THE TELESCOPE--PRACTICAL
Having thus briefly sketched the history of the telescope, we turn now
to consider the optical means which are most likely to be in the hands
or within the reach of the beginner in astronomical observation. Let
us, first of all, make the statement that any telescope, good, bad, or
indifferent, is better than no telescope. There are some purists who
would demur to such a statement, who make the beginner's heart heavy
with the verdict that it is better to have no telescope at all
than one that is not of the utmost perfection, and, of course, of
corresponding costliness, and who seem to believe that the performance
of an inferior glass may breed disgust at astronomy altogether. This
is surely mere nonsense. For most amateurs at the beginning of their
astronomical work the question is not between a good telescope and an
inferior one, it is between a telescope and no telescope. Of course,
no one would be so foolish as willingly to observe with an inferior
instrument if a better could be had; but even a comparatively poor
glass will reveal much that is of great interest and beauty, and
its defects must even be put up with sometimes for the sake of its
advantages until something more satisfactory can be obtained. An
instrument which will show fifty stars where the naked eye sees five
is not to be despised, even though it may show wings to Sirius that
have no business there, or a brilliant fringe of colours round Venus
to which even that beautiful planet can lay no real claim. Galileo's
telescope would be considered a shockingly bad instrument nowadays;
still, it had its own little influence upon the history of astronomy,
and the wonders which it first revealed are easily within the reach of
anyone who has the command of a shilling or two, and, what is perhaps
still more important, of a little patience. The writer has still in
his possession an object-glass made out of a simple single eyeglass,
such as is worn by Mr. Joseph Chamberlain. This, mounted in a
cardboard tube with another single lens in a sliding tube as an
eye-piece, proved competent to reveal the more prominent lunar
craters, a number of sunspots, the phases of Venus, and the existence,
though not the true form, of Saturn's ring. Its total cost, if memory
serve, was one shilling and a penny. Of course it showed, in addition,
a number of things which should not have been seen, such as a lovely
border of colour round every bright object; but, at the same time,
it gave a great deal more than thirteen pence worth of pleasure and
instruction.
Furthermore, there is this to be said in favour of beginning with a
cheap and inferior instrument, that experience may thus be gained
in the least costly fashion. The budding astronomer is by nature
insatiably curious. He wants to know the why and how of all the things
that his telescope does or does not do. Now this curiosity, while
eminently laudable in itself, is apt in the end to be rather hard
upon his instrument. A fine telescope, whatever its size may be, is
an instrument that requires and should receive careful handling; it is
easily damaged, and costly to replace. And therefore it may be better
that the beginner should make his earlier experiments, and find
out the more conspicuous and immediately fatal of the many ways of
damaging a telescope, upon an instrument whose injury, or even whose
total destruction, need not cause him many pangs or much financial
loss.
It is not suggested that a beginning should necessarily be made on
such a humble footing as that just indicated. Telescopes of the sizes
mainly referred to in these pages--_i.e._, refractors of 2 or 3 inches
aperture, and reflectors of 4-1/2 to 6 inches--may frequently be
picked up second-hand at a very moderate figure indeed. Of course, in
these circumstances the purchaser has to take his chance of defects
in the instrument, unless he can arrange for a trial of it, either by
himself, or, preferably, by a friend who has some experience; yet even
should the glass turn out far from perfect, the chances are that it
will at least be worth the small sum paid for it. Nor is it in the
least probable, as some writers seem to believe, that the use of
an inferior instrument will disgust the student and hinder him from
prosecuting his studies. The chances are that it will merely create
a desire for more satisfactory optical means. Even a skilled observer
like the late Rev. T. W. Webb had to confess of one of his telescopes
that 'much of its light went the wrong way'; and yet he was able to
get both use and pleasure out of it. The words of a well-known English
amateur observer may be quoted. After detailing his essays with
glasses of various degrees of imperfection Mr. Mee remarks: 'For the
intending amateur I could wish no other experience than my own. To
commence with a large and perfect instrument is a mistake; its owner
cannot properly appreciate it, and in gaining experience is pretty
sure to do the glass irreparable injury.'
Should the beginner not be willing or able to face the purchase
of even a comparatively humble instrument, his case is by no means
desperate, for he will find facilities at hand, such as were not
thought of a few years ago, for the construction of his own telescope.
