TELESCOPES.
THE REFRACTING TELESCOPE.—The function of a telescope is two-fold.
First, to magnify the heavenly bodies, or, what comes to the same thing,
to make them look as if they were nearer to us, so that we can see them
better. Second, to collect a much greater number of rays of light than
the unassisted eye alone can grasp, so that objects too dim to be
otherwise perceptible are brought within our range of vision.
There are two forms of telescope, distinguished as _Refractors_ and
_Reflectors_. The simplest form of refracting telescope is exemplified
by the common opera-glass, and large refractors are not essentially
different. Such instruments depend for their action upon the formation
of an image by a lens. One can easily illustrate this by producing upon
the wall of a room an inverted image of a candle or gas flame with a
spectacle lens (one adapted for a long-sighted person), or with one of
the larger lenses from an opera-glass. Having such an image, it may be
magnified by means of another lens, just as one may magnify a photograph
with an ordinary reading glass. Technically, the lens which forms the
primary image is called the _object-glass_ of the telescope, and that
which is used to magnify this image is called the _eye-piece_. The
object-glass is usually a large lens, which is placed at one end of a
tube, while the eye-piece is a much smaller lens, placed at the other
end. Means are provided for adjusting the distance between the two
lenses so as to admit of distinct vision.
[Illustration:
FIG. 49.—_The Achromatic Object-Glass._
]
Matters are, however, not quite so simple as has been stated. There is a
very great difficulty introduced by the fact that a lens made out of a
single piece of glass gives an image which is surrounded by fringes of
colour, so that some device has to be adopted in order to destroy, as
far as possible, this enemy of good definition. In the early history of
the telescope, this so-called _chromatic aberration_ was considerably
reduced by making small object-glasses of very great focal length.[4]
Lenses of 100-feet focus, however, are not easy to employ as
object-glasses, and astronomy was, therefore, greatly benefited by
Dollond’s invention of the _achromatic lens_ in 1760. This is a compound
lens, usually consisting of a double convex crown-glass lens and a
concavo-convex, or double concave, lens of flint glass. The curvatures
of the lenses, and the optical properties of the two kinds of glass
composing them, are such that the colour due to one of them is
practically neutralised by that due to the other acting in opposition. A
section of such an object-glass, with the “cell” in which it rests, is
shown in Fig. 49.
In this way the focal length of the lens, and, therefore, the length of
the telescope tube, can be kept within reasonable dimensions, while the
definition is improved. There is, however, usually a little outstanding
colour, due to the imperfect matching of the two lenses, and if one
looks through a large refractor, even of a good quality, a purple fringe
will be noticed round all very bright objects. This only affects a few
of the brighter objects, while millions of others which are dimmer may
be seen free from spurious colour.
It may be remarked that the curved surfaces of the lenses forming
telescopic object-glasses must not be parts of spheres. If they are, the
images will be rendered indistinct by _spherical aberration_, and the
optician has to design his curves to get rid of this defect at the same
time as chromatic aberration.
A new form of telescopic objective, consisting of three lenses, which
has many important advantages, has recently been invented by Mr. Dennis
Taylor, of the well-known firm of T. Cooke & Sons, York.
Such a lens as this illustrates the perfection which the optician’s art
has now attained. Six surfaces of glass have to be so accurately figured
that every ray of light falling upon the surface of the lens shall pass
through the finest pinhole at a distance of eighteen times the diameter
of the lens.
THE REFLECTOR.—In a reflecting telescope, the object-glass of the
refractor is replaced by a concave mirror. In order that such a mirror
may reflect all the rays from a star to a single point, its concave
surface must be part of a paraboloid of revolution, that is, a surface
produced by the revolution of a parabola on its axis. If a spherical
surface be employed, all the rays will not be reflected to a single
point, and the images which it gives will be ill-defined. Yet it is
astonishing to find that the difference between a parabolic and
spherical surface, even in the case of a large mirror, is exceedingly
small. Sir John Herschel states that in the case of a mirror four feet
in diameter, and forming an image at a distance of forty feet, the
parabolic only departs from the spherical form at the edges by less than
a twenty-one thousandth part of an inch.
