_NOTES ON TELESCOPES AND THEIR ACCESSORIES._
Choice of Telescopes.—Refractors and Reflectors.—Observer’s
Aims.—Testing Telescopes.—Mounting.—Eyepieces.—Requisite
Powers.—Overstating Powers.—Method of finding the Power.—Field of
Eyepiece.—Limited Means no obstacle.—Observing-Seats.-Advantage
of Equatoreals.—Test-Objects.—Cheapness and increasing number of
Telescopes.—Utility of Stops.—Cleaning Lenses.—Opera-Glasses.—Dewing
of Mirrors.—Celestial Globe.—Observatories.
_Choice of Telescopes._—The subject of the choice of telescopes
has exercised every astronomer more or less, and the question as
to the best form of instrument is one which has occasioned endless
controversy. The decision is an important one to amateurs, who at
the outset of their observing careers require the most efficient
instruments obtainable at reasonable cost. It is useless applying to
scientific friends who, influenced by different tastes, will give
an amount of contradictory advice that will be very perplexing.
Some invariably recommend a small refractor and unjustly disparage
reflectors, as not only unfitted for very delicate work, but as
constantly needing re-adjustment and resilvering[6].
Others will advise a moderate-sized reflector as affording wonderfully
fine views of the Moon and planets. The question of cost is greatly in
favour of the latter construction, and, all things considered, it may
claim an unquestionable advantage. A man who has decided to spend a
small sum for the purpose not merely of gratifying his curiosity but
of doing really serviceable work, must adopt the reflector, because
refractors of, say, 5 inches and upwards are far too costly, and become
enormously expensive as the diameter increases. This is not the case
with reflectors; they come within the reach of all, and may indeed
be constructed by the observer himself with a little patience and
ingenuity.
_Refractors and Reflectors._—The relative merits of refractors and
reflectors[7] have been so frequently compared and discussed that we
have no desire to re-open the question here. These comparisons have
been rarely free from bias, or sufficiently complete to afford really
conclusive evidence either way. There is no doubt that each form of
instrument possesses its special advantages: aperture for aperture
the refractor is acknowledged to be superior in light-grasping power,
but the ratio given by different observers is not quite concordant.
A silver-on-glass mirror of 8-inches aperture is certainly equal to
a 7-inch objective in this respect, while as regards dividing power
and the definition of planetary markings, the reflector is equal to a
refractor of the same aperture. The much shorter focal length of the
reflector is an advantage not to be overlooked. A century ago Sir W.
Herschel figured his specula to foci of more than a foot to every inch
of aperture, except in the case of his largest instruments. Thus he
made specula of 18½-inches and 24-inches diameter, the former of which
had a focal length of 20 feet and the latter of 25 feet. The glass
mirrors of the present time are much shorter, and the change has not
proved incompatible with excellent performance. Calver has made two
good mirrors of 17-1/4-inches aperture, and only 8 ft. 4 in. focus. Mr.
Common’s 5-foot mirror is only 27½ feet, so that in these instances the
length of the tube is less than six times the diameter.
[Illustration: Fig. 12.
“The Popular Reflector” by Calver.]
It has long been proved that refractors and reflectors alike are, in
good hands, capable of producing equally good results; and we may
depend upon it that, in spite of all argument and experiment, both
kinds of telescope will continue to hold their own until superseded
by a new combination, which hardly seems likely. If the observer is
free from prejudice, he will have no cause to deplore the character
of his instrument, always supposing it to be by a good maker. Be
it object-glass or speculum, he will rarely find it lacking in
effectiveness. It happens only too often that the telescope or the
atmosphere is hastily blamed when the fault rests with the observer
himself. Let him be persistent in waiting opportunities, and let the
instrument be nicely adjusted and in good condition, and in the great
majority of cases it will perform all that can reasonably be expected
of it.
In choosing appliances for observational purposes, the observer will of
course be guided by his means and requirements. If his inclination lead
him to enter a particular department of research, he will take care to
provide himself with such instruments as are specially applicable to
the work in hand. Modern opticians have effected so many improvements,
and brought out so many special aids to smooth the way of an observer,
that it matters little in which direction he advances; he will scarcely
find his progress impeded by want of suitable apparatus. In size, as
also in character, the observer should be careful to discriminate
as to what is really essential. Large instruments and high powers
are not necessary to show what can be sufficiently well seen in a
small telescope with moderate power. Of course there is nothing like
experience in such matters, and practice soon renders one more or less
proficient in applying the best available means.