Two-inch achromatic object-glasses, with suitable lenses for the
making up of the requisite eye-pieces, are to be had for a few
shillings, together with cardboard tubes of sizes suitable for
fitting up the instrument; and such a volume as Fowler's 'Telescopic
Astronomy' gives complete directions for the construction of a glass
which is capable of a wonderful amount of work in proportion to
its cost. The substitution of metal tubes for the cardboard ones is
desirable, as metal will be found to be much more satisfactory if the
instrument is to be much used. The observer, however, will not long
be satisfied with such tools as these, useful though they may be. The
natural history of amateur astronomers may be summed up briefly in the
words 'they go from strength to strength.' The possessor of a small
telescope naturally and inevitably covets a bigger one; and when the
bigger one has been secured it represents only a stage in the search
for one bigger still, while along with the desire for increased size
goes that for increased optical perfection. No properly constituted
amateur will be satisfied until he has got the largest and best
instrument that he has money to buy, space to house, and time to use.
Let us suppose, then, that the telescope has been acquired, and that
it is such an instrument as may very commonly be found in the hands
of a beginner--a refractor, say, of 2, 2-1/2, or 3 inches aperture
(diameter of object-glass). The question of reflectors will fall to
be considered later. Human nature suggests that the first thing to do
with it is to unscrew all the screws and take the new acquisition to
pieces, so far as possible, in order to examine into its construction.
Hence many glasses whose career of usefulness is cut short before
it has well begun. 'In most cases,' says Webb, 'a screw-driver is a
dangerous tool in inexperienced hands'; and Smyth, in the Prolegomena
to his 'Celestial Cycle,' utters words of solemn warning to the
'over-handy gentlemen who, in their feverish anxiety for meddling
with and making instruments, are continually tormenting them with
screw-drivers, files, and what-not.' Unfortunately, it is not only
the screw-driver that is dangerous; the most deadly danger to the most
delicate part of the telescope lies in the unarmed but inexperienced
hands themselves. You may do more irreparable damage to the
object-glass of your telescope in five minutes with your fingers than
you are likely to do to the rest of the instrument in a month with
a screw-driver. Remember that an object-glass is a work of art,
sometimes as costly as, and always much more remarkable than, the
finest piece of jewellery. It may be unscrewed, _carefully_, from
the end of its tube and examined. Should the examination lead to the
detection of bubbles or even scratches in the glass (quite likely the
latter if the instrument be second-hand), these need not unduly vex
its owner's soul. They do not necessarily mean bad performance, and
the amount of light which they obstruct is very small, unless the
case be an extreme one. But on no account should the two lenses of the
object-glass itself be separated, for this will only result in making
a good objective bad and a bad one worse. The lenses were presumably
placed in their proper adjustment to one another by an optician before
being sent out; and should their performance be so unsatisfactory
as to suggest that this adjustment has been disturbed, it is to an
optician that they should be returned for inspection. The glass may,
of course, be carefully and gently cleaned, using either soft chamois
leather, or preferably an old silk handkerchief, studiously kept
from dust; but the cleaning should never amount to more than a gentle
sweeping away of any dust which may have gathered on the surface.
Rubbing is not to be thought of, and the man whose telescope has been
so neglected that its object-glass needs rubbing should turn to some
other and less reprehensible form of mischief. For cleaning the
small lenses of the eye-pieces, the same silk may be employed; Webb
recommends a piece of blotting-paper, rolled to a point and aided by
breathing, for the edges which are awkward to get at. Care must, of
course, be taken to replace these lenses in their original positions,
and the easiest way to ensure this is to take out only one at a time.
In replacing them, see that the finger does not touch the surface of
the glass, or the cleaning will be all to do over again.
[Illustration: FIG. 8.
_a_, O.G. in perfect adjustment; _b_, O.G. defectively centred.]
Next comes the question of testing the quality of the objective. (The
stand is meanwhile assumed, but will be spoken of later.) Point the
telescope to a star of about the third magnitude, and employ
the eye-piece of highest power, if more than one goes with the
instrument--this will be the shortest eye-piece of the set. If the
glass be of high quality, the image of the star will be a neat round
disc of small size, surrounded by one or two thin bright rings (Fig.
8, _a_). Should the image be elliptical and the rings be thrown to the
one side (Fig. 8, _b_), the glass may still be quite a good one, but
is out of square, and should be readjusted by an optician. Should
the image be irregular and the rings broken, the glass is of inferior
quality, though it may still be serviceable enough for many purposes.
