To an amateur photographer who desires to obtain the picture of a comet
which may appear perchance in the near future, their capricious
appearance at unexpected periods being one of their charms, the
following hints may be most acceptable. For work of this kind an
equatorial telescope is used with a photographic lens and camera
strapped thereon. The telescope is mounted on an axis that is parallel
to the earth’s axis, and is made to rotate westward by what is called a
driving-clock just as fast as the earth turns to the east. It will
follow the motion of the sky (which apparently drifts westward), and
keep every star approximately fixed in the field of view, or on the
photographic plate in the attached camera.
Otherwise, the stars will appear as trails of light, caused by the
rotation of the earth as it moves onward at the rate of nineteen miles a
second, which is rather disturbing to an astronomer who may be desirous
of obtaining a photograph of the stars overhead. What is he to do? Here
is our planet turning eastward and the stars apparently drifting
westward, and unless the telescope is made to keep up with the stars by
means of clockwork the results are disastrous. Consequently, the
telescope is made to follow the star, comet, or whatever the desired
celestial trophy may be, and it is kept in such a position that the
object in view is centrally placed at the intersection of two threads
obtained from a spider’s web. For this reason, spiders are treated with
due respect in observatories, and may explain the expression of dismay
the writer saw on the face of an Indian assistant at the Kodaikanal
observatory in southern India, when she nearly dispatched one of these
noxious insects, which succeeded, however, in deftly eluding
destruction.
[Illustration:
PHOTOGRAPH OF A BRIGHT METEOR BY DR. W. J. S. LOCKYER
]
The star trails shown in the photograph taken by Dr. W. J. S. Lockyer
give an excellent idea of what happens to a photograph of the stars when
the clockwork is allowed to run down. In this instance the telescope,
with the accompanying camera, was stationary during the exposure of a
little over two hours, with the result that the stars photographed are
not points of light, but bright and faint lines in sections of circles,
since the telescope was pointed to the pole of the heavens. The interest
in this photograph is increased by the fact that a meteor dashed across
that part of the sky during the course of the exposure, thus resulting
in one of the finest photographs of a meteor ever obtained.[8] Had the
exposure lasted during twenty-four hours, and the photograph been taken
in Norway some time during the course of their long winter night, the
trails would have been complete circles. By this means we do not get a
picture of the stars, but simply a photograph illustrating the rotation
of the earth. To obtain a picture of the stars, therefore, an equatorial
attachment, as above described, is an absolute necessity.
Some people have an idea that all an astronomer has to do in making
photographs of a comet, or other celestial object, is to turn the
telescope in the direction wanted, strap on the camera, wind up the
clock, and then go homeward for a good night’s rest. Unfortunately, no
driving-clock has yet been devised so perfect as to move the telescope
exactly with the stars. According to Professor E. E. Barnard, who was an
expert on such matters—
“There is always more or less irregularity of motion, all of which
would be recorded on the plate, and the stars, instead of showing as
merely points of light, would be elongated and blurred. The fainter
ones would not show at all, because they could not be still long
enough to have their pictures taken. That is why you see in the
photograph the observer with his eye ‘glued to the telescope,’
watching a star, a guiding star which he constantly keeps behind the
intersection of two illuminated spider threads in the eyepiece, by
the slow-motion rods which are controlled by his hands.”
Thus, every star or comet is kept immovable on the sensitive plate, and
it paints its own portrait as long as the telescope is made to turn
westward as fast as the earth rotates eastward. That is why a driving
clock is absolutely necessary for the amateur comet photographer who is
desirous of obtaining accurate results. Many hours are required in
obtaining a successful photograph of such comets as the Morehouse comet,
the sensitive plate requiring sometimes an exposure of many hours before
it reveals satisfactory results. The observer must sit patiently hour
after hour, guiding the instrument, and the writer has some idea of what
this must mean, from a brief five minutes’ experience at Mount Wilson in
connection with the sixty-inch reflector. As a great favor she was
allowed to hold the bulb in her hand which by the slightest pressure
brought back an erring star which had attempted to stray momentarily
from the center of the field of view of the telescope.
When one considers the hours spent by the late Professor Barnard in this
nerve-racking work, the patient endurance of the astronomers who
specialize in celestial photography becomes evident. It is an arduous
task, and one doubtless subject to many disappointments, to avoid which
Professor Barnard tried to formulate some set of rules that would be
dependent on the local time and position of the comet, but these were
finally rejected.
“So much would depend on the purity of the atmosphere at the time,
the size and light ratio of the lens, the kind of plate used, etc.,
that they would probably lead to the very errors against which we
wish to guide.”
The position of the comet with respect to the point of sunrise or
sunset, and freedom from any form of haze in the sky, are important
factors in the exposure of comet plates. Moreover, it is necessary that
they should not be exposed too early in the evening or too late in the
morning, in either case resulting in unsatisfactory negatives. The best
of all rules is the judgment of the observer at the moment, but only
long experience will warn one by a glance at the sky when there is
danger of failure in this class of work. It is the few moments at the
beginning or end of the exposure that will injure or ruin the plate.