[Illustration:
FIG. 50.—_The Newtonian Reflector._
]
An image being formed by a mirror, it is next to be viewed with an
eye-piece just as in the case of a refracting telescope. Here there is a
little difficulty, for if the eye-piece be applied in the direct line of
the mirror, the interposition of the observer’s head will block out the
light. Several ways of overcoming this have been devised, but the plan
most generally followed is that which Newton adopted in the first
reflecting telescope which was ever constructed. With his own hands
Newton made a small reflector, 6¼ inches long and having an aperture of
1⅓ inches, with which he was able to study the phases of Venus, and the
phenomena of Jupiter’s satellites. This precious little instrument is
now one of the greatest treasures in the collection of the Royal Society
of London. The general design of this telescope is shown in Fig. 50. The
concave mirror is at the bottom of the telescope tube, and normally it
would form an image of a star near the end of the tube. A plane mirror,
however, of small size intercepts the rays and reflects them to the
side, where they converge to a focus. This image is observed and
magnified by an eye-piece, as in the refractor. It is true that in this
arrangement the plane mirror, or _flat_, renders the central part of the
principal mirror ineffective, but the loss of light is very much less
than would be the case if the eye-piece were placed in position to view
the image centrally.
In the hands of Sir William Herschel the reflecting telescope was
greatly developed. The great telescope with which he enriched
astronomical science had a mirror four feet in diameter, and its tube
was 40 feet in length. With the view of utilising the whole surface of
the mirror and dispensing with a second reflecting surface, the four
foot mirror was placed at a small angle to the bottom of the tube, so
that its principal focal point was no longer at the centre, but at the
side of the tube.
In practice, however, it is found that the Herschellian form of
reflector does not give the best definition, and it is now very seldom
seen.
Among other forms, the “Cassegrain” is perhaps the most important.
During the last year or two this form has received a great deal of
attention, more especially in regard to its special adaptability for
photographic purposes.
In the Cassegrain telescope, the plane mirror of the Newtonian form is
replaced by a small convex mirror which is part of a hyperboloid of
revolution, its axis and focal point being coincident with those of the
primary mirror. The rays are in this way reflected back to the mirror at
the bottom of the tube, and in order that the image may be seen, it is
necessary to cut out the middle part of the mirror to admit the
eye-piece.
Although the small mirror must theoretically be hyperbolic, tolerable
definition is obtained even if it be spherical or ellipsoidal, and its
actual departure from these forms is so slight as to be beyond detection
by measurement, so that the figuring of such mirrors can only be tested
in the telescope. For photographic purposes this telescope has the very
important advantage that a short telescope is equivalent to a very long
one of the Newtonian form, or refracting telescope, so that the image of
sun, moon, or planets formed at the focus is very large in comparison
with the size of the telescope. A modification of this form of
telescope, in which the small mirror is out of the path of the rays
falling upon the larger one, and no longer obstructing the central part,
has been recently revived by Dr. Common, and has become generally known
as the “Skew Cassegrain.”
In reflecting telescopes the mirrors were formerly made of _speculum_
metal (an alloy of copper and tin), and the word speculum is even now
commonly employed to signify a telescopic mirror, although it is usual
to make the mirror of glass, with the concave surface silvered and
highly polished.
[Illustration:
FIG. 51.—_The Cassegrain Reflector._
]
One is frequently asked for an opinion as to which is the better form of
telescope, the reflector or refractor, and it is a question that one
finds some little difficulty in answering. On one point, however, all
are agreed, namely, that the reflector has the advantage in regard to
its achromatism; it is indeed perfectly achromatic, while the so-called
“achromatic” refractor is at best only a compromise. For the rest, one
cannot do better than quote the evidence of Dr. Isaac Roberts before the
International Astro-photographic Congress:—“The reflector requires the
exercise of great care and patience, and a thorough personal interest on
the part of the observer using it. In the hands of such a person it
yields excellent results, but in other hands it might be a bad
instrument. The reflector gives results at least equal, if not superior,
to those obtained with the refractor, if the observer be careful of the
centering, and of the polish of the mirror, and keeps the instrument in
the highest state of efficiency; but when entrusted to an ordinary
assistant the conditions necessary for its best performance cannot be so
well fulfilled as the same could be in the case of the refractor.” One
great practical advantage of the reflector is that there are fewer
optical surfaces, so that a large reflector may be obtained for the
price of a much smaller refractor.
EYE-PIECES.—So far we have regarded the eye-piece of a telescope as a
simple lens, but it is evident that the spherical and chromatic
aberration of such a lens will interfere with its performance. For
occasional use, however, even a simple lens is very serviceable if the
object observed is brought to the centre of the field of view.