Next throw the image of the star out of focus by racking the eye-piece
in towards the objective, and then repeat the process by racking it
again out of focus away from the objective. The image will, in either
case, expand into a number of rings of light, and these rings should
be truly circular, and should present precisely the same appearance at
equal distances within and without the focus. A further conception of
the objective's quality may be gained by observing whether the image
of a star or the detail of the moon or of the planets comes sharply
to a focus when the milled head for focussing is turned. Should it
be possible to rack the eye-tube in or out for any distance without
disturbing the distinctness of the picture to any extent, then the
glass is defective. A good objective will admit of no such range, but
will come sharply up to focus, and as sharply away from it, with any
motion of the focussing screw. A good glass will also show the details
of a planet like Saturn, such as are within its reach, that is, with
clearness of definition, while an inferior one will soften all the
outlines, and impart a general haziness to them. The observer may now
proceed to test the colour correction of his objective. No achromatic,
its name notwithstanding, ever gives an absolutely colourless image;
all that can be expected is that the colour aberration should have
been so far eliminated as not to be unpleasant. In a good instrument
a fringe of violet or blue will be seen around any bright object, such
as Venus, on a dark sky; a poor glass will show red or yellow. It
is well to make sure, however, should bad colour be seen, that the
eye-piece is not causing it; and, therefore, more than one eye-piece
should be tried before an opinion is formed. Probably more colour will
be seen at first than was expected, more particularly with an object
so brilliant as Venus. But the observer need not worry overmuch about
this. He will find that the eye gets so accustomed to it as almost
to forget that it is there, so that something of a shock may be
experienced when a casual star-gazing friend, on looking at some
bright object, remarks, as friends always do, 'What beautiful
colours!' Denning records a somewhat extreme case in which a friend,
who had been accustomed to observe with a refractor, absolutely
resented the absence of the familiar colour fringe in the picture
given by a reflector, which is the true achromatic in nature, though
not in name. The beginner is recommended to read the article 'The
Adjustment of a Small Equatorial,' by Mr. E. W. Maunder, in the
_Journal of the British Astronomical Association_, vol. ii., p. 219,
where he will find the process of testing described at length and with
great clearness.
In making these tests, allowance has, of course, to be made for the
state of the atmosphere. A good telescope can only do its best on a
good night, and it is not fair to any instrument to condemn it until
it has been tested under favourable conditions. The ideal test would
be to have its performance tried along with that of another instrument
of known good quality and of as nearly the same size as possible. If
this cannot be arranged for, the tests must be made on a succession
of nights, and good performance on one of these is sufficient to
vindicate the reputation of the glass, and to show that any deficiency
on other occasions was due to the state of the air, and not to the
instrument. Should his telescope pass the above tests satisfactorily,
the observer ought to count himself a happy man, and will until he
begins to hanker after a bigger instrument.
The mention of the pointing of the telescope to a star brings up the
question of how this is to be done. It seems a simple thing; as a
matter of fact, with anything like a high magnifying power it is next
to impossible; and there are few things more exasperating than to see
a star or a planet shining brightly before your eyes, and yet to find
yourself quite unable to get it into the field of view. The simple
remedy is the addition of a finder to the telescope. This is a small
telescope of low magnifying power which is fastened to the larger
instrument by means of collars bearing adjusting screws, which enable
it to be laid accurately parallel with the large tube (Fig. 10). Its
eye-piece is furnished with cross-threads, and a star brought to
the intersection of these threads will be in the field of the large
telescope. In place of the two threads crossing at right angles there
may be substituted three threads interlacing to form a little triangle
in the centre of the finder's field. By this device the star can
always be seen when the glass is being pointed instead of being
hidden, as in the other case, behind the intersection of the two
threads. A fine needle-point fixed in the eye-piece will also be found
an efficient substitute for the cross-threads. In the absence of
a finder the telescope may be pointed by using the lowest power
eye-piece and substituting a higher one when the object is in the
field; but beyond question the finder is well worth the small
addition which it makes to the cost of an instrument. A little care
in adjusting the finder now and again will often save trouble and
annoyance on a working evening.
