If it could be used as a spy-glass by simply holding it in the hand,
and pointing at the object we wish to observe, there would be little
need of any very elaborate support. But if a telescope, even of the
smallest size, is to be used with regularity, a proper "mounting" is as
essential as a good instrument. Persons unpractised in the use of such
instruments are very apt to underrate the importance of those
accessories which merely enable us to point the telescope. An idea of
what is wanted in the mounting may readily be formed if the reader will
try to look at a star with an ordinary good-sized spy-glass held in the
hand, and then imagine the difficulties he meets with multiplied by
fifty.
The smaller and cheaper telescopes, as commonly sold, are mounted on a
simple little stand, on which the instrument admits of a horizontal and
vertical motion. If one only wants to get a few glimpses of a celestial
object, this mounting will answer his purpose. But to make anything
like a study of a celestial body, the mounting must be an equatorial
one; that is, one of the axes around which the telescope moves must be
inclined so as to point towards the pole of the heavens, which is near
the polar star. This axis will then make an angle with the horizon
equal to the latitude of the place. The telescope cannot, however, be
mounted directly on this axis, but must be attached to a second one,
itself fastened to this one.
[Illustration with caption: SECTION OF THE PRIMITIVE MOUNTING. P P.
Polar axis, bearing a fork at the upper end A. Declination axis passing
through the fork E. Section of telescope tube C. Weight to balance the
tube.]
When mounted in this way, an object can be followed in its diurnal
motion from east to west by turning on the polar axis alone. But if the
greatest facility in use is required, this motion must be performed by
clock-work. A telescope with this appendage will commonly cost one
thousand dollars and upward, so that it is not usually applied to very
small ones.
We will now suppose that the reader wishes to purchase a telescope or
an object-glass for himself, and to be able to judge of its
performance. He must have the object-glass properly adjusted in its
tube, and must use the highest power; that is, the smallest eye-piece,
which he intends to use in the instrument. Of course he understands
that in looking directly at a star or a celestial object it must appear
sharp in outline and well defined. But without long practice with good
instruments, this will not give him a very definite idea. If the person
who selects the telescope is quite unpractised, it is possible that he
can make the best test by ascertaining at what distance he can read
ordinary print. To do this he should have an eye-piece magnifying about
fifty times for each inch of aperture of the telescope. For instance,
if his telescope is three inches clear aperture, then his eye-piece
should magnify one hundred and fifty times; if the aperture is four
inches, one magnifying two hundred times may be used. This magnifying
power is, as a general rule, about the highest that can be
advantageously used with any telescope. Supposing this magnifying power
to be used, this page should be legible at a distance of four feet for
every unit of magnifying power of the telescope. For example, with a
power of 100, it should be legible at a distance of 400 feet; with a
power of 200, at 800 feet, and so on. To put the condition into another
shape: if the telescope will read the print at a distance of 150 feet
for each inch of aperture with the best magnifying power, its
performance is at least not very bad. If the magnifying power is less
than would be given by this rule, the telescope should perform a little
better; for instance, a three-inch telescope with a power of 60 should
make this page legible at a distance of 300 feet, or four feet for each
unit of power.
The test applied by the optician is much more exact, and also more
easy. He points the instrument at a star, or at the reflection of the
sun's rays from a small round piece of glass or a globule of
quicksilver several hundred yards away, and ascertains whether the rays
are all brought to a focus. This is not done by simply looking at the
star, but by alternately pushing the eye-piece in beyond the point of
distinct vision and drawing it out past the point. In this way the
image of the star will appear, not as a point, but as a round disk of
light. If the telescope is perfect, this disk will appear round and of
uniform brightness in either position of the eye-piece. But if there is
any spherical aberration or differences of density in different parts
of the glass, the image will appear distorted in various ways. If the
spherical aberration is not correct, the outer rim of the disk will be
brighter than the centre when the eye-piece is pushed in, and the
centre will be the brighter when it is drawn out. If the curves of the
glass are not even all around, the image will appear oval in one or the
other position. If there are large veins of unequal density, wings or
notches will be seen on the image. If the atmosphere is steady, the
image, when the eye-piece is pushed in, will be formed of a great
number of minute rings of light. If the glass is good, these rings will
be round, unbroken, and equally bright. We present several figures
showing how these spectral images, as they are sometimes called, will
appear; first, when the eye-piece is pushed in, and secondly, when it
is drawn out, with telescopes of different qualities.
We have thus far spoken only of the refracting telescope, because it is
the kind with which an observer would naturally seek to supply himself.
At the same time there is little doubt that the construction of a
reflector of moderate size is easier than that of a corresponding
refractor. The essential part of the reflector is a slightly concave
mirror of any metal which will bear a high polish. This mirror may be
ground and polished in the same way as a lens, only the tool must be
convex.
[Illustration with caption: SPECTRAL IMAGES OF STARS; THE UPPER LINE
SHOWING HOW THEY APPEAR WITH THE EYE-PIECE PUSHED IN, THE LOWER WITH
THE EYE-PIECE DRAWN OUT.
A The telescope is all right B Spherical aberration shown by the light
and dark centre C The objective is not spherical but elliptical D The
glass not uniform--a very bad and incurable case E One side of the
objective nearer than the other. Adjust it]
Of late years it has become very common to make the mirror of glass and
to cover the reflecting face with an exceedingly thin film of silver,
which can be polished by hand in a few minutes. Such a mirror differs
from our ordinary looking-glass in that the coating of silver is put on
the front surface, so that the light does not pass through the glass.
Moreover, the coating of silver is so thin as to be almost transparent:
in fact, the sun may be seen through it by direct vision as a faint
blue object. Silvered glass reflectors made in this way are extensively
manufactured in London, and are far cheaper than refracting telescopes
of corresponding size. Their great drawback is the want of permanence
in the silver film. In the city the film will ordinarily tarnish in a
few months from the sulphurous vapors arising from gaslights and other
sources, and even in the country it is very difficult to preserve the
mirror from the contact of everything that will injure it. In
consequence, the possessor of such a telescope, if he wishes to keep it
in order, must always be prepared to resilver and repolish it. To do
this requires such careful manipulation and management of the chemicals
that it is hardly to be expected that an amateur will take the trouble
to keep his telescope in order, unless he has a taste for chemistry as
well as for astronomy.
The curiosity to see the heavenly bodies through great telescopes is so
wide-spread that we are apt to forget how much can be seen and done
with small ones. The fact is that a large proportion of the
astronomical observations of past times have been made with what we
should now regard as very small instruments, and a good deal of the
solid astronomical work of the present time is done with meridian
circles the apertures of which ordinarily range from four to eight
inches. One of the most conspicuous examples in recent times of how a
moderate-sized instrument may be utilized is afforded by the
discoveries of double stars made by Mr. S. W. Burnham, of Chicago.
Provided with a little six-inch telescope, procured at his own expense
from the Messrs. Clark, he has discovered many hundred double stars so
difficult that they had escaped the scrutiny of Maedler and the
Struves, and gained for himself one of the highest positions among the
astronomers of the day engaged in the observation of these objects. It
was with this little instrument that on Mount Hamilton,
California--afterward the site of the great Lick Observatory--he
discovered forty-eight new double stars, which had remained unnoticed
by all previous observers. First among the objects which show
beautifully through moderate instruments stands the moon. People who
want to see the moon at an observatory generally make the mistake of
looking when the moon is full, and asking to see it through the largest
telescope. Nothing can then be made out but a brilliant blaze of light,
mottled with dark spots, and crossed by irregular bright lines. The
best time to view the moon is near or before the first quarter, or when
she is from three to eight days old. The last quarter is of course
equally favorable, so far as seeing is concerned, only one must be up
after midnight to see her in that position. Seen through a three or
four inch telescope, a day or two before the first quarter, about half
an hour after sunset, and with a magnifying power between fifty and one
hundred, the moon is one of the most beautiful objects in the heavens.
Twilight softens her radiance so that the eye is not dazzled as it will
be when the sky is entirely dark. The general aspect she then presents
is that of a hemisphere of beautiful chased silver carved out in
curious round patterns with a more than human skill. If, however, one
wishes to see the minute details of the lunar surface, in which many of
our astronomers are now so deeply interested, he must use a higher
magnifying power. The general beautiful effect is then lessened, but
more details are seen. Still, it is hardly necessary to seek for a very
large telescope for any investigation of the lunar surface. I very much
doubt whether any one has ever seen anything on the moon which could
not be made out in a clear, steady atmosphere with a six-inch telescope
of the first class.
Next to the moon, Saturn is among the most beautiful of celestial
objects. Its aspect, however, varies with its position in its orbit.
Twice in the course of a revolution, which occupies nearly thirty
years, the rings are seen edgewise, and for a few days are invisible
even in a powerful telescope. For an entire year their form may be
difficult to make out with a small telescope. These unfavorable
conditions occur in 1907 and 1921. Between these dates, especially for
some years after 1910, the position of the planet in the sky will be
the most favorable, being in northern declination, near its perihelion,
and having its rings widely open. We all know that Saturn is plainly
visible to the naked eye, shining almost like a star of the first
magnitude, so that there is no difficulty in finding it if one knows
when and where to look. In 1906-1908 its oppositions occur in the month
of September. In subsequent years, it will occur a month later every
two and a half years. The ring can be seen with a common, good
spy-glass fastened to a post so as to be steady. A four or five-inch
telescope will show most of the satellites, the division in the ring,
and, when the ring is well opened, the curious dusky ring discovered by
Bond. This "crape ring," as it is commonly called, is one of the most
singular phenomena presented by that planet.
It might be interesting to the amateur astronomer with a keen eye and a
telescope of four inches aperture or upward to frequently scrutinize
Saturn, with a view of detecting any extraordinary eruptions upon his
surface, like that seen by Professor Hall in 1876. On December 7th of
that year a bright spot was seen upon Saturn's equator. It elongated
itself from day to day, and remained visible for several weeks. Such a
thing had never before been known upon this planet, and had it not been
that Professor Hall was engaged in observations upon the satellites, it
would not have been seen then. A similar spot on the planet was
recorded in 1902, and much more extensively noticed. On this occasion
the spot appeared in a higher latitude from the planet's equator than
did Professor Hall's. At this appearance the time of the planet's
revolution on its axis was found to be somewhat greater than in 1876,
in accordance with the general law exhibited in the rotations of the
sun and of Jupiter. Notwithstanding their transient character, these
two spots have afforded the only determination of the time of
revolution of Saturn which has been made since Herschel the elder.
[Illustration with caption: THE GREAT REFRACTOR OF THE NATIONAL
OBSERVATORY AT WASHINGTON]
Of the satellites of Saturn the brightest is Titan, which can be seen
with the smallest telescope, and revolves around the planet in fifteen
days. Iapetus, the outer satellite, is remarkable for varying greatly
in brilliancy during its revolution around the planet. Any one having
the means and ability to make accurate photometrical estimates of the
light of this satellite in all points of its orbit, can thereby render
a valuable service to astronomy.
The observations of Venus, by which the astronomers of the last century
supposed themselves to have discovered its time of rotation on its
axis, were made with telescopes much inferior to ours. Although their
observations have not been confirmed, some astronomers are still
inclined to think that their results have not been refuted by the
failure of recent observers to detect those changes which the older
ones describe on the surface of the planet. With a six-inch telescope
of the best quality, and with time to choose the most favorable moment,
one will be as well equipped to settle the question of the rotation of
Venus as the best observer. The few days near each inferior conjunction
are especially to be taken advantage of.
The questions to be settled are two: first, are there any dark spots or
other markings on the disk? second, are there any irregularities in the
form of the sharp cusps? The central portions of the disk are much
darker than the outline, and it is probably this fact which has given
rise to the impression of dark spots. Unless this apparent darkness
changes from time to time, or shows some irregularity in its outline,
it cannot indicate any rotation of the planet. The best time to
scrutinize the sharp cusps will be when the planet is nearly on the
line from the earth to the sun. The best hour of the day is near
sunset, the half-hour following sunset being the best of all. But if
Venus is near the sun, she will after sunset be too low down to be well
seen, and must be looked at late in the afternoon.
The planet Mars must always be an object of great interest, because of
all the heavenly bodies it is that which appears to bear the greatest
resemblance to the earth. It comes into opposition at intervals of a
little more than two years, and can be well seen only for a month or
two before and after each opposition. It is hopeless to look for the
satellites of Mars with any but the greatest telescopes of the world.
But the markings on the surface, from which the time of rotation has
been determined, and which indicate a resemblance to the surface of our
own planet, can be well seen with telescopes of six inches aperture and
upward. One or both of the bright polar spots, which are supposed to be
due to deposits of snow, can be seen with smaller telescopes when the
situation of the planet is favorable.
The case is different with the so-called canals discovered by
Schiaparelli in 1877, which have ever since excited so much interest,
and given rise to so much discussion as to their nature. The astronomer
who has had the best opportunities for studying them is Mr. Percival
Lowell, whose observatory at Flaggstaff, Arizona, is finely situated
for the purpose, while he also has one of the best if not the largest
of telescopes. There the canals are seen as fine dark lines; but, even
then, they must be fifty miles in breadth, so that the word "canal" may
be regarded as a misnomer.
Although the planet Jupiter does not present such striking features as
Saturn, it is of even more interest to the amateur astronomer, because
he can study it with less optical power, and see more of the changes
upon its surface. Every work on astronomy tells in a general way of the
belts of Jupiter, and many speculate upon their causes. The reader of
recent works knows that Jupiter is supposed to be not a solid mass like
the earth, but a great globe of molten and vaporous matter,
intermediate in constitution between the earth and the sun. The outer
surface which we see is probably a hot mass of vapor hundreds of miles
deep, thrown up from the heated interior. The belts are probably
cloudlike forms in this vaporous mass. Certain it is that they are
continually changing, so that the planet seldom looks exactly the same
on two successive evenings. The rotation of the planet can be very well
seen by an hour's watching. In two hours an object at the centre of the
disk will move off to near the margin.