With a small portrait lens (the most useful size is about six inches)
essentially everything about a comet will be shown as quickly as with a
larger one. The main advantage of the large lens would lie in its
greater scale—which of itself is of great importance. Another source of
danger is moonlight, especially in the case of a long exposure.
Nevertheless, according to Professor Barnard, important results may be
obtained in full moonlight, if the comet is not too near the moon. Much,
however, will depend upon the clearness of the atmosphere; the purer it
is the less will the moonlight affect the plate. In this case a dew-cap
helps much. On an ordinary moonlit sky an exposure of half an hour with
a quick portrait lens will not ruin a fast plate if the comet is not too
near the moon. In full moonlight, however, a longer exposure, unless
under exceptional conditions, will seriously injure or ruin the plate.
With the half-hour exposure the plate will be fogged, and of course the
best quality of negatives cannot be obtained therefrom. All plates
should be backed to prevent halation. A backing made of sugar and burnt
sienna is recommended as entirely satisfactory, and can be kept in
stock.
The formula as supplied by the Cramer Dry Plate Company is as follows:
Cook two pounds of granulated sugar in a saucepan, without the addition
of any water, until it is nearly in the caramel or fudge stage. Then
stir in one pound of burnt sienna and cook a little longer, stirring
well. Do not let the backing get sticky, or it will be difficult to
handle and will not soften so readily when removed from the plate.
Finally add about half an ounce of alcohol to each pint as a drier. Put
away in a wide-mouthed stoppered bottle or jar. When needed for use,
dilute a little of this with water to the consistency of a thick but not
too wet paste. Apply (not wet enough to run) to the back of the plate
with a wide camel’s hair brush. It is not necessary to back heavily. A
sheet of soft paper (an old newspaper) pressed on the backed surface
will prevent injury to the plate, which should be freshly backed when
ready for use. If kept in stock a long time after being backed, an
unequal fogging is likely to occur.
Before developing, remove the backing while it is still damp, with a
moist piece of absorbent cotton. Should a small amount remain it will
not affect the developer seriously. The plates should be carefully
dusted with a broad soft camel’s hair brush, after being put in the
plate-holder. The camera tube should be wiped out frequently with a damp
cloth to free it from dust. It should also have a tight-fitting cover at
the plate end to keep it closed when the plate-holder is not in
position. There should be four springs, one at each corner, on the back
of the plate-holders, to press the plate forward in a constant position.
On account of its greater sensitiveness the Lumière Sigma plate is
recommended by Professor Barnard, although he draws attention to the
fact that this plate has frequently been found defective in having
small, round, transparent and opaque spots. It is also more subject to
“chemical fog” than the Cramer or Seed. Otherwise, it is a beautiful and
very rapid plate.
When the comet is at its brightest, the Seed 27 Gilt Edge plates are
recommended on account of their general freedom from defects and finer
grain. With these few suggestions in respect to photographing comets,
made by Professor Barnard in _Popular Astronomy_, No. 170, the amateur
comet-hunter is enabled to make an attempt, at any rate, at
photographing those wonders of the heavens which have proved so
attractive on account of their varying appearance from night to night.
For those who may not have a ready access to astronomical libraries, the
above condensed account from Professor Barnard’s article on the subject
should prove invaluable.
In an account of his life-work given by Professor Barnard during the
course of an after-dinner speech in January, 1907, at Nashville, and
entitled by him, “Some Unastronomical Experiences of a Lecturer,” he
referred to his interest in comets as follows:
“I have always been interested in comets. These remarkable objects,
which sometimes sweep across the heavens with their wonderful trains
of light, and which in all ages have been objects of superstition
and terror, are among the most interesting in the heavens. Little by
little the mystery attached to them is being solved. This has been
done mainly through the aid of photography. Many of the physical
phenomena of the tails of comets are too faint to be seen with the
eye, although it may be aided by a powerful telescope; but the
photographic plate secures a permanent record of these in all their
complexity and beauty. These photographs show that the form and
other peculiarities of a comet’s tail are often utterly transformed
from night to night. It is therefore highly important that a
continuous series of photographs should be obtained of every active
comet that can be observed, for their phenomena are as evanescent as
smoke itself.
“In 1892, at the Lick Observatory, I was engaged in photographing a
comet (Swift’s) then visible in the morning sky just before
daylight. Every morning’s picture increased the interest and
importance of the work. Unfortunately, I had arranged for a lecture
in the Normal School at San José for the night of May 6. I did not
want to disappoint the people, and I certainly could not let the
comet go by unphotographed. San José was nearly a mile below us in
vertical height and twenty-seven miles distant by stage road. The
only possible way for me to secure my photograph and not disappoint
my audience was to return to Mt. Hamilton that night after the
lecture. At ten o’clock I hired a horse and buggy in San José and
drove up that lonely mountain road, the journey taking five hours,
and arrived at the summit at three o’clock in the morning, in time
to make a photograph of the comet.
[Illustration: