CELESTIAL PHOTOGRAPHY.—THE WAYS AND MEANS.
We come now last of all to that branch of the work of the physical
astronomer which bids fair in the future to replace all existing methods
of observation.
In the introductory chapter we referred to the introduction of
photographic records of astronomical phenomena as marking an epoch in
the development of the science. In the last ones we have to dwell
briefly on the _modus operandi_ of the various methods by which the eye
is thus being gradually replaced.
The point of celestial photography is that it not only enables us to
determine form and place, absolutely irrespective of personal equation
so far as the eye is concerned, but that, properly done, it gives us a
faithful and lasting record of the operation, so that it is not
forgotten; Mr. De La Rue has called the photographic plate the _retina
which does not forget_, and an excellent name it is.
We may pass over altogether the ordinary photographic processes, which
have been carried on with a degree of skill and patience which is beyond
all praise, and confine our attention exclusively to the instrumental
processes. Be it remembered, we have no longer to consider the visual
rays, but the so-called chemical rays, which lie at the violet end of
the spectrum.
We must also recollect that, in a former chapter, we have seen that the
optician’s business was to throw aside the violet rays altogether—to
discard them, caring nothing for them, because, so far as the visible
form of the objects is concerned, they help very little. But we shall
see in a moment that, if we wish to use refractors for photographing, we
must abolish this idea, and undo everything we did to get a perfect
telescope to see the body, because in the case of the photographic
processes employed at present, the visible rays have as little to do
with building up the image on the photographic plate as the blue rays
have to do with building up the image on the retina of the eye. We shall
see presently how admirably this has been done by Mr. Rutherfurd. If,
however, we use reflectors instead of refractors, we are able to utilize
all the rays by means of the same mirror without alteration, as the
focus is the same for all rays, so that a reflector is equally good for
all classes of observation.
Let us first consider the cases in which the plate is made to replace
the retina with the ordinary telescope. We shall see in the sequel that
whether the spectroscope, polariscope, or other physical instrument be
added to the telescope—when we pass, that is to say, from mechanical to
physical astronomy—the plate can still replace the eye with advantage.
The body of the telescope, with the object-glass or mirror at one end
and the plate at its focus in place of the eyepiece, forms the camera,
corresponding to those we find in photographic studies. The plate-holder
shown in section in the accompanying figure is therefore the only
addition required to make a telescope into a camera for ordinary work.
Fig. 208.
[Illustration:
FIG. 208.—Section of Plate-holder.
]
A is a screw of such a size that it can be inserted into the eyepiece
end of the telescope; the sensitive plate is held between a lid at the
back, which opens for the plate to be inserted, and a slide in front,
which is drawn out so as to expose the face of the plate to the object.
A piece of ground glass of extreme fineness is inserted in the slide, on
which the object is focussed before the sensitive plate is put in. It is
easy then by the eyepiece focussing-screw to put this nearer or further
away from the object-glass, so that the image is thrown sharply on the
ground glass. When that is done the ground glass is taken away, and the
sensitive plate put there in its place, and then exposed as required, so
that the methods are similar to the ordinary photographic process.
We have here an arrangement that enables us to photograph the moon,
stars, and planets. M. Faye has proposed that for the transit circle
also the photographic method should be applied, the chronograph
registering the time of the instantaneous opening of the slide, instead
of the time the star is seen to transit, so that the position of the
star with respect to the wires is registered at a certain known time;
therefore, not only for physical astronomy have we the means of making
observations without an observer at all, but also for position
observations.
Every one knows sufficient of photography to be aware that, if we wish
to secure the image of a faint object, such as a faint star or a faint
part of the moon, we must expose the plate for some little time, as we
have to do in ordinary photography if the day is dull, and therefore the
larger the aperture of the telescope the more light passes; and the
shorter the focus is, and the more rapid the process, the shorter will
be the exposure; if the focus is short, the image will be small; but as
we can magnify the image afterwards, rapidity becomes of greater moment,
as the shorter the time of exposure is the less atmospheric and other
disturbances and errors in driving the telescope come into play. Still,
if we photograph the moon or other object, we do not wish to limit
ourselves to the size of the original negative obtained at the focus. If
the negative is well defined—that is, if it possesses the quality of
enlargeableness—there is no difficulty in getting enlarged prints.