The satellites of this planet, in their ever-varying phases, are
objects of perennial interest. Their eclipses may be observed with a
very small telescope, if one knows when to look for them. To do this
successfully, and without waste of time, it is necessary to have an
astronomical ephemeris for the year. All the observable phenomena are
there predicted for the convenience of observers. Perhaps the most
curious observation to be made is that of the shadow of the satellite
crossing the disk of Jupiter. The writer has seen this perfectly with a
six-inch telescope, and a much smaller one would probably show it well.
With a telescope of this size, or a little larger, the satellites can
be seen between us and Jupiter. Sometimes they appear a little brighter
than the planet, and sometimes a little fainter.
Of the remaining large planets, Mercury, the inner one, and Uranus and
Neptune, the two outer ones, are of less interest than the others to an
amateur with a small telescope, because they are more difficult to see.
Mercury can, indeed, be observed with the smallest instrument, but no
physical configurations or changes have ever been made out upon his
surface. The question whether any such can be observed is still an open
one, which can be settled only by long and careful scrutiny. A small
telescope is almost as good for this purpose as a large one, because
the atmospheric difficulties in the way of getting a good view of the
planet cannot be lessened by an increase of telescopic power.
Uranus and Neptune are so distant that telescopes of considerable size
and high magnifying power are necessary to show their disks. In small
telescopes they have the appearance of stars, and the observer has no
way of distinguishing them from the surrounding stars unless he can
command the best astronomical appliances, such as star maps, circles on
his instrument, etc. It is, however, to be remarked, as a fact not
generally known, that Uranus can be well seen with the naked eye if one
knows where to look for it. To recognize it, it is necessary to have an
astronomical ephemeris showing its right ascension and declination, and
star maps showing where the parallels of right ascension and
declination lie among the stars. When once found by the naked eye,
there will, of course, be no difficulty in pointing the telescope upon
it.
Of celestial objects which it is well to keep a watch upon, and which
can be seen to good advantage with inexpensive instruments, the sun may
be considered as holding the first place. Astronomers who make a
specialty of solar physics have, especially in this country, so many
other duties, and their view is so often interrupted by clouds, that a
continuous record of the spots on the sun and the changes they undergo
is hardly possible. Perhaps one of the most interesting and useful
pieces of astronomical work which an amateur can perform will consist
of a record of the origin and changes of form of the solar spots and
faculae. What does a spot look like when it first comes into sight?
Does it immediately burst forth with considerable magnitude, or does it
begin as the smallest visible speck, and gradually grow? When several
spots coalesce into one, how do they do it? When a spot breaks up into
several pieces, what is the seeming nature of the process? How do the
groups of brilliant points called faculae come, change, and grow? All
these questions must no doubt be answered in various ways, according to
the behavior of the particular spot, but the record is rather meagre,
and the conscientious and industrious amateur will be able to amuse
himself by adding to it, and possibly may make valuable contributions
to science in the same way.
Still another branch of astronomical observation, in which industry and
skill count for more than expensive instruments, is the search for new
comets. This requires a very practised eye, in order that the comet may
be caught among the crowd of stars which flit across the field of view
as the telescope is moved. It is also necessary to be well acquainted
with a number of nebulae which look very much like comets. The search
can be made with almost any small telescope, if one is careful to use a
very low power. With a four-inch telescope a power not exceeding twenty
should be employed. To search with ease, and in the best manner, the
observer should have what among astronomers is familiarly known as a
"broken-backed telescope." This instrument has the eye-piece on the end
of the axis, where one would never think of looking for it. By turning
the instrument on this axis, it sweeps from one horizon through the
zenith and over to the other horizon without the observer having to
move his head. This is effected by having a reflector in the central
part of the instrument, which throws the rays of light at right angles
through the axis.
[Illustration: THE "BROKEN-BACKED COMET-SEEKER"]
How well this search can be conducted by observers with limited means
at their disposal is shown by the success of several American
observers, among whom Messrs. W. R. Brooks, E. E. Barnard, and Lewis
Swift are well known. The cometary discoveries of these men afford an
excellent illustration of how much can be done with the smallest means
when one sets to work in the right spirit.
The larger number of wonderful telescopic objects are to be sought for
far beyond the confines of the solar system, in regions from which
light requires years to reach us. On account of their great distance,
these objects generally require the most powerful telescopes to be seen
in the best manner; but there are quite a number within the range of
the amateur. Looking at the Milky Way, especially its southern part, on
a clear winter or summer evening, tufts of light will be seen here and
there. On examining these tufts with a telescope, they will be found to
consist of congeries of stars. Many of these groups are of the greatest
beauty, with only a moderate optical power. Of all the groups in the
Milky Way the best known is that in the sword-handle of Perseus, which
may be seen during the greater part of the year, and is distinctly
visible to the naked eye as a patch of diffused light. With the
telescope there are seen in this patch two closely connected clusters
of stars, or perhaps we ought rather to say two centres of condensation.