Compound eye-pieces are of various forms, each having certain
advantages, the desiderata being freedom from colour and “flatness of
field”—that is, stars in different parts of the field are to be equally
well in focus. Those most commonly employed are the Ramsden and
Huyghenian eye-pieces. The former consists of two plano-convex lenses of
equal focal lengths, having their curved faces towards each other, and
being placed at a distance apart equal to two-thirds of the focal length
of either lens. Such an eye-piece can be used as a magnifying-glass, and
it is therefore placed outside the focal image formed by the telescope
with which it is used; on this account it is called a _positive
eye-piece_. This kind of eye-piece is not quite achromatic, but its flat
field of view gives it a special value for many purposes.
In the Huyghenian eye-piece there are again two lenses, made of the same
kind of glass. That which comes nearest to the eye has a focal length of
only one-third that of the _field_ lens, and the distance between the
two lenses is half the sum of the focal lengths. This form of eye-piece
cannot be used as a magnifying-glass in the ordinary sense, and as the
field lens must be placed on the object-glass or mirror side of the
focus, it is called a _negative eye-piece_. The Huyghenian eye-piece is
more achromatic than the Ramsden, and is more widely used when it is
only required to view the heavenly bodies. In instruments employed for
purposes of measurement, a positive eye-piece is essential in order that
the spider threads may be placed at the focus of the telescope. The
images formed by an astronomical telescope are upside down, and neither
of the eye-pieces described reinverts them.
A special form of eye-piece is therefore used when a telescope is
employed for terrestrial sight-seeing. The desired result is obtained by
the introduction of additional lenses, but there is a corresponding
reduction of brightness.
For viewing the sun some device is necessary to reduce the quantity of
light entering the eye. To look at the sun directly, even with a small
instrument, is very dangerous. The arrangement usually adopted is a
_solar diagonal_, in which the light is reflected from a piece of plane
glass before entering the eye-piece; the piece of glass is wedge-shaped,
so that the reflection from one surface only is effective; if the glass
had parallel sides, the solar image would be double.
MAGNIFYING POWER.—The magnifying power of a telescope depends upon the
focal length of the object-glass, or speculum, and that of the
eye-piece. Optically, it is equal to the former divided by the latter,
so that the greater the focal length of an object-glass, or the smaller
the focal length of the eye-piece, the greater will be the magnifying
power. In a given telescope, the object-glass, or speculum, is a
constant factor, and the magnifying power can only be varied by changing
the eye-piece. The focal length of the Lick telescope, for example, is
about 600 inches; with an eye-piece which is equivalent to a lens of
one-inch focus, the magnifying power would be 600; with a lens of half
an inch focus, it would be 1,200, and so on.
The magnifying power which can be effectively employed, however, depends
upon a great variety of circumstances. First, the clearness and
steadiness of the air; then there is the quality of the object-glass, or
speculum, to be considered; and also the brightness of the object to be
observed, for when the object is very dim, its light will be spread out
into invisibility if too high a power be used.
In practice, good refractors perform well with powers ranging up to 80
or 100 for each inch in the diameter of the object-glass. Thus, on
sufficiently bright objects, a six-inch telescope will work well with a
power of about 500, while a 30-inch may be effectively employed with
powers between 2,000 and 3,000.
ILLUMINATING POWER.—It has already been pointed out that magnification
is not the only function of a telescope. As a matter of fact, the most
powerful telescopes in the world fail to produce the slightest increase
in the apparent size of a star, for even if these objects be brought to
apparently a 3,000th part of their real distances, they are still too
far away to have any visible size. But although a star cannot be
magnified, it can be rendered more visible by the telescope, for the
reason that the object-glass collects a greater number of rays than the
naked eye. The pupil of the eye may be taken to have a diameter of
one-fifth of an inch; a lens one inch in diameter will have 25 times the
_area_ of the pupil, and will therefore collect 25 times the amount of
light from a star; a two-inch lens will grasp 100 times, and a 36-inch
32,400 times as much light as the pupil alone. Practically all these
rays collected by the object-glass, or speculum, of a telescope cannot
be brought into the eye; some are lost through the imperfect
transparency of the glass, or the imperfect reflecting power of the
speculum. Still, allowing a considerable percentage for loss, there is
an enormous concentration of light when a large telescope is employed.
