THE STORY OF ASTRONOMICAL PHOTOGRAPHY
The most powerful ally of both telescope and spectroscope is
photography. Without it the marvelous researches carried on with both
these types of instrument would have been essentially impossible. Even
the great telescopes of Herschel and Lord Rosse, notwithstanding their
splendid record as optical instruments, might have achieved vastly more
had photography been developed in their time to the point where the
astronomer could have employed its wonderful capabilities as he does
to-day. And, with the spectroscope, it is hardly too much to say that no
investigator ever observes visually with that instrument any more:
practically every spectrum is made a matter of photographic record
first. The observing, or nowadays the measuring, is all done afterward.
All telescopes and cameras are alike, in that each must form or have
formed within it an image by means of a lens or mirror. In the telescope
the eye sees the fleeting image, in the camera the process of
registering the image on a plate or film is known as photography.
Daguerre first invented the process (silver film on a copper plate) in
1839. The year following it was first employed on the moon, in 1850 the
first star was photographed, in 1851 the first total eclipse of the sun;
all by the primitive daguerreotype process, which, notwithstanding its
awkwardness and the great length of exposure required, was found to
possess many advantages for astronomical work.
About the middle of the last century the wet plate process, so called
because the sensitized collodion film must be kept moist during
exposure, came into general use, and the astronomers of that period were
not slow to avail themselves of the advantages of a more sensitive
process, which in 1872, in the skillful hands of Henry Draper, produced
the first spectrum of a star. In 1880 a nebula was first photographed,
and in 1881 a comet.
Before this time, however, the new dry-plate process had been developed
to the point where astronomers began to avail of its greater convenience
and increased sensitiveness, even in spite of the coarseness of grain of
the film. Forty years of dry-plate service have brought a wealth of
advantages scarcely dreamed of in the beginning, and nearly every
department of astronomical research has been enhanced thereby, while
many entirely new photographic methods of investigation have been worked
out.
Continued improvement in photographic processes has provided the
possibility of pictures of fainter and fainter celestial objects, and
all the larger telescopes have photographed stars and nebulae of such
exceeding faintness that the human eye, even if applied to the same
instrument, would never be able to see them. This is because the eye, in
ten or twelve seconds of keen watching, becomes fatigued and must be
rested, whereas the action of very faint light rays is cumulative on the
highly sensitive film; so that a continuous exposure of many hours'
duration becomes readily visible to the eye on development. So a
supersensitive dry plate will often record many thousand stars in a
region where the naked eye can see but one.
Perhaps the greatest amplification of photography has taken place at the
Harvard Observatory under Pickering, where a library of many hundred
thousand plates has accumulated; and at Groningen, Holland, where
Kapteyn has established an astronomical laboratory without instruments
except such as are necessary to measure photographic plates, whenever
and wherever taken. So it is possible to select the clearest of skies,
all over the world, for exposure of the plates, and bring back the
photographs for expert discussion.
Of course the sun was the celestial body first photographed, and its
surpassing brilliance necessitates reduction of exposure to a minimum.
In moments of exceptional steadiness of the atmosphere, a very high
degree of magnification of the solar surface on the photographic plate
is permitted, and the details in formation, development, and ending of
sun spots are faithfully registered. Nevertheless, it cannot be said
that photography has yet entirely replaced the eye in this work, and
careful drawings of sun spots at critical stages in their life are
capable of registering fine detail which the plate has so far been
unable to record. Janssen of Paris took photographs of the solar
photosphere so highly magnified that the granulation or willow-leaf
structure of the surface was clearly visible, and its variations
traceable from hour to hour.
The advantages of sun spot photography in ascertaining the sun's
rotation, keeping count of the spots, and in a permanent record for
measurement of position of the sun's axis and the spot zones, are
obvious. In direct portrayal of the sun's corona during total eclipses,
photography has offered superior advantages over visual sketching, in
the form and exact location of the coronal streamers; but the
extraordinary differences of intensity between the inner corona and its
outlying extensions are such that halation renders a complete picture on
a single plate practically impossible. The filamentous detail of the
inner corona, and the faintest outlying extensions or streamers, the eye
must still reveal directly.
In solar spectrum photography, research has been especially benefited;
indeed, exact registry of the multitudinous lines was quite impossible
without it. Photographic maps of the spectrum by Thollon, McClean and
Rowland are so complete and accurate that no visual charts can approach
them. Rowland's great photographic map of the solar spectrum spread out
into a band about forty feet in length; and in the infra-red, Langley's
spectrobolometer extended the invisible heat spectrum photographically
to many times that length. At the other end of the spectrum, special
photographic processes have extended the ultra-violet spectrum far
beyond the ocular limit, to a point where it is abruptly cut off by
absorption of the earth's atmosphere. On the same plate with certain
regions of the sun's spectrum, the spectra of terrestrial metals are
photographed side by side, and exact coincidences of lines show that
about forty elemental substances known to terrestrial chemistry are
vaporized in the sun.