The question of a stand on which to mount the telescope now falls to
be considered, and is one of great importance, though apt to be rather
neglected at first. It will soon be found that little satisfaction or
comfort can be had in observing unless the stand adopted is steady. A
shaky mounting will spoil the performance of the best telescope that
ever was made, and will only tantalize the observer with occasional
glimpses of what might be seen under better conditions. Better have a
little less aperture to the object-glass, and a good steady mounting,
than an extra inch of objective and a mounting which robs you of all
comfort in the using of your telescope. Beginners are indeed rather
apt to be misled into the idea that the only matters of importance are
the objective and its tube, and that money spent on the stand is money
wasted. Hence many fearful and wonderful contrivances for doing badly
what a little saved in the size of the telescope and expended on the
stand would have enabled them to do well. It is very interesting, no
doubt, to get a view of Jupiter or Saturn for one field's-breadth,
and then to find, on attempting to readjust the instrument for another
look, that the mounting has obligingly taken your star-gazing into its
own hands, and is now directing your telescope to a different object
altogether; but repetition of this form of amusement is apt to pall. A
radically weak stand can never be made into a good one; the best plan
is to get a properly proportioned mounting at once, and be done with
it.
[Illustration: FIG. 9.--SMALL TELESCOPE ON PILLAR AND CLAW STAND.]
For small instruments, such as we are dealing with, the mounting
generally adopted is that known as the Altazimuth, from its giving two
motions, one in altitude and one in azimuth, or, to use more familiar
terms, one vertical and the other horizontal. There are various types
of the Altazimuth. If the instrument be of not more than 3 feet focal
length, the ordinary stand known as the 'pillar and claw' (Fig. 9)
will meet all the requirements of this form of motion. Should the
focal length be greater than 3 feet, it is advisable to have the
instrument mounted on a tripod stand, such as is shown in Fig. 10. In
the simpler forms of both these mountings the two motions requisite
to follow an object must be given by hand, and it is practically
impossible to do this without conveying a certain amount of tremor to
the telescope, which disturbs clearness of vision until it subsides,
by which time the object to be viewed is generally getting ready to
go out of the field again. To obviate this inconvenience as far as
possible, the star or planet when found should be placed just outside
the field of view, and allowed to enter it by the diurnal motion
of the earth. The tremors will thus have time to subside before the
object reaches the centre of the field, and this process must
be repeated as long as the observation continues. In making this
adjustment attention must be paid to the direction of the object's
motion through the field, which, of course, varies according to its
position in the sky. If it be remembered that a star's motion through
the telescopic field is the exact reverse of its true direction across
the sky, little difficulty will be found, and use will soon render
the matter so familiar that the adjustment will be made almost
automatically.
[Illustration: FIG. 10.--TELESCOPE ON TRIPOD, WITH FINDER AND SLOW
MOTIONS.]
A much more convenient way of imparting the requisite motions is by
the employment of tangent screws connected with Hooke's joint-handles,
which are brought conveniently near to the hands of the observer as
he sits at the eye-end. These screws clamp into circles or portions of
circles, which have teeth cut on them to fit the pitch of the screw,
and by means of them a slow and steady motion may be imparted to the
telescope. When it is required to move the instrument more rapidly, or
over a large expanse of sky, the clamps which connect the screws with
the circles are slackened, and the motion is given by hand. Fig. 10
shows an instrument provided with these adjuncts, which, though not
absolutely necessary, and adding somewhat to the cost of the mounting,
are certainly a great addition to the ease and comfort of observation.
[Illustration: FIG. 11.--EQUATORIAL MOUNTING FOR SMALL TELESCOPE.]
The Altazimuth mounting, from its simplicity and comparative
cheapness, has all along been, and will probably continue to be, the
form most used by amateurs. It is, however, decidedly inferior in
every respect to the equatorial form of mount. In this form (Fig. 11)
the telescope is carried by means of two axes, one of which--the Polar
axis--is so adjusted as to be parallel to the pole of the earth's
rotation, its degree of inclination being therefore dependent upon the
latitude of the place for which it is designed. At the equator it will
be horizontal, will lie at an angle of 45 degrees half-way between
the equator and either pole, and will be vertical at the poles. At its
upper end it carries a cross-head with bearings through which there
passes another axis at right angles to the first (the declination
axis). Both these axes are free to rotate in their respective
bearings, and thus the telescope is capable of two motions, one of
which--that of the declination axis--enables the instrument to be set
to the elevation of the object to be observed, while the other--that
of the polar axis--enables the observer to follow the object, when
found, from its rising to its setting by means of a single movement,
the telescope sweeping out circles on the sky corresponding to those
which the stars themselves describe in their journey across the
heavens. This single movement may be given by means of a tangent screw
such as has already been described, and the use of a telescope thus
equipped is certainly much easier and more convenient than that of an
Altazimuth, where two motions have constantly to be imparted. To gain
the full advantage of the equatorial form of mounting, the polar axis
must be placed exactly in the North and South line, and unless the
mounting can be adjusted properly and left in adjustment, it is robbed
of much of its superiority. For large fixed instruments it is, of
course, almost universally used; and in observatories the motion in
Right Ascension, as it is called, which follows the star across the
sky, is communicated to the driving-wheel of the polar axis by means
of a clock which turns the rod carrying the tangent screw (Plate II.).