THE ALTAZIMUTH MOUNTING.—Having got a telescope, we have next to see how
it can be best supported, for unless it be a very small instrument
indeed, it will be impossible to hold it in the hand like a spy-glass.
However a telescope be mounted, provision must be made for turning it to
any part of the sky whatsoever. Very frequently one of the axes on which
the instrument turns is vertical, while the other is horizontal. Such a
stand for a telescope is called an _altazimuth mounting_, for the reason
that it permits the instrument to be moved in altitude and in azimuth.
As a rule, one finds only small telescopes mounted in this manner. The
objection to it is that, as one continues to observe a heavenly body,
two independent movements must be given to the telescope in order to
follow the body in its diurnal movement across the heavens. If we
commence observing a star newly risen, for example, the telescope must
trace a stair-like path in order to follow it, as it ascends into the
heavens.
THE EQUATORIAL TELESCOPE.—A much more convenient method of setting up a
telescope is to mount it as an _equatorial_. The essential feature of
this instrument is that one of the axes of movement, instead of being
vertical, is placed parallel to the axis of the earth. This is called
the _polar axis_, and, when the telescope is turned around such an axis,
it traces out curves in the sky which are identical with those described
by the stars in their diurnal motions. If, then, the telescope be
directed to a star or other heavenly body, it can be made to follow the
object and keep it in view by a single movement. The axis at right
angles to the polar axis is called the declination axis, and is
necessary in order that the telescope may be moved towards and from the
Poles so that all the heavenly bodies above the horizon may be included
in its sweep.
One very important advantage of the equatorial is that as only one
motion is required to keep a star in view, so long as it is above the
horizon, the necessary movement may be furnished by clock-work. A good
equatorial is accordingly provided with a driving clock, which is
regulated so that it would drive the telescope through a whole
revolution once a day. Unlike an ordinary clock, the driving clock of a
telescope is regulated by a governor, in order that the instrument may
have a continuous and not a jerky movement.
The telescope is also provided with clamps and fine adjustments, one
each in R. A. and declination, in order that it may be under the control
of the observer. It is evident that the telescope must be capable of
moving independently of the driving gear, so that it may first be placed
in the desired direction; when this is accomplished, the R. A. clamp is
used to put the telescope in gear with the clock. The declination clamp
is them made to fix the telescope firmly to the declination axis. Fine
adjustments in both directions are necessary, because it is impossible
to sight a large instrument with such precision as to bring an object
exactly to the centre of the field of view.
Some of the driving clocks fitted to equatorials are very elaborate. As
clocks regulated by governors are not such reliable time-keepers as
those regulated by pendulums, arrangements are made by which the
accuracy of a pendulum can be electrically communicated to a governor
clock. One of the best forms of electrically-controlled clocks is that
devised by Sir Howard Grubb.
Another important feature of an equatorial is that it can be provided
with circles which enable the telescope to be pointed to any desired
object of known right ascension and declination. One of these is the
declination circle, attached to the declination axis and read by a
vernier fixed to the sleeve in which the axis turns; this is adjusted so
as to read 0° when the telescope points to any part of the celestial
equator, and 90° when it is directed to the Pole. The other circle is
attached to the polar axis, and determines the position of the telescope
with regard to the meridian; this is called the _hour circle_, and is
divided into 24 hours. When the telescope is on the meridian, the hour
circle reads zero, so that its reading in any other position gives the
hour angle of the telescope. Having given the right ascension and
declination of a heavenly body which it is desired to observe, the
telescope is turned until the declination circle reads the proper angle,
and the hour circle indicates the hour angle which is calculated for the
particular moment of pointing the telescope. [The hour angle is the
difference between the right ascension of the object and the sidereal
time of observation.] In this way it is easy to find objects of known
position which are invisible to the naked eye, and one can even pick up
the planets and brighter stars in full sunshine. Conversely one can
determine from the circles the right ascension and declination of any
object under observation, but for various reasons only approximate
results can be obtained in this way. The chief use of the circles on an
equatorial is therefore to provide a means of pointing the telescope.
Telescopes of 4 inches aperture and upwards are usually provided with a
smaller companion called a _finder_. This has a larger field of view
than the main telescope, so that objects which are of sufficient
brightness can readily be picked up and brought to the centre of the
finder, the adjustments being such that the object is then also at the
centre of the field of the large telescope.