The method of enlarging photographs is very simple; all that is required
is a large camera, the negative to be copied being placed nearer the
lens than the prepared paper, so that the image is larger than the
original. Fig. 209 shows an enlarging camera: the body, A, can be made
of wood, or better still, of a soft material, bellows-fashion, so that
the length can be altered at pleasure. In the end, at B, is fixed a
lens—an ordinary portrait lens will do, but a proper copying lens is
preferable; and E is a piece of wood with a hole in its centre, over
which the negative is placed, the distance of E to B being also
adjustible; then, by altering the lengths of B E and B C, the image of
the negative can be made to appear of suitable size. At the end, C, a
piece of sensitive paper is placed, and the light of the sun being
allowed to fall through the negative and lens, the paper soon becomes
printed, and can be toned and fixed as an ordinary paper positive. The
camera may be carried on a rough equatorial mounting, consisting of an
axis pointing to the pole, and pulled round with the sun by attaching a
string to an equatorial telescope, moved by clockwork; or a heliostat
can be used with more advantage, thereby allowing the camera to be
stationary; a good enlarging lens is a very desirable thing, for most
lenses seem to distort the image considerably.
[Illustration:
FIG. 209.—Enlarging Camera. F, heliostat for throwing beam of sunlight
on the reflector, which throws it into the camera; E, negative; B,
focussing-lens; C, plate- or paper-holder; D, focussing-screw.
]
If we wish to obtain a large direct image of the moon, we must, as said
before, employ a telescope of as long a focal length as possible; for
reasons just mentioned, this is not always desirable. If, however, large
images can be obtained as good as small ones, they can of course be
enlarged to a much greater size. The primary image of the moon taken by
Mr. De La Rue’s exquisite reflector is not quite an inch in diameter. In
one of Mr. Rutherfurd’s telescopes of fifteen feet focus, the image of
the moon is somewhat larger—about one and a half inch in diameter. In
Mr. Newall’s magnificent refractor, the focal length of which is thirty
feet, the diameter is over three inches. In the Melbourne reflector the
image obtained is larger still.
In celestial photography we have not only to deal with faint objects.
With the sun the difficulty is of no ordinary character in the opposite
direction, because the light is so powerful that we have to get rid of
it. Now there are two methods of doing this, and as in a faint object we
get more light by increasing the aperture, so with a bright light like
that of the sun we can get rid of a large amount of it by reducing the
aperture of our telescope; but it is found better to reduce
infinitesimally the time of exposure, and methods have been adopted by
which that has been brought down to the one-hundredth part of a second.
Let us show the simple way in which this can be done by the means of an
addition to an ordinary plate-holder.
Fig. 208 shows the ordinary plate-holder, like those used generally for
photography. What is termed the instantaneous slide, B, Fig. 210,
consists of a plate with an adjustible slit in it inserted between the
object itself and the focus. This can be drawn rapidly across the path
of the rays by means of a spring, D; we can bring it to one side, and
fix it by a piece of cotton, E, and then we can release it by burning
the cotton, when the spring draws it rapidly across. The velocity of the
rush of the aperture across the plate, and the time of exposure, can be
determined by the strength of the spring and the aperture of the slit.
If the velocity is too great, we can alter the size of the slit, C. If
we absorb some of the superabundant light by means of yellow glass, or
some similar material, we can keep the opening wide enough to prevent
any bad effects of diffraction coming into play.
[Illustration:
FIG. 210.—Instantaneous Shutter.
]
The light of the sun is so intense that another method may be employed.
Instead of having the plate at the focus of the object-glass we may
introduce a secondary magnifier in the telescope itself, and thus obtain
an enlarged image, the time necessary for its production being still so
short (1/50th of a second) that nothing is lost from the disturbances of
the air.
A telescope with this addition is called a photoheliograph. The first
instrument of this kind was devised by Mr. De La Rue, and for many years
was regularly employed in taking photographs of the sun at Kew.
[Illustration:
FIG. 211.—Photoheliograph as erected in a Temporary Observatory for
Photographing the Transit of Venus in 1874.
]
Some astronomers object to this secondary magnifier, and to obtain large
images use very long focal lengths, and of course a siderostat is
employed. In this way Professor Winlock obtained photographs of the sun
which have surpassed the limits of Mr. Newall’s refractor; the negatives
have a good definition, and show a considerable amount of detail about
the spots; they were taken by a lens, inserted at the end of a gas-pipe
forty feet long. The pipe was fixed in a horizontal position, facing the
north, and at the extreme north part of it was the lens, a single one of
crown glass, with no attempt to correct it. In front of it was a
siderostat, moved by a clock, reflecting the light down the tube, so
that the image of the sun could be focussed on the ground glass at the
opposite end.
One will see the importance of shortening the time for even the
brightest object. Those who are favoured with many opportunities of
looking through large telescopes know that the great difficulty we have
to deal with is the atmosphere; because we have to wait for definition,
and the sum total of the photograph of any one particular thing depends
upon these atmospheric fits. If we require to photograph an object, it
will be obvious that the more fits we have, the worse it will be,
because we get a number of images partially superposed which would
otherwise give as good an effect as we could get by an ordinary eye
observation. It is therefore most important to reduce the interval as
much as possible.