These are matters which in most circumstances are outside the sphere
of the amateur; it may be interesting for him, however, to see
examples of the way in which large instruments are mounted. The
frontispiece, accordingly, shows the largest and most perfect
instrument at present in existence, while Plate II., with Figs. 4
and 12, give further examples of fine modern work. The student can
scarcely fail to be struck by the extreme solidity of the modern
mountings, and by the way in which all the mechanical parts of the
instrument are so contrived as to give the greatest convenience
and ease in working. Comparing, for instance, Plate II., a 6-inch
refractor by Messrs. Cooke, of York, available either for visual or
photographic work, with the Dorpat refractor (Fig. 3), it is seen that
the modern maker uses for a 6-inch telescope a stand much more
solid and steady than was deemed sufficient eighty years ago for an
instrument of 9-6/10 inches. Attention is particularly directed to the
way in which nowadays all the motions are brought to the eye-end so
as to be most convenient for the observer, and frequently, as in this
case, accomplished by electric power, while the declination circle is
read by means of a small telescope so that the large instrument can
be directed upon any object with the minimum of trouble. The
driving clock, well shown on the right of the supporting pillar, is
automatically controlled by electric current from the sidereal clock
of the observatory.
[Illustration:
PLATE II.
6-inch Photo-Visual Refractor, equatorially mounted. Messrs. T. Cooke
& Sons.]
We have now to consider the reflecting form of telescope, which,
especially in this country, has deservedly gained much favour, and has
come to be regarded as in some sense the amateur's particular tool.
[Illustration: FIG. 12.--8-INCH REFRACTOR ON EQUATORIAL MOUNTING.]
As a matter of policy, one can scarcely advise the beginner to make
his first essay with a reflector. Its adjustments, though simple
enough, are apt to be troublesome at the time when everything has
to be learned by experience; and its silver films, though much more
durable than is commonly supposed, are easily destroyed by careless
or unskilful handling, and require more careful nursing than the
objective of a refractor. But, having once paid his first fees to
experience, the observer, if he feel so inclined, may venture upon a
reflector, which has probably more than sufficient advantages to
make up for its weaker points. First and foremost of these advantages
stands the not inconsiderable one of cheapness. A 10-1/2-inch
reflector may be purchased new for rather less than the sum which will
buy a 4-inch refractor. True, the reflector has not the same command
of light inch for inch as the refractor, but a reflector of 10-1/2
inches should at least be the match of an 8-inch refractor in this
respect, and will be immeasurably more powerful than the 4-inch
refractor, which comes nearest to it in price. Second stands the ease
and comfort so conspicuous in observing with a Newtonian. Instead
of having almost to break his neck craning under the eye-piece of a
telescope pointed to near the zenith, the observer with a Newtonian
looks always straight in front of him, as the eye-piece of a reflector
mounted as an altazimuth is always horizontal, and when the instrument
is mounted equatorially, the tube, or its eye-end, is made to rotate
so that the line of vision may be kept horizontal. Third is the
absence of colour. Colour is not conspicuous in a small refractor,
unless the objective be of very bad quality; but as the aperture
increases it is apt to become somewhat painfully apparent. The
reflector, on the other hand, is truly achromatic, and may be relied
upon to show the natural tints of all objects with which it deals.
This point is of considerable importance in connection with planetary
observation. The colouring of Jupiter, for instance, will be seen in a
reflector as a refractor can never show it.
Against these advantages there have to be set certain disadvantages.