There are, of course, many practical details connected with the working
of an equatorial with which space does not permit us to deal. It may be
remarked, however, that the adjustment of the polar axis is very simply
performed by first inclining it at an angle approximately equal to the
latitude of the place where it is set up, and setting it as nearly as
possible in the meridian by means of a compass or by observations of the
sun at noon. The final adjustment is then made by a series of
observations of stars of known position.
SOME OF THE WORLD’S GREAT TELESCOPES.—Thanks to the wide public interest
taken in astronomical matters, a large number of powerful telescopes has
been set up in various parts of the world. To the British Islands
belongs the honour of possessing the largest telescope in the world.
This is the giant reflector erected by Lord Rosse, in 1842, at
Parsonstown, the mirror being 6 feet in diameter, and the focal length
60 feet. Many very valuable observations were made with this instrument
in its early days, but of late years it seems to have fallen into
disuse. One reason may be that the mounting is not of the most
convenient form, and makes the telescope unsuitable for photographic
work.
Coming next in point of size to the Rosse telescope is the reflector
erected at Ealing, by Dr. A. A. Common. The glass mirror of this
telescope is 5 feet in diameter, 5 inches thick, and weighs more than
half a ton. Dr. Common aimed specially at constructing the largest
possible telescope which could be equatorially mounted and provided with
a driving clock, and he was only limited to an aperture of 5 feet by the
impossibility of obtaining a glass disc of larger size. He has attained
such great skill in this work that he was able to produce a perfect
mirror 5 feet in diameter in three months time, although no less than
410,000 strokes of the polishing machine were required.
The telescope is of the Newtonian form, and the mounting is quite
unique. The polar axis consists of an iron cylinder, made up of boiler
plates, 7 feet 8 inches in diameter, and about 15 feet long. From the
top of the cylinder, near its outer edge, two horns, each 6 feet long,
project outwards, and the tube of the telescope swings on trunnions
attached to the ends of the horns. The main part of the telescope tube
is square, built up of steel angle iron, and carries the mirror at its
lower end; the upper part of the tube, which carries the “flat” and
eye-piece, is round, and of tinned steel strengthened by a skeleton
framework.
It is evident that such an enormous instrument as this cannot be made to
travel by clock-work with the necessary uniformity without some very
efficient arrangement for reducing friction. Dr. Common’s plan—and it is
here that his instrument is unlike others—is to make the hollow polar
axis water-tight, and to fix it in a tank of water. At the bottom of the
polar axis is a ball and socket joint to keep it in position, and at the
top is another bearing, which can be adjusted so that the polar axis
lies truly in the meridian. It was found necessary to introduce 9 tons
of iron into the bottom of the hollow polar axis in order to sink it to
the proper angle, and to put sufficient weight on the bearings to give
stability to the instrument. In this way the great mass is brought into
the region of manageability, and the driving clock, which is driven by a
weight of 1½ tons, is able to do its work efficiently. Such, in general
outline, is this wonderful telescope, which, although not so large as
Lord Rosse’s famous instrument, is undoubtedly its superior in
light-grasping power and general utility, and more especially in its
adaptability for photographing the heavens.
Among other large reflecting telescopes now in use are the four-foot
reflectors at Melbourne and Paris, and the three-foot reflectors at
South Kensington and the Lick Observatory, California.
The largest refracting telescope yet constructed is one of 40 inches
aperture for the new Yerkes Observatory of the University of Chicago. It
is interesting to note here that Professor Keeler, in his report as an
expert upon the performance of the object-glass, considers that there is
“evidence for the first time that we are approaching the limit of size
in the construction of great objectives.” Unlike a mirror, a lens can be
supported only upon its circumference, and it is the bending by its own
weight that proves detrimental to its defining power. If the lens be
made thicker with a view of overcoming this defect, the absorption of
light by the glass increases, so that there is in the end no special
gain by increasing the size.
The length of the Yerkes telescope is 62 feet, and it will be provided
with all accessories pertaining to astrophysical research. The Yerkes
telescope, however, is not yet in actual use, and meanwhile the
world-renowned Lick telescope, of 36 inches aperture, keeps the lead
among active big refractors. The story of the foundation of this monster
instrument is not much less wonderful than the telescope itself. Brought
up in poor circumstances, with few opportunities for intellectual
development, James Lick, nevertheless, amassed a fortune in business,
and having few relations, he was anxious to dispose of his wealth in
such a way as to bring him that fame which he had failed to achieve in
other directions. Although it is very probable that he had never looked
through a telescope in his life, the idea of a large telescope had taken
a very firm hold upon his mind, and, thanks to the influence of his
advisers, it was definitely announced in 1873 that Mr. Lick’s bid for
immortality was to take this form. Several sites were examined by
experts, and finally Mount Hamilton, California, 4,200 feet above
sea-level, was selected. An excellent road, 26 miles in length, made at
the cost of the county authorities, connects the observatory with the
nearest town, San José, 13 miles distant.