[Illustration: Fig. 13.
3-inch Refracting-Telescope, by Newton & Co.]
An amateur who really wants a competent instrument and has to
consider cost, will do well to purchase a Newtonian reflector. A
4½-inch refractor will cost about as much as a 10-inch reflector,
but, as a working tool, the latter will possess a great advantage.
A small refractor, if a good one, will do wonders, and is a very
handy appliance, but it will not have sufficient grasp of light for
it to be thoroughly serviceable on faint objects. Anyone who is
hesitating in his choice should look at the cluster about χ Persei
through instruments such as alluded to, and he will be astonished at
the vast difference in favour of the reflector. For viewing sun-spots
and certain lunar objects small refractors are very effective, and
star-images are usually better seen than in reflectors, but the latter
are much preferable for general work on account of three important
advantages, viz., cheapness, illuminating power, and convenience of
observation. When high magnifiers are employed on a refractor of small
aperture, the images of planets become very faint and dusky, so that
details are lost.
_Observer’s Aims._—If the intending observer merely requires a
telescope to exhibit glimpses of the wonders which he has seen
portrayed in books, and has no intention of pursuing the subject
further than as an occasional hobby, he will do well to purchase a
small refractor between 3 and 4 inches in aperture. Such instruments
are extremely effective on the Sun and Moon, which are naturally the
chief objects to attract attention, and, apart from this, appliances
of the size alluded to may be conveniently used from an open window.
The latter is an important consideration to many persons; moreover,
a small telescope of this kind will reveal an astonishing number of
interesting objects in connection with the planets, comets, &c., and
it may be employed by way of diversion upon terrestrial landscape, as
such instruments are almost invariably provided with non-inverting
eyepieces. Out-of-door observing is inconvenient in many respects, and
those who procure a telescope merely to find a little recreation will
soon acknowledge a small refractor to be eminently adapted to their
purposes and conveniences.
Those who meditate going farther afield, and taking up observation
habitually as a means of acquiring practical knowledge, and possibly
of doing original work, will essentially need different means. They
will require reflectors of about 8 or 10 inches aperture; and, if
mounted in the open on solid ground, so much the better, as there
will be a more expansive view, and a freedom from heated currents,
which renders an apartment unsuited to observations, unless with small
apertures where the effects are scarcely appreciable. A reflector
of the diameter mentioned will command sufficient grasp to exhibit
the more delicate features of planetary markings, and will show many
other difficult objects in which the sky abounds. If the observer be
specially interested in the surface configuration of Mars and Jupiter
he will find a reflector a remarkably efficient instrument. On the Moon
and planets it is admitted that its performance is, if not superior,
equal to that of refractors. If, however, the inclination of the
observer leads him in the direction of double stars, their discovery
and measurement, he will perhaps find a refractor more to be depended
upon, though there is no reason why a well-mounted reflector should not
be successfully employed in this branch; and the cost of a refractor
of the size to be really useful as an instrument of discovery must
be something very considerable—perhaps ten times as great as that of
a reflector of equal capacity. As far as my own experience goes the
refractor gives decidedly the best image of a star. In the reflector,
a bright star under moderately high power is seen with rays extending
right across the field, and these appear to be caused by the supports
of the flat.
_Testing Telescopes._—No amateur should buy an instrument, especially
a second-hand one, without testing it, and this is a delicate process
involving many points to be duly weighed. Experience is of great
service in such matters, and is, in fact, absolutely necessary.
Even old observers are sometimes misled as to the real worth of a
glass. In such cases, there is nothing like having a reliable means
of comparison, _i. e._ another telescope of acknowledged excellence
with which to test the doubtful instrument. In the absence of such a
standard judgment will be more difficult, but with care a satisfactory
decision may be arrived at. The Moon is too easy an object for the
purpose of such trials; the observer should rather select Venus or
Jupiter. The former is, however, so brilliant on a dark sky, and so
much affected with glare, that the image will almost sure to be faulty
even if the glass is a good one. Let the hour be either near sunrise or
sunset, and if the planet has a tolerably high altitude, her disk ought
to be seen beautifully sharp and white. Various powers should be tried,
increasing them each time, and it should be noticed particularly
whether the greater expansion of the image ruins the definition or
simply enfeebles the light. In a thoroughly good glass faintness will
come on without seriously impairing the definite contour of the object
viewed, and the observer will realize that the indistinctness is merely
occasioned by the power being relatively in excess of the light-grasp.