First, the question of adjustments. A small refractor requires
practically none; but a reflector, whatever its size, must be
occasionally attended to, or else its mirrors will get out of square
and bad performance will be the result. It is easy, however, to
make too much of this difficulty. The adjustments of the writer's
8-1/2-inch With reflector have remained for months at a time as
perfect as when they had been newly attended to. Second, the renewal
of the silver films. This may cause some trouble in the neighbourhood
of towns where the atmosphere is such as to tarnish silver quickly;
and even in the country a film must be renewed at intervals. But these
may be long enough. The film on the mirror above referred to has stood
without serious deterioration for five years at a time. Third, the
reflector, with its open-mouthed tube, is undoubtedly more subject
to disturbance from air currents and changes of temperature, and its
mirrors take longer to settle down into good definition after the
instrument has been moved from one point of the sky to another. This
difficulty cannot be got over, and must be put up with; but it is not
very conspicuous with the smaller sizes of telescopes, such as are
likely to be in the hands of an amateur at the beginning of his work.
There are probably but few nights when an 8-1/2-inch reflector will
not give quite a good account of itself in this respect by comparison
with a refractor of anything like equal power. On the whole, the state
of the question is this: If the observer wishes to have as much power
as possible in proportion to his expenditure, and is not afraid to
take the risk of a small amount of trouble with the adjustments and
films, the reflector is probably the instrument best suited to him.
If, on the other hand, he is so situated that his telescope has to be
much moved, or, which is almost as bad, has to stand unused for any
considerable intervals of time, he will be well advised to prefer
a refractor. One further advantage of the reflecting form is that,
aperture for aperture, it is very much shorter. The average refractor
will probably run to a length of from twelve to fifteen times the
diameter of its objective. Reflectors are rarely of a greater length
than nine times the diameter of the large mirror, and are frequently
shorter still. Consequently, size for size, they can be worked in less
space, which is often a consideration of importance.
[Illustration: FIG. 13.--FOUR-FOOT REFLECTOR EQUATORIALLY MOUNTED.]
The mountings of the reflector are in principle precisely similar to
those of the refractor already described. The greater weight, however,
and the convenience of having the body of the instrument kept as low
as possible, owing to the fact of the eye-piece being at the upper end
of the tube, have necessitated various modifications in the forms to
which these principles are applied. Plates III. and IV., and Fig. 13,
illustrate the altazimuth and equatorial forms of mounting as applied
to reflectors of various sizes, Fig. 13 being a representation of
Lassell's great 4-foot reflector.
[Illustration:
PLATE III.
20-inch Reflector, Stanmore Observatory.]
And now, having his telescope, whatever its size, principle, or
form of mounting, the observer has to proceed to use it. Generally
speaking, there is no great difficulty in arriving at the manner of
using either a refractor or a reflector, and for either instrument
the details of handling must be learned by experience, as nearly all
makers have little variations of their own in the form of clamps and
slow motions, though the principles in all instruments are the same.
With regard to these, the only recommendation that need be made is one
of caution in the use of the glass until its ways of working have
been gradually found out. With a knowledge of the principles of its
construction and a little application of common-sense, there is no
part of a telescope mounting which may not be readily understood.
Accordingly, what follows must simply take the form of general hints
as to matters which every telescopist ought to know, and which
are easier learned once and for all at the beginning than by slow
experience. These hints are of course the very commonplaces of
observation; but it is the commonplace that is the foundation of good
work in everything.
If possible, let the telescope be fixed in the open air. Where
money is no object, a few pounds will furnish a convenient little
telescope-house, with either a rotating or sliding roof, which enables
the instrument to be pointed to any quarter of the heavens. Such
houses are now much more easily obtained than they once were, and
anyone who has tried both ways can testify how much handier it is to
have nothing to do but unlock the little observatory, and find the
telescope ready for work, than to have to carry a heavy instrument out
into the open. Plate IV. illustrates such a shelter, which has done
duty for more than twelve years, covering an 8-1/2-inch With, whose
tube and mounting are almost entirely the work of a local smith; and
in the _Journal of the British Astronomical Association_, vol.
xiv., p. 283, Mr. Edwin Holmes gives a simple description of a small
observatory which was put up at a cost of about £3, and has proved
efficient and durable. The telescope-house has also the advantage
of protecting the observer and his instrument from the wind, so that
observation may often be carried on on nights which would be quite too
windy for work in the open.
[Illustration:
PLATE IV.
Telescope House and 8-1/2-inch 'with' Reflector.]
Should it not be possible to obtain such a luxury, however,
undoubtedly the next best is fairly outside. No one who has garden
room should ever think of observing from within doors. If the
telescope be used at an open window its definition will be impaired
by air-currents. The floor of the room will communicate tremors to the
instrument, and every movement of the observer will be accompanied by
a corresponding movement of the object in the field, with results
that are anything but satisfactory. In some cases no other position is
available. If this be so, Webb's advice must be followed, the window
opened as widely and as long beforehand as possible, and the telescope
thrust out as far as is convenient. But these precautions only
palliate the evils of indoor observation. The open air is the best,
and with a little care in wrapping up the observer need run no risk.