Owing to various delays, operations were not commenced until 1880, and
five years were consumed in clearing away 72,000 tons of rocks and in
erecting the buildings.
Mr. Lick had stipulated for the erection of “a telescope superior to and
more powerful than any telescope yet made,” and Messrs. Alvan, Clark &
Co. contracted to supply a lens of 36 inches aperture for the sum of
50,000 dollars. It turned out, however, that it was much easier to make
such a contract than to fulfil it. To produce large discs of optically
perfect glass, even in the rough, requires the greatest possible skill
and patience, and this part of the work was undertaken by Feil & Co. of
Paris. The flint glass disc was safely delivered in America in 1882, but
the crown disc was cracked in packing. The elder Feil having retired
from business, the duty of providing a new block of crown glass devolved
upon his sons, who, after two years spent in vain attempts, ended in
bankruptcy, and it was only through the elder Feil again resuming
business that the much-required disc was finally completed in 1885.
After the lapse of another year, the rough discs were fashioned, in the
workshops of the Clarks, into the most marvellous of telescopic lenses.
The mounting of the object-glass is worthy of the occasion, as will be
seen from our illustration (see page 40). The tube is no less than 57
feet long, and 4 feet in diameter in the middle part. An iron pier, 38
feet high, beneath which lie the remains of Mr. Lick, supports the
equatorial head, and a winding staircase enables the observer to reach
the setting circles. Inside the hollow pier is the powerful driving
clock which turns the telescope to follow the heavenly bodies in their
apparent movements. Finders of 6, 4, and 3 inches diameter, rods for the
manipulation of the instrument, and all necessary accessories, complete
what must long remain one of the most perfect instruments at the service
of astronomical science. The 200,000 dollars expended upon it have
already been amply justified by the work accomplished, while Mr. Lick’s
dream of immortality has become a reality.
The following list indicates some of the large refractors now (Feb.,
1897) doing active service:—
_Aperture._ _Observatory._
36 inch [Lick] California.
30 „ Pulkowa, Russia.
30 „ [Bischoffeim] Nice.
28 „ Greenwich.
27 „ Vienna.
26 „ Washington.
25 „ [Newall] Cambridge.
24 „ [Lowell] Mexico.
23 „ Princeton, New Jersey.
It is right to add, however, that opinion is still greatly divided as to
whether these telescopes of large aperture really repay the expense and
labour involved in their erection and use. On the very rare occasion
when the “seeing” is practically perfect—which occurs perhaps only a few
hours in a year—it is probable that the superiority of a large telescope
is very marked, but under average conditions there seems to be little
advantage over instruments of moderate size for many classes of
observations.
Certain it is that a great deal of valuable work is done with
comparatively small telescopes, ranging from six to fifteen inches
aperture, and this in all departments of astronomical research. Hence,
some of the most active observatories do not figure in the above list;
among them may be mentioned the observatories of Harvard College
(U.S.A.), Potsdam, Paris, Heidelberg, Cape of Good Hope, Edinburgh,
South Kensington, Stonyhurst College, and the observatory of Dr. Isaac
Roberts at Crowborough, Sussex.
HOUSING OF EQUATORIALS.—The building which accommodates an equatorial
telescope must evidently be designed to admit of giving a clear opening
to any part of the sky. Usually this is accomplished by making the roof,
or _dome_, with a circular base, provided with wheels, which run on
rails. It is then only necessary to open a narrow portion of the dome,
extending from top to base, and to turn the dome until this aperture is
in the required direction. One of the most elaborate domes now in
existence is that built by M. Eiffel for the great refractor of the Nice
Observatory. The lower part of the building is in the form of a square
(see Frontispiece), having a side of about 87 feet, and a height of
about 30 feet. The dome itself is 74 feet in diameter, and the moving
parts alone weigh 95 tons.