But in a defective telescope, a press of magnifying power at once
brings out a mistiness and confuses the details of the image in a very
palpable manner. Try how he will, the observer will find it impossible
to get rid of this, except, perhaps, by a “stop” which cuts off so much
light that the instrument is ineffective for the work required of it.
The blurred image is thought, at the moment of its first perception,
to be caused by the object being out of focus, and the observer vainly
endeavours to get a sharper image until he finds the source of error
lies elsewhere. A well-figured glass ought to come very sharply to a
focus. The slightest turn of the adjusting-screw should make a sensible
difference. On the other hand, an inferior lens will permit a slight
alteration of focusing without affecting the distinctness, because the
rays from the image are not accurately thrown to a point. Jupiter is
also a good test. The limbs of the planet, if shown clean and hard,
and the belts, if they are pictured like the finely cut details of an
engraving, will at once stamp a telescope as one of superior quality.
Saturn can also be examined though not, perhaps, so severe a test.
The belts, crape ring, Cassini’s division, ought to be revealed in
any telescope of moderate aperture. If, with regard to any of these
objects, the details apparently run into each other and there is a
“fuzzy” or woolly aspect about them which cannot be eliminated by
careful focusing, then either the atmosphere or the telescope is in
fault. If the former, another opportunity must be awaited. An observer
of experience will see at a glance whether the cause lies in the air
or the instrument. The images will be agitated by obnoxious currents,
if the defects are due to the atmosphere, but if the glass itself is
in error, then the objects will be comparatively tranquil but merged
in hazy outlines, and a general lack of distinctness will be apparent.
Perhaps the best test of all as to the efficiency of a telescope is
that of a moderately bright star, say of the 2nd or 3rd magnitude. With
a high power the image should be very small, circular, and surrounded
by two or three rings of light lying perfectly concentric with each
other. No rays, wings, or extraneous appearance other than the
diffraction rings should appear.
This, however, specially applies to refractors, for in reflectors the
arms of the flat occasion rays from any bright star; I have also seen
them from Mars, but of course this does not indicate an imperfect
mirror. If there is any distortion on one side of the image, then the
lenses are inaccurately centred though the instrument may be otherwise
good, and a little attention may soon set matters right. When testing a
glass the observer should choose objects at fairly high altitudes, and
not condemn a telescope from a single night’s work unless the evidence
is of unusually convincing character. If false colour is seen in a
silver-on-glass reflector it is originated by the eyepiece, though not
necessarily so in a refractor. The object-glass of the latter will be
sure to show some uncorrected colour fringing a bright object. A good
lens, when exactly focused, exhibits a claret tint, but within the
focus purple is seen and beyond the focus green comes out. In certain
cases the secondary spectrum of an object-glass is so inadequately
corrected that the vivid colouring of the images is sometimes
attributed by inexperienced observers to a real effect. A friend who
used a 3-inch refractor once called on me to have a glimpse of Jupiter
through my 10-inch With-reflector. On looking at the planet he at once
exclaimed “But where are the beautiful colours, Mr. Denning?” I replied
to his question by asking another, viz., “What colours?” he answered,
“Why, the bright colours I see round Jupiter in my refractor?” I said,
“Oh, they exist in your telescope only!” He looked incredulous, and
when he left me that night did not seem altogether pleased with the
appearance of Jupiter shorn of his false hues!
_Mounting._—Too much care cannot be given to the mounting of
telescopes, for the most perfectly figured glass will be rendered
useless by an inefficient stand; a faulty lens, if thoroughly
well mounted, will do more than a really good one on a shaky or
unmanageable mounting. Whatever form is adopted, the arrangement should
ensure the utmost steadiness, combined with every facility for readily
following objects. A man who has every now and then to undergo a
great physical exertion in bodily shifting the instrument is rendered
unfit for delicate work. The telescope should be provided with every
requisite for carrying on prolonged work with slight exertion on the
part of the observer. Unless the stand is firm there will be persistent
vibrations, especially if the instrument is erected in the open, for
there are very few nights in the year when the air is quite calm. These
contingencies should be provided against with scrupulous attention if
the observer would render his telescope most effective for the display
of its powers, and avoid the constant annoyance that must otherwise
follow.
