. Gordin 3
What is the Role of Abstract, or Representational, Images in Unveiling
the Underlying Physics of “Nebulae”?
Ebenezer Porter Mason (1819–1840) was a young American astronomer who died from
tuberculosis when only 21 years old. He is little known. John Herschel admired Mason’s
accurate and methodical work on “nebulae.” He wrote, “Mr. Mason, a young and ardent
astronomer, a native of the United States of America, whose premature death is the more to
be regretted, as he was (as far as I am aware) the only other recent observer who has given
himself, with the assiduity which the subject requires, the exact delineations of nebulae,
and whose figures I find at all satisfactory.”4
Mason was working with college friends to learn more about “nebulae.” To conduct
their pioneering work, Mason and Yale College friends built a 30-cm reflector, at the time
the largest telescope in the Americas. Mason’s goal was to advance the study of “nebulae”
1 E. T. Tufte, The Visual Display of Quantitative Information, Cheshire: Graphic Press, 1983, p. 9.
2 C. Easton, A Photographic Chart of the Milky Way and the Spiral Theory of the Galactic System, The Astrophysical Journal, 1913, Vol. 37, p. 105.
3 M. D. Gordin, A Well-Ordered Thing, Dmitrii Mendeleev and the Shadow of the Periodic Table, New York: Basic Books, 2004, p. xvii.
4 J. Herschel, Results of Astronomical Observations Made During the Years 1834, 5, 6, 7, 8 at the Cape of Good Hope, London: Smith, Elder & Co., p. 1847.
92
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4. Portraying “Nebulae” for the Mind
93
beyond just description and cataloguing. He was involved in geodesic work, participating
in the field survey of the Maine–Canadian border. As we will see later, his knowledge of
topography may have influenced his approach to astronomical work, and nebular observing
in particular. He applied the technique of contour lines to his study of “nebulae.” It was a first
step in representational imaging to study “nebulae” which would become a most powerful
tool of twentieth-century astrophysics.
Representing the Immense with Synoptic Imaging
The celestial vault is immense and unfathomable. As observing tools, telescopes, photo-
graphic plates and camera systems improved and became more powerful, the number of
astronomical objects recorded increased phenomenally. Let us recall the Carte du ciel and
Astrographic Catalogue project. Already in 1918, Heber Curtis had completed a rapid pho-
tographic survey of the sky with the 36-inch Crossley telescope at the Lick Observatory. He
had inferred the number of spiral galaxies in the observable volume of the Crossley to be
at least one million.5 Beyond the sheer number, astronomical photography revealed a stag-
gering amount of details and features. Astronomers also found many new forms of sidereal
objects: asteroids in huge numbers, patches of darkness or “empty regions,” groups and
clusters of objects, nebulosities of many kinds and shapes and galaxies in ever increasing
numbers.
It is no surprise that researchers felt the need to integrate the vast amount of informa-
tion in a more synthetic form, which could be visualized differently. Synoptic charts and
maps came to the rescue; they were effective means to summarize, average and distil large
amounts of information. As implied by Edward Tufte’s quote, new images were created,
but they were images of the mind and for the mind, not of a natural phenomenon.6
Synoptic charts and maps are called non-homomorphic representations, as opposed to
direct, homomorphic, images that the eye sees.7 Geographical maps are the simplest and
most familiar expressions of non-homomorphic representations; they may show contours of
equal heights to emphasize and quantify the geographical relief of the Earth’s continents or
seafloors. Meteorologists make extensive use of synoptic maps of temperature and pressure
distributions across land and sea for weather forecasts. There are also maps of hours of
sunshine, height of snow or rainfall, foliage coverage and much else that integrate data
over long periods of time.
Non-homomorphic representations are visual forms, visual “languages,” that integrate
or summarize large quantities of data, which can also be handled in tabular form. Images
of transformed data are created to impress on the mind and to convey quantified informa-
tion in a single synthetic view. Dmitrii Mendeleev’s periodic table of chemical elements
may be considered as one of the finest and most powerful non-homomorphic scientific
5 H. D. Curtis, The Number of the Spiral Nebulae, Publications of the Astronomical Society of the Pacific, 1918, Vol. 30, 159–161.