As will be seen from the illustration, there are two shutters, each a
little wider than half the possible opening: these run on short rails,
and are moved simultaneously by means of an endless rope. The whole of
the dome is built up of steel angle iron, covered with very thin sheet
steel. In order to facilitate the manipulation of the dome, its great
weight is buoyed up by means of a float attached to its base and
immersed in a circular tank of water of a little greater size than the
base of the dome. If any mishap occurs with this gigantic tank, the dome
rests on wheels which run on a circular rail, so that the work need not
be interrupted. The whole arrangement is very easily turned with the aid
of a winch by one man when the dome is floating, but when resting on the
wheels several men are required at the winch.
This brief description will serve to illustrate some of the problems
which confront the possessor of a very large telescope. For smaller
instruments, the observatories follow pretty nearly the same plan,
except that it is unnecessary to provide an arrangement for floating the
dome.
The observatory which shelters a reflecting telescope need not differ
very greatly from one which contains a refractor. If the instrument be a
Newtonian, it is generally convenient to sink the polar axis below the
level of the floor in order that the observer may not be at too great a
height from the ground, and in that case, the dome, or its equivalent,
is all that is necessary. For his five-foot reflector, Dr. Common
designed an observatory which is not of the ordinary form, but gives the
necessary opening partly by means of large shutters, and partly by a
revolution of the whole house. It is not everyone who is able to lay out
£8,000 on such a dome as that erected at Nice by M. Bischoffeim.
The varying position of the eye end of a telescope, when it is turned to
different parts of the sky, makes it necessary to provide comfortable
and safe seating accommodation for the observer, more especially when
the telescope is a very large one. In the case of the Yerkes telescope,
the eye-piece will be 30 feet higher when observing near the horizon
than when observing near the zenith, and the observer must necessarily
follow the telescope. The most convenient arrangement in such a case is
to raise or lower the floor of the observatory as occasion demands. The
floor of the Yerkes Observatory is 75 feet in diameter, and by means of
electric motors it can be given a vertical motion of 22 feet. A similar
arrangement was provided for the Lick telescope from the designs of Sir
Howard Grubb. With smaller instruments, observing ladders and adjustable
chairs of various forms are employed.
THE EQUATORIAL COUDÉ.—A form of equatorial telescope which has possibly
a great future before it is one introduced at Paris under the name of
the _equatorial coudé_, or elbowed telescope. Its practical advantage is
that the observer remains in a constant and comfortable position, so
that revolving domes and elevating floors, or other arrangements serving
similar purposes, are no longer necessary. The telescope tube is of two
parts of nearly equal length, and what is ordinarily the lower half of
the tube forms part of the polar axis, while the other half is attached
to it at right angles. At the point of intersection of the two halves of
the tube is a plane mirror, and there is another mirror in front of the
object-glass. If the latter mirror were removed, such a telescope would
only enable the observer to see objects lying along the celestial
equator, but by its means objects in all parts of the heavens can be
brought within range to an observer gazing down the hollow polar axis.
The largest instrument is that at the Paris Observatory, which has an
object-glass 23½ inches in diameter for visual observations, and another
of the same size for photographic purposes.
FIXED TELESCOPES.—There is still another method of using a telescope.
The telescope itself may be fixed, and the light of the heavenly bodies
may be reflected into it by means of a mirror which is made to revolve
so as to keep pace with their movements. Foucault devised an instrument
called the _siderostat_ for this purpose, and although it is not largely
employed for telescopic observations, it is very widely utilised for
spectroscopic work, where the spectroscope is of a kind not readily
attached to a telescope.
Another instrument used for the same purpose has recently been brought
forward under the name of the _coelostat_. This is simply a mirror which
is made to turn on a polar axis in its own plane, and since a reflected
ray of light moves through twice the angle that the reflecting surface
turns through, the mirror is made to revolve at the rate of one
revolution in two days. As the name indicates, the whole heavens appear
stationary in such an instrument, whereas in a siderostat, only one star
at a time appears at rest, while its neighbours slowly revolve round it.
PHOTOGRAPHIC TELESCOPES.—The application of photography to the study of
the heavenly bodies marks one of the greatest advances of the present
century. The instruments which are employed for this purpose range from
the ordinary tourist camera to the largest telescope. Unlike a person
sitting for a portrait, the heavenly bodies cannot be made to stand
still for the purpose, and as instantaneous photographs can only be
obtained in the case of the sun and moon, it is usually necessary to
make the camera follow the stars very exactly during the time of
exposure, in order that the images may fall on precisely the same parts
of the photographic plate.
