REFLECTORS--MIRROR TELESCOPES
Cherished with the utmost care in the rooms of the Royal Society of
London is a world-famous telescope, a diminutive reflector made by the
hands of Sir Isaac Newton. We have already mentioned his connection with
the refractor; and how he abandoned that type of telescope in favor of
the reflecting mirror, or reflector in which the obstacles to great size
appeared to be purely mechanical. By many, indeed, Newton is regarded as
the inventor of the reflector.
By the principles of optics, all the rays from a star that strike a
concave mirror will be reflected to the geometric focal point, provided
a section of that mirror is a parabola. Such a mirror is called a
speculum, and is an alloy of tin, copper, and bismuth. Its surface takes
a very high polish, reflecting when newly polished nearly 90 per cent of
the light that falls upon it.
But the focus where the eyepiece must be used is in front of the mirror,
and if the eye were placed there, the observer's head would intercept
all or much of the light that would otherwise reach the mirror. Gregory,
probably the real inventor of the reflector, was the first to dodge this
difficulty by perforating the mirror at the center and applying the
eyepiece there, at the back of the speculum; but it was necessary to
first send the rays to that point by reflection from a second or
smaller mirror, in the optical axis of the speculum. This reflects the
rays backward down the tube to the eyepiece, or spectroscope, or camera.
Another English optician, Cassegrain, improved on this design somewhat
by placing the secondary mirror inside the focus of the speculum, or
nearer to it, so that the tube is shorter. This form is preferable for
many kinds of astronomical work, especially photography. Herschel sought
to do away with the secondary reflector entirely and save the loss of
light by tilting the speculum slightly, so as to throw the image at one
side of the tube; but this modification introduces bad definition of the
image and has never been much used.
A better plan is that of Newton, who placed a small plane speculum at an
angle of 45 degrees in the optical axis where the secondary mirror of
the Gregory-Cassegrainian type is placed. The rays are then received by
the eyepiece at the side of the upper end of the tube, the observer
looking in at right angles to the axis. And a modern improvement first
used by Draper is a small rectangular prism in place of the little plane
speculum, effecting a saving of five to ten per cent of the light.
It is not easy to say which type of telescope, the refractor or the
reflector, is the more famous. Nor which is the better or more useful,
or the more likely to lead in the astronomy of the future. When the
successors of Dollond had carried the achromatic refractor to the limit
enforced by the size of the glass disks they were able to secure, they
found these instruments not so great an improvement after all. The
single-lens telescopes of great focal length were nearly as good
optically, though much more awkward to handle. But the quality of the
glass obtainable in that day appeared to set an arbitrary limit to that
great amplification of size and power which progress in observational
astronomy demanded.
Then came the elder Herschel, best known and perhaps the greatest of all
astronomers. At Bath, England, music was his profession, especially the
organ. But he was dissatisfied with his little Gregorian reflector, and
being a very clever mechanician he set out to build a reflector for
himself. It is said that he cast and polished nearly 200 mirrors, in the
course of experiments on the most highly reflective type of alloys, and
the sort of mechanism that would enable him to give them the highest
polish. In all his work he was ably and enthusiastically aided by his
sister, Caroline Herschel, most famous of all women astronomers.
Upward in size of his mirrors he advanced, till he had a speculum of two
feet diameter with a tube 20 feet long. Twelve to fifteen years had
elapsed when in 1781, while testing one of these reflectors on stars in
the constellation Gemini, he made the first discovery of a planet since
the invention of the telescope--the great planet now known as Uranus.
Under the patronage of King George, he advanced to telescopes of still
greater size, his largest being no less than forty feet in length, with
a speculum of four feet in diameter. Two new satellites of Saturn were
discovered with this giant reflector, which was dismantled by Sir John
Herschel with appropriate ceremonies, including the singing of an ode by
the Herschel family assembled inside of the tube, on New Year's Eve,
1839-40.
We have record of but few attempts to improve the size and definition of
great reflectors by the continental astronomers during this era. In
England and Ireland, however, great progress was made. About 1860
Lassell built a two-foot reflector, with which he discovered two new
satellites of Uranus, and which he subsequently set up in the island of
Malta. Ten years later Thomas Grubb and Son of Dublin constructed a
four-foot reflector, now at the Observatory in Melbourne, Australia.
Calver in conjunction with Common of Ealing, London, about 1880-95 built
several large reflectors, the largest of five feet diameter, now owned
by Harvard College Observatory; and, rather earlier, Martin of Paris
completed a four-foot reflector.
The mirrors of these latter instruments were not made of speculum metal,
but of solid glass, which must be very thick (one-seventh their
diameter) in order to prevent flexure or bending by their own weight. So
sensitive is the optical surface to distortion that unless a complicated
series of levers and counterpoises is supplied, to support the under
surface of the mirror, the perfection of its optical figure disappears
when the telescope is directed to objects at different altitudes in the
sky. The upper or outer surface of the glass is the one which receives
the optical polish on a heavy coat of silver chemically deposited on the
polished glass after its figure has been tested and found satisfactory.
But far and away the most famous reflecting telescope of all is the
"Leviathan" of Lord Rosse, built at Birr Castle, Parsonstown, Ireland,
about the middle of the last century. His Lordship made many ingenious
improvements in grinding the mirror, which was of speculum metal, six
feet in diameter and weighed seven tons. It was ground to a focal
length of fifty-four feet and mounted between heavy walls of masonry, so
that the motion of the great tube was restricted to a few degrees on
both sides of the meridian. The huge mechanism was very cumbersome in
operation, and photography was not available in those days; nevertheless
Lord Rosse's telescope made the epochal discovery of the spiral nebulae,
which no other telescope of that day could have done.