6 E. T. Tufte, op. cit., 1983.
7 P. Galison, Image & Logic, A Material Culture of Microphysics, Chicago: University of Chicago Press, 1997, p. 19.
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Fig. 4.1 Synoptic chart of the solar magnetic field assembled from individual magnetograms covering one full solar rotation in April 2013. Light shading shows the positive magnetic regions, and dark shading the negative regions. Credit: National Solar Observatory Integrated Synoptic Program.
4. Portraying “Nebulae” for the Mind
95
representations of modern chemistry.8 Maps of compiled data are now extensively used
and produced; they serve many practical purposes. In the social or economic sciences, non-
homomorphic representations help visualize vast sums of data: demographic indicators of
population, natural resources, or the spread of endemic diseases, etc.
Because of their efficient summarizing power, non-homomorphic representations have
been used extensively in the physical and natural sciences to synthesize or illustrate complex
sets of data. They average several features in order for the intellect to process large quan-
tities of information and to comprehend reality at a higher level. The photograph might
raise an unneeded barrier for the neophyte. Geological maps and geological sections are
highly complex, abstract and formalized kinds of representations.9 Photographs of the ter-
rain would not necessarily or easily reveal the richness and complexity of the geomorpho-
logical or stratigraphic landscape. The maps and charts are used to highlight features and
guide the researcher or the student through the complex natural scenery.10,11 Remarkable
insights into geohistory have emerged by using higher-level representations.
Mapping the Heavens by Counting
In astronomy, the role of synoptic images has been to bring together or to average quantities
of data or to provide a unifying picture of the systems or phenomena observed. For example,
synoptic maps of the solar surface magnetic fields have been very useful for visualizing the
large-scale magnetic properties of the Sun, for revealing the reversal of the overall magnetic
field polarity every 11 years, and for making sense of the full solar activity 22-year cycle
(Fig. 4.1). On a larger scale, objects external to the Milky Way appear much
more numerous
at high galactic latitudes. Edwin Hubble produced a “zone of avoidance” map to illustrate
the obscuring material in the main plane of our Milky Way (Fig. 4.2; see Plate 6.1).12 This
obscuration limits our viewing ability in several directions as dust clouds hide a significant
part of the universe at distances greater than a few hundred parsecs.
An early example of a synoptic representation in astronomy was the 1785 outline of the
Milky Way star system by William Herschel (Fig. 4.3). His schematic model illustrated his
ambitious approach of “constructing the heavens.” Herschel observed countless stars, at
whatever the direction he pointed his telescope. Being an astute observer, he noticed dif-
ferences in the distribution of stars across the sky, finding variations subtler than the naked
eye could perceive. Consequently, Herschel set himself the staggering task of reproducing
the full three-dimensional distribution of the stars as he could see them with the aid of his
telescopes. He decided to count stars in different directions, to make a census of all the stars
in each given beam.
8 M. D. Gordin, A Well-Ordered Thing, Dmitrii Mendeleev and the Shadow of the Periodic Table, New York: Basic Books, 2004.
9 M. J. S. Rudwick, The Emergence of a Visual Language for Geological Science 1740–1840, History of Science, 1976, Vol. XIV, p. 159.
10 T. Sharpe, The Birth of the Geological Map, Science, 2015, Vol. 347, pp. 230–232.
11 M. J. S. Rudwick, Earth’s Deep History: How It Was Discovered and Why It Matters, Chicago: University of Chicago Press, 2014, p. 140–142.
12 E. P. Hubble, The Distribution of Extra-Galactic Nebulae, The Astrophysical Journal, 1934, Vol. 79, pp. 8–76.
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Part I – Images and the Cosmos
Fig. 4.2 Dust in the plane of the Milky Way absorbs stellar light and blocks our view to distant
portions of the universe. Very few galaxies can be seen in that part of the sky, hence the appellation
of “zone of avoidance.” The zone of avoidance is sketched against the distribution of galaxies across
the sky. From Hubble (1934), The Astrophysical Journal. C
AAS. Reproduced with permission.