Some guiding arrangement is, therefore, essential, and generally the
photographic camera or telescope is attached to an ordinary equatorial
which is driven by clock-work, or very carefully by hand if the camera
be a small one. In the guiding telescope are two spider threads at right
angles to each other, and it is by constantly keeping the image of a
star at the intersection of these “wires” that the operator ensures the
images remaining in a constant position upon the sensitive plate.
An ordinary portrait camera, in the hands of a skilled observer, yields
very beautiful pictures, but they are naturally on a small scale. The
field of view of such an instrument is so large that a whole
constellation may be photographed with a single exposure.
Portrait lenses of 6 inches aperture in the hands of Dr. Max Wolf and
Professor Barnard have given magnificent delineations of the Milky Way,
and of the extremely faint nebulosities which are to be found in many
parts of the heavens.
For many purposes, however, telescopes of greater power are required,
and here it may be remarked that the distance between the images of any
two adjacent stars will vary in direct proportion to the focal length of
the telescope. In the same way the size of the image of a planet, the
moon, or a comet, increases as the focal length of the objective is
increased.
Refracting telescopes which are employed for photography require
object-glasses which are specially “corrected” for the photographic
rays. White light is compounded of light of all colours, but it is the
blue and violet constituents which are effective in producing
photographic action on an ordinary sensitive plate. Now, an object-glass
which is intended for visual purposes is made to focus at the same point
as many as possible of the rays which are most effective to the human
eye, that is the green, yellow, and red, and usually there is a blue or
purple halo round the images of the brighter objects, which is, however,
too feeble as a rule to interfere with visual observations. This blue
halo, will evidently result in defective definition if the lens be
employed for photography. By putting the plate at the point where the
blue rays are most nearly focused, a better image is obtained; but for
really good work a photographic object-glass must be so designed that
all the blue and violet rays are brought to one and the same focus. Such
a lens will consequently be a very poor one for visual observations. At
the present time, 18 photographic telescopes, each of 13 inches
aperture, and corrected in this way, are at work in various parts of the
world for the international star chart.
[Illustration:
FIG. 52.—_The Photographic Telescope employed by Dr. Isaac Roberts._
]
The new “photo telescopic” object-glass now manufactured by Messrs.
Cooke appears to be full of promise. In this lens all the colours of the
spectrum are brought to almost exactly the same focal point, so that it
serves equally well for photographic or visual purposes.
This difficulty in regard to achromatism does not exist in the case of
the reflecting telescope, since rays of light of every colour are
reflected at precisely the same angles. For this reason reflectors, when
properly managed, give the best photographic results. Dr. Isaac Roberts
and Dr. Common are especially identified with the application of the
reflecting telescope for celestial photography. The instrument employed
by the former consists of a 20-inch reflector, and a 7-inch guiding
telescope of the refracting form. The two telescopes are mounted on the
extreme ends of the declination axis of an equatorial, a photograph of
which we owe to the kindness of Dr. Roberts.
Dr. Common does not employ a guiding telescope at all. The photographic
plate which he places at the focus of the reflector is smaller than the
field of view, so that by means of an eye-piece fitted with a cross wire
at the side of the dark slide, he is able to watch a star near the edge
of the field. Both eye-piece and dark slide are attached to a frame
which can be controlled by two screws at right angles to each other. If
the guiding star leaves the cross wire through errors in driving, or
other causes, the eye-piece and dark slide are bodily moved after it by
means of the adjusting screws. This method not only has the advantage of
saving the cost of a guiding telescope, but reduces the effects of
vibration consequent upon the correction of errors by moving the whole
telescope.
For photographing the sun a special instrument called a
_photoheliograph_ is usually employed. This differs only from an
ordinary photographic telescope in being provided with a secondary
magnifier, by which means the focal image formed by the object-glass is
amplified before falling upon the photographic plate. On a bright clear
day, pictures of the sun 8 inches in diameter can be taken with an
exposure of about ¹⁄₅₀₀th of a second, and such a photograph will
frequently record more facts as to the state of the solar surface than a
whole day’s observation. Lenses or mirrors of very long focus are also
occasionally employed in solar photography, and in this way a large
image is obtained without the use of a secondary magnifier.
Photographs of the moon and planets may be taken either with or without
a secondary magnifier, but in either case the exposures are longer than
for the sun.
Finally, it may be added that the sensitive plates and processes used in
astronomical photography do not differ from those employed by ordinary
photographers.