Provide the telescope, if a refractor, with a dew-cap. Without this
precaution dew is certain to gather upon the object-glass, with
the result of stopping all observation until it is removed, and the
accompanying risk of damage to the objective itself. Some instruments
are provided by their makers with dew-caps, and all ought to be; but
in the absence of this provision a cap may be easily contrived. A
tube of tin three or four times as long as the diameter of the
object-glass, made so as to slide fairly stiffly over the object end
of the tube where the ordinary cap fits, and blackened inside to a
dead black, will remove practically all risk. The blackening may be
done with lamp-black mixed with spirit varnish. Some makers--Messrs.
Cooke, of York, for instance--line both tube and dew-cap with black
velvet. This ought to be ideal, and might be tried in the case of the
dew-cap by the observer. Finders are rarely fitted with dew-caps,
but certainly should be; the addition will often save trouble and
inconvenience.
Be careful to cover up the objective or mirror with its proper cap
before removing it into the house. If this is not done, dewing at once
results, the very proper punishment for carelessness. This may seem a
caution so elementary as scarcely to be worth giving; but it is easier
to read and remember a hint than to have to learn by experience, which
in the case of a reflector will almost certainly mean a deteriorated
mirror film. Should the mirror, if you are using a reflector, become
dewed in spite of all precautions, do not attempt to touch the film
while it is moist, or you will have the pleasure of seeing it scale
off under your touch. Bring it into a room of moderate temperature,
or stand it in a through draught of dry air until the moisture
evaporates; and should any stain be left, make sure that the mirror is
absolutely dry before attempting to polish it off. With regard to this
matter of polishing, touch the mirror as seldom as possible with the
polishing-pad. Frequent polishing does far more harm than good, and
the mirror, if kept carefully covered when not in use, does not need
it. A fold of cotton-wool between the cap and the mirror will, if
occasionally taken out and dried, help greatly to preserve the film.
Next comes a caution which beginners specially need. Almost everyone
on getting his first telescope wants to see everything as big as
possible, and consequently uses the highest powers. This is an entire
mistake. For a telescope of 2-1/2 inches aperture two eye-pieces,
or at most three, are amply sufficient. Of these, one may be low in
power, say 25 to 40, to take in large fields, and, if necessary, to
serve in place of a finder. Such an eye-piece will give many star
pictures of surprising beauty. Another may be of medium power, say 80,
for general work; and a third may be as high as 120 for exceptionally
fine nights and for work on double stars. Nominally a 2-1/2 inch, if
of very fine quality, should bear on the finest nights and on stars
a power of 100 to the inch, or 250. Practically it will do nothing
of the sort, and on most nights the half of this power will be found
rather too high. Indeed, the use of high powers is for many reasons
undesirable. A certain proportion of light to size must be preserved
in the image, or it will appear faint and 'clothy.' Further, increased
magnifying power means also increased magnification of every tremor of
the atmosphere; and with high powers the object viewed passes through
the field so rapidly that constant shifting of the telescope is
required, and only a brief glimpse can be obtained before the
instrument has to be moved again. It is infinitely more satisfactory
to see your object of a moderate size and steady than to see it much
larger, but hazy, tremulous, and in rapid motion. 'In inquiring about
the quality of some particular instrument,' remarks Sir Howard Grubb,
'a tyro almost invariably asks, "What is the highest power you can
use?" An experienced observer will ask, "What is the lowest power with
which you can do so and so?"'
Do not be disappointed if your first views of celestial objects do not
come up to your expectations. They seldom do, particularly in respect
of the size which the planets present in the field. A good deal of
the discouragement so often experienced is due to the idea that the
illustrations in text-books represent what ought to be seen by anyone
who looks through a telescope. It has to be remembered that these
pictures are, for one thing, drawn to a large scale, in order to
insure clearness in detail, that they are in general the results of
observation with the very finest telescopes, and the work of skilled
observers making the most of picked nights. No one would expect to
rival a trained craftsman in a first attempt at his trade; yet most
people seem to think that they ought to be able at their first essay
in telescopic work to see and depict as much as men who have
spent half a lifetime in an apprenticeship to the delicate art of
observation. Given time, patience, and perseverance, and the skill
will come. The finest work shown in good drawings represents, not
what the beginner may expect to see at his first view, but a standard
towards which he must try to work by steady practice both of eye and
hand. In this connection it may be suggested that the observer should
take advantage of every opportunity of seeing through larger and finer
instruments than his own. This will teach him two things at least.