[Illustration: A VIEW OF THE 100-FOOT DOME IN WHICH THE LARGEST
TELESCOPE IN THE WORLD IS HOUSED. (_Courtesy, Mt. Wilson Solar
Observatory._)]
[Illustration: MOUNT CHIMBORAZO, NEAR THE EQUATOR. An observatory
located on this mountain would make it possible to study the
phenomena of northern and southern skies from the same point.
(_Courtesy, Pan-American Union._)]
[Illustration: LICK OBSERVATORY, ON THE SUMMIT OF MT. HAMILTON,
ABOUT TWENTY-FIVE MILES S. W. OF SAN JOSE, CALIFORNIA. It
contains the famous Lick telescope, a 36-inch refractor.]
[Illustration: NEAR VIEW OF THE EYE-END OF THE YERKES TELESCOPE.
The eyepiece is removed and its place taken by a photographic
plate.]
Young was the first to photograph a solar prominence in 1870, and twenty
years later Deslandres of Paris and Hale of Chicago independently
invented the spectroheliograph, by which the chromosphere and
prominences of the sun, as well as the disk of the sun itself, are all
photographed by monochromatic light on a single plate. Hale has
developed this instrument almost to the limit, first at the Yerkes
Observatory of the University of Chicago, and more recently at the Mount
Wilson Observatory of the Carnegie Institution, where spectroheliograms
of marvelous perfection are daily taken. It was with this instrument
that Hale discovered the effect of an electro-magnetic field in sun
spots which has revolutionized solar theories, a research impossible to
conceive of without the aid of photography.
When we apply Doppler's principle, photography becomes doubly
advantageous, whether we determine, as Duner did and more recently
Adams, the sun's own rotation and find it to vary in different solar
latitudes, the equator going fastest; or apply the method to the sun's
corona at the east and west limbs of the sun, which Deslandres in 1893
proved to be rotating bodily with the sun, because of the measured
displacement of spectral lines of the corona in juxtaposition on the
photographic plate.
In the solar astronomy of measurement, too, photography has been
helpfully utilized, as in registering the transits of Mercury over the
sun's disk, for correcting the tables of the planet's orbital motion;
and most prominently in the action taken by the principal governments of
the world in sending out expeditions to observe the transits of Venus in
1874 and 1882, for the purpose of determining the parallax of Venus and
so the distance of the earth from the sun.
In our studies of the moon, photography has almost completely superseded
ocular work during the past sixty years. Rutherfurd and Draper of New
York about 1865 obtained very excellent lunar photographs with wet
plates, which were unexcelled for nearly half a century. The Harvard,
Lick, and Paris Observatories have published pretty complete
photographic atlases of the moon, and the best negatives of these series
show nearly everything that the eye can discern, except under unusual
circumstances. Later lunar photography was taken up at the Yerkes
Observatory, and exceptionally fine photographs on a large scale were
obtained with the 40-inch refractor, using a color screen. More recently
the 60-inch and 100-inch mirrors of the Mount Wilson Observatory have
taken a series of photographs of the moon far surpassing everything
previously done, as was to be expected from the unique combination of a
tranquil mountain atmosphere with the extraordinary optical power of the
instruments, and a special adaptation of photographic methods. During
lunar eclipses, Pickering has made a photographic search for a possible
satellite of the moon, occultations of stars by the moon have been
recorded by photography, and Russell of Princeton has shown how the
position of the moon among the stars can be determined by the aid of
photography with a high order of precision.
The story of planetary photography is on the whole disappointing. Much
has been done, but there is much that is within reach, or ought to be,
that remains undone. From Mercury nothing ought perhaps to be expected.
On many of the photographs of the transit of Venus, especially those
taken under the writer's direction at the Lick Observatory in 1882, we
have unmistakable evidence of the planet's atmosphere. Here again the
wet plate process, although more clumsy, demonstrated its superiority
over the dry process used by other expeditions.
In spectroscopy, Belopolsky has sought to determine the period of
rotation of Venus on her axis. At the Lowell Observatory, Douglass
succeeded in photographing the faint zodiacal light, and very successful
photographs of Mars were taken at this institution as early as 1905 by
Slipher. Two years later these were much improved upon by the writer's
expedition to the Andes of Chile, when 12,000 exposures of Mars were
made, many of them showing the principal _canali_, and other prominent
features of the planet's disk. At subsequent oppositions of the planet,
Barnard at the Yerkes Observatory and the Mount Wilson observers have
far surpassed all these photographs.