In America the reflector has always kept at least even pace with the
refractor. As early as 1830, Mason and Smith, two students at Yale
College, enthused by Denison Olmsted, built a 12-inch speculum with
which they made unsurpassed observations of the nebulae. Dr. Henry
Draper, returning from a visit to Lord Rosse, began about 1865 the
construction of two silver-on-glass reflectors, one of 15 inches
diameter, the other of 28 inches, with which he did important work for
many years in photography and spectroscopy, and his mirrors are now the
property of Harvard College Observatory. Alvan Clark and Sons have in
later years built a 40-inch mirror for the Lowell Observatory in
Arizona, and very recently a 6-foot silver-on-glass mirror has been set
up in the Dominion of Canada Astrophysical Observatory at Victoria,
British Columbia, where it is doing excellent work in the hands of
Plaskett, its designer.
The huge glass disk for the reflector weighs two tons, and it must be
cast so that there are no internal strains; otherwise it is liable to
burst in fragments in the process of grinding. It should be free from
air-bubbles, too; so the glass is cast in one melting, if possible. This
disk was made by the St. Gobain Plate Glass Company, whose works have
been ruthlessly destroyed by the enemy during the war; but fortunately
the great disk had been shipped from Antwerp only a week before
declaration of hostilities.
Brashear of Allegheny was intrusted with the optical parts, which
occupied many months of critical work. The finished mirror is 73 inches
in diameter, its focal length is 30 feet, and its thickness 12 inches. A
central hole 10 inches in diameter makes possible its use as a Gregorian
or Cassegrainian type, as well as Newtonian. The mechanical parts of
this great telescope are by Warner and Swasey of Cleveland, after the
well-known equatorial mounting of the Melbourne reflector by Grubb of
Dublin. Friction of the polar and declination axes is reduced by ball
bearings. The 66-foot dome has an opening 15 feet wide and extending six
feet beyond the zenith. All motions of the telescope, dome shutters, and
observing platform are under complete control by electric motors.
Spectroscopic binaries form one of the special fields of research with
this powerful instrument, and many new binaries have already been
detected.
The great reflectors designed and constructed by Ritchey, formerly of
Chicago and now of Pasadena, deserve especial mention. While connected
with the Yerkes Observatory he constructed a two-foot reflector for that
institution, with which he had exceptional success in photography of the
stars and nebulae. Later he built a 5-foot reflector, now at the Carnegie
Observatory on Mount Wilson, California, with which the spiral nebulae
and many other celestial objects have been especially well photographed.
Ritchey's later years have been spent on the construction of an even
greater mirror, no less than 100 inches in diameter, which was completed
in 1919, and has already yielded photographic results dealt with farther
on, and far surpassing anything previously obtained. Theoretically this
huge mirror, if its surface were perfectly reflective so that it would
transmit all the rays falling upon it, would gather 160,000 times as
much light as the unaided eye alone.
Whether a 72-inch refractor, should it ever be constructed, would
surpass the 100-inch reflector as an all-round engine for astronomical
research, is a question that can only be fully answered by building it
and trying the two instruments alongside.
Probably three-quarters of all the really great astronomical work in the
past has been done by refractors. They are always ready and convenient
for use, and the optical surfaces rarely require cleaning and
readjustment. With increase of size, however, the secondary spectrum
becomes very bothersome in the great lenses; and the larger they are,
the more light is lost by absorption on account of the increasing
thickness of the lenses. With the reflector on the other hand, while
there is clearly a greater range of size, the reflective surface retains
its high polish only a brief period, so that mere tarnish effectively
reduces the aperture; and the great mirror is more or less ineffective
in consequence of flexure uncompensated by the lever system that
supports the back of the mirror.
Both types of telescope still have their enthusiastic devotees; and the
next great reflector would doubtless be a gratifying success, if mounted
in some elevated region of the world, like the Andes of northern Chile,
where the air is exceptionally steady and the sky very clear a large
part of the year. The highest magnifying powers suitable for work with
such a telescope could then be employed, and new discoveries added as
well as important work done in extension of lines already begun on the
universe of stars.
On the authority of Clark, even a six-foot objective would not
necessitate a combined thickness of its glasses in excess of six inches.
Present disks are vastly superior to the early ones in transparency, and
there is reason to expect still greater improvement. The engineering
troubles incident to execution of the mechanical side of the scheme need
not stand in the way; they never have, indeed the astronomer has but
just begun to invoke the fertile resources of the modern engineer. Not
long before his death the younger Clark who had just finished the great
lenses of the 40-inch Yerkes telescope, ventured this prevision, already
in part come true: "The new astronomy, as well as the old, demands more
power. Problems wait for their solution, and theories to be
substantiated or disproved. The horizon of science has been greatly
broadened within the last few years, but out upon the borderland I see
the glimmer of new lights that await for their interpretation, and the
great telescopes of the future must be their interpreters."
Practically all the great telescopes of the world have in turn
signalized the new accession of power by some significant astronomical
discovery: to specify, one of Herschel's reflectors first revealed the
planet Uranus; Lord Rosse's "Leviathan" the spiral nebulae; the 15-inch
Cambridge lens the crape, or dusky ring of Saturn; the 18-1/2-inch
Chicago refractor the companion of Sirius; the Washington 26-inch
telescope the satellites of Mars; the 30-inch Pulkowa glass the
nebulosities of the Pleiades; and the 36-inch Lick telescope brought to
light a fifth satellite of Jupiter. At the time these discoveries were
made, each of these great telescopes was the only instrument then in
existence with power enough to have made the discovery possible. So we
may advance to still farther accessions of power with the expectation
that greater discoveries will continue to gratify our confidence.