First, to respect his own small telescope, as he sees how bravely it
stands up to the larger instrument so far as regards the prominent
features of the celestial bodies; and, second, to notice how the
superior power of the large glass brings out nothing startlingly
different from that which is shown by his own small one, but a wealth
of delicate detail which must be looked for (compare Plate XV. with
Fig. 22). A little occasional practice with a large instrument will be
found a great encouragement and a great help to working with a small
one, and most possessors of large glasses are more than willing to
assist the owners of small ones.
Do not be ashamed to draw what you see, whether it be little or much,
and whether you can draw well or ill. At the worst the result will
have an interest to yourself which no representation by another hand
can ever possess; at the best your drawings may in course of time come
to be of real scientific value. There are few observers who cannot
make some shape at a representation of what they see, and steady
practice often effects an astonishing improvement. But draw only what
you see with certainty. Some observers are gifted with abnormal powers
of vision, others with abnormal powers of imagination. Strange to say,
the results attained by these two classes differ widely in appearance
and in value. You may not be endowed with faculties which will enable
you to take rank in the former class; but at least you need not
descend to the latter. It is after all a matter of conscience.
Do not be too hasty in supposing that everybody is endowed with a zeal
for astronomy equal to your own. The average man or woman does not
enjoy being called out from a warm fireside on a winter's night, no
matter how beautiful the celestial sight to be seen. Your friend
may politely express interest, but to tempt him to this is merely to
encourage a habit of untruthfulness. The cause of astronomy is not
likely to be furthered by being associated in any person's mind with
discomfort and a boredom which is not less real because it is veiled
under quite inadequate forms of speech. It is better to wait until the
other man's own curiosity suggests a visit to the telescope, if you
wish to gain a convert to the science.
When observing in the open be sure to wrap up well. A heavy ulster or
its equivalent, and some form of covering for the feet which will keep
them warm, are absolute essentials. See that you are thoroughly warm
before you go out. In all probability you will be cold enough before
work is over; but there is no reason why you should make yourself
miserable from the beginning, and so spoil your enjoyment of a fine
evening.
Having satisfied his craving for a general survey of everything in
the heavens that comes within the range of his glass, the beginner
is strongly advised to specialize. This is a big word to apply to the
using of a 2-1/2- or 3-inch telescope, but it represents the only
way in which interest can be kept up. It does no good, either to
the observer or to the science of astronomy, for him to take out
his glass, have a glance at Jupiter and another at the Orion nebula,
satisfy himself that the two stars of Castor are still two, wander
over a few bright clusters, and then turn in, to repeat the same
dreary process the next fine night. Let him make up his mind to stick
to one, or at most two, objects. Lunar work presents an attractive
field for a small instrument, and may be followed on useful lines, as
will be pointed out later. A spell of steady work upon Jupiter will at
least prepare the way and whet the appetite for a glass more adequate
to deal with the great planet. Should star work be preferred, a fine
field is opened up in connection with the variable stars, the chief
requirement of work in this department being patience and regularity,
a small telescope being quite competent to deal with a very large
number of interesting objects.
The following comments in Smyth's usual pungent style are worth
remembering: 'The furor of a green astronomer is to possess himself
of all sorts of instruments--to make observations upon everything--and
attempt the determination of quantities which have been again and
again determined by competent persons, with better means, and
more practical acquaintance with the subject. He starts with an
enthusiastic admiration of the science, and the anticipation of new
discoveries therein; and all the errors consequent upon the momentary
impulses of what Bacon terms "affected dispatch" crowd upon him. Under
this course--as soon as the more hacknied objects are "seen up"--and
he can decide whether some are greenish-blue or bluish-green--the
excitement flags, the study palls, and the zeal evaporates in
hyper-criticism on the instruments and their manufacturers.'
This is a true sketch of the natural history, or rather, of the
decline and fall, of many an amateur observer. But there is no reason
why so ignominious an end should ever overtake any man's pursuit of
the study if he will only choose one particular line and make it his
own, and be thorough in it. Half-study inevitably ends in weariness
and disgust; but the man who will persevere never needs to complain of
sameness in any branch of astronomical work.