For future oppositions a more sensitive film is highly desired, in
connection with instruments possessing greater light-gathering power, so
permitting a briefer exposure that will be less influenced by
irregularities and defects of the atmosphere. The spectrum of Mars is of
course that of sunlight, very much reduced, and modified to a slight
extent by its passing twice through the atmosphere of Mars. What amount
of aqueous vapor that atmosphere may contain is a question that can be
answered only by critical comparison of the Martian spectrum with the
spectrum of the moon, and photography affords the only method by which
this can be done.
Many are the ways in which photography has aided research on the
asteroid group. Since 1891 more than 600 of them have been discovered by
photography, and it is many times easier to find the new object on the
photographic plate than to detect it in the sky as was formerly done by
means of star charts. The planet by its motion during the exposure of
the plate produces a trail, whereas the surrounding stars are all round
dots or images. Or by moving the plate slightly during exposure, as in
Metcalf's ingenious method, we may catch the planet at that point where
it will give a nearly circular image, and thus be quite as easy to
detect, because all the stars on the same plate will then be trails.
Photographic photometry of the asteroids has revealed marked variations
in their light, due perhaps to irregularities of figure. On account of
their faint light, the asteroids are especially suited, as Mars is not,
to exact photography for ascertaining their parallax, and from this the
sun's distance when the asteroid's distance has been found. Many
asteroids have been utilized in this way, in particular Eros (433). In
1931 it approaches the earth within 13 million miles, when the
photographic method will doubtless give the sun's distance with the
utmost accuracy.
Photographs of Jupiter have been very successfully taken at the Yerkes
and Lowell Observatories and elsewhere, but the great depth of the
planet's atmosphere is highly absorptive, so that the impression is very
weak in the neighborhood of the limb, if the exposure is correctly timed
for the center of the disk. The striking detail of the belts, however,
is excellently shown. Wood of Baltimore has obtained excellent results
by monochromatic photography of Jupiter and Saturn with the 60-inch
reflector on Mount Wilson. Jupiter's satellites have not been neglected
photographically, and Pickering has observed hundreds of the eclipses
of the satellites by a sort of cinematographic method of repeated
exposures, around the time of disappearance and reappearance by eclipse.
The newest outer satellites of Jupiter were all discovered by
photography, and it is extremely doubtful if they would have been found
otherwise.
Saturn has long been a favorite object with the astronomical
photographer, and there are many fine pictures in spite of his yellowish
light, relatively weak photographically. The marvelous ring system with
the Cassini division, the oblateness of the ball, the occasional
markings on it--all are well shown in the best photographs; but the call
is for more light and a more sensitive photographic process. Pickering's
ninth satellite (Phoebe) was discovered by photography, one of the
faintest moons in the solar system. Like the faint outer moons of
Jupiter, few existing telescopes are powerful enough to show it. Its
orbit has been found from photographic observations, and its position is
checked up from time to time by photography.
But the crowning achievement of spectrum photography in the Saturnian
system is Keeler's application of Doppler's principle in determining the
rate of orbital motion of particles in different zones of the rings,
thereby establishing the Maxwellian theory of the constitution of the
rings beyond the possibility of doubt. For Uranus and Neptune
photography has availed but little, except to negative the existence of
additional satellites of these planets, which doubtless would have been
discovered by the thorough photographic search which has been made for
them by W. H. Pickering without result.
As with the asteroids, so with comets: several of these bodies have been
discovered by photography; none more spectacular than the Egyptian comet
of May 17th, 1882, which impressed itself on the plates of the corona of
that date. Withdrawal of the sun's light by total eclipse made the comet
visible, and it had never been seen before, nor is it known whether it
will ever return. In cometary photography, much the same difficulties
are present as in photographing the corona: if the plate is exposed long
enough to get the faint extensions of the tail, the fine filaments of
the coma or head are obliterated by halation and overexposure.
No one has had greater success in this work than Barnard, whose
photographs of comets, particularly at the Lick Observatory, are
numerous and unexcelled. His photographs of the Brooks Comet of 1893
revealed rapid and violent changes in the tail, as if shattered by
encounter with meteors; and the tail of Halley's comet in 1910 showed
the rapid propagation of luminous waves down the tail, similar to
phenomena sometimes seen in streamers of the aurora. Draper obtained the
first photograph of a comet's spectrum in 1881, disclosing an identity
with hydrocarbons burning in a Bunsen flame, also bands in the violet
due to carbon compounds. The photographic spectra of subsequent comets
have shown bright lines due to sodium and the vapor of iron and
magnesium.
Even the elusive meteor has been caught by photography, first by Wolf in
1891, who was exposing a plate on stars in the Milky Way. On developing
it, he found a fine, dark nearly uniform line crossing it, due to the
accidental flight across the field of a meteor of varying brightness.
Since then meteor trails have been repeatedly photographed, and even
the trail spectra of meteors have been registered on the Harvard plates.
At Yale in 1894 Elkin employed a unique apparatus for securing
photographic trails of meteors: six photographic cameras mounted at
different angles on a long polar axis driven by clockwork, the whole
arranged so as to cover a large area of the sky where meteors were
expected.
When we pass from the solar system to the stellar universe the
advantages of photography and the amplification of research due to its
employment as accessory in nearly every line of investigation are
enormous. So extensively has photography been introduced that plates,
and to a slight extent films, are now almost exclusively used in
securing original records. Regrettably so in case of the nebulae, because
the numerous photographs of the brighter nebulae taken since 1880 when
Draper got the first photograph of the nebula of Orion, are as a rule
not comparable with each other. Differences of instruments, of plates,
of exposure, and development--all have occasioned differences in
portrayal of a nebula which do not exist. When we consider faithful
accuracy of portrayal of the nebulae for purposes of critical comparison
from age to age, many of our nebular photographs of the past forty
years, fine as they are and marvelous as they are, must fail to serve
the purpose of revealing progressive changes in nebular features in the
future.
Roberts and Common in England were among the first to obtain nebular
photographs with extraordinary detail, also the brothers Henry of Paris.
As early as 1888 Roberts revealed the true nature of the great nebula in
Andromeda, which had never been suspected of being spiral; and Keeler
and Perrine at the Lick Observatory pushed the photographic discovery of
spiral nebulae so far that their estimates fill the sky with many hundred
thousands of these objects.
In the southern hemisphere the 24-inch Bruce telescope of Harvard
College Observatory has obtained many very remarkable photographs of
nebulae, particularly in the vicinity of Eta Carinae. But the great
reflectors of the Mount Wilson Observatory, on account of their
exceptional location and extraordinary power, have surpassed all others
in the photographic portrayal of these objects, especially of the spiral
nebulae which appear to show all stages in transition from nebula to
star. No less remarkable are the photographs of such wonderful clusters
as Omega Centauri, a perfect visual representation of which is wholly
impossible. Intercomparison of the photographs of clusters has afforded
Bailey of Harvard, Shapley of Mount Wilson and others the opportunity of
discovery that hundreds of the component stars are variable.
What is the longest photographic exposure ever made? At the Cape of Good
Hope, under the direction of the late Sir David Gill, exposures on
nebulae were made, utilizing the best part of several nights, and
totaling as high as seventeen, or even twenty-three hours. But the Mount
Wilson observers have far surpassed this duration. To study the rotation
and radial velocity of the central part of the nebula of Andromeda, an
exposure of no less than 79 hours' total duration was made on the
exceedingly faint spectrum, and even that record has since been
exceeded. The eye cannot be removed from the guiding star for a moment
while the exposure is in progress, and this tedious piece of work was
rewarded by determining the velocity of the center of the nucleus as a
motion of approach at the rate of 316 kilometers per second.
But when the stars, their magnitudes and their special peculiarities are
to be investigated _en masse_, photography provides the facile means for
researches that would scarcely have been dreamed of without it. The
international photographic chart of the entire heavens, in progress at
twenty observatories since 1887, the photographic charts of the northern
heavens at Harvard and of the southern sky at Cape Town, the manifold
investigations that have led up to the Harvard photometry, and the
unparalleled photographic researches of the Henry Draper Memorial,
enabling the spectra of many hundred thousand stars to be examined and
classified--all this is but a part of the astronomical work in stellar
fields that photography has rendered possible.
Then there are the stellar parallaxes, now observed for many stars at
once photographically, when formerly only one star's parallax could be
measured at a time and with the eye at the telescope. And photo-electric
photometry, measuring smaller differences of light than any other
method, and providing more accurate light-curves of the variable stars.
And perhaps most remarkable of all, the radial velocity work on both
stars and nebulae, giving us the distance of whole classes of stars,
discovering large numbers of spectroscopic binaries and checking up the
motion of the solar system toward Lyra within a fraction of a mile per
second.
All told, photography has been the most potent adjunct in astronomical
research, and it is impossible to predict the future with more powerful
apparatus and photographic processes of higher sensitiveness. The field
of research is almost boundless, and the possibilities practically
without limit.
What would Herschel have done with L100,000--and photography!