[Illustration: Fig. 14.
Huygens’s negative eyepiece.]
[Illustration: Fig. 15.
Ramsden’s positive eyepiece.]
_Eyepieces._—Good eyepieces are absolutely essential. Many
object-glasses and specula have been deprecated for errors really
originated by the eyepiece. Again, telescopes have not unfrequently
been blamed for failures through want of discrimination in applying
suitable powers. A consistent adaptation of powers according to the
aperture of the telescope, the character of the object, the nature
of the observation, and the atmospheric conditions prevailing at the
time, is necessary to ensure the best results. If it is required to
exhibit a general view of Jupiter and his satellites to a friend, we
must utilize a low power with a large field; if, on the other hand,
we desire to show the red spot and its configuration in detail, we
must apply the highest power that is satisfactorily available. The
_negative_ or Huygenian eyepiece is the one commonly used, and it
forms good colourless images, though the field is rather small. The
_positive_ or Ramsden eyepiece gives a flatter and larger field, but
it is not often achromatic. A Kellner eyepiece, the feature of which
is a very large field, is often serviceable in observations of nebulæ,
clusters, and comets. Telescopes are sometimes stated to bear 100 to
the inch on planets, but this is far beyond their capacities even in
the very best condition of air. Amateurs soon find from experience
that it is best to employ those powers which afford the clearest and
most comprehensive views of the particular objects under scrutiny. Of
course when abnormally high powers are mentioned in connection with an
observation, they have an impressive sound, but this is all, for they
are practically useless for ordinary work. I find that 40, or at the
utmost 50 to the inch, is ample, and generally beyond the capacities
of my 10-inch reflector. A Barlow lens used in front of the eyepiece
raises the power about one third, and thus a whole set of eyepieces may
be increased by its insertion. It is said to improve the definition,
while the loss of light is very trifling. I formerly used a Barlow lens
in all planetary observations, but finally dispensed with it, as I
concluded the improved distinctness did not compensate for the fainter
image. A great advantage, both in light and definition, results from
the employment of a single lens as eyepiece. True, the field is very
limited, and, owing to the spherical aberration, the object so greatly
distorted near the edges that it must be kept near the centre, but, on
the whole, the superiority is most evident. By many careful trials I
find it possible to glimpse far more detail in planetary markings than
with the ordinary eyepiece. Dawes, and other able observers, also found
a great advantage in the single lens, and Sir W. Herschel, more than a
century ago, expressed himself thus:—“I have tried both the double and
single lens eye-glass of equal powers, and always found that the single
eye-glass had much the superiority in light and distinctness.”
_Requisite Powers._—For general purposes I believe three eyepieces
are all that is absolutely requisite, viz., a low power with large
field for sweeping up nebulæ and comets; a moderate power for viewing
the Moon and planets; and a high power for double stars and the more
delicate forms on the planets. For a 3-inch refractor, eyepieces of
about 15, 75, and 150 would be best, and for a 10-inch reflector 40,
150, and 300. For very difficult double stars a still higher power will
be occasionally useful, say 250 for the refractor, and 500 for the
reflector. The definition usually suffers so much under high powers,
and the tremors of the atmosphere are brought out so conspicuously,
that the greater expansion of the image of a planet does not
necessarily enable it to present more observable detail. The features
appear diluted and merged in hazy outlines, and there is a lack of the
bright, sharply determinate forms so steadily recognized under lower
magnifiers. In special cases great power may become essential, and,
under certain favourable circumstances, will prove really serviceable,
but, in a general way, it is admitted that the lowest power which shows
an object well is always the best. I have occasionally obtained very
fair views of Saturn with a power of 865, but find that I can perceive
more of the detail with 252. Some daylight observations of Venus
were also effected under very high power, and, though the definition
remained tolerably good, I found as the result of careful comparison
that less power answered more satisfactorily. But it would be absurd to
lay down inviolable rules in such cases. Special instruments, objects,
and circumstances require special powers, and observers may always
determine with a little care and experience the most eligible means to
support their endeavours. One thing should be particularly remembered,
that the power used must not be beyond the illuminating capacity of
the instrument, for planetary features appear so faint and shady under
excessive magnifiers that nothing is gained. To grasp details there
must be a fair amount of light. I have seen more with 252 on my 10-inch
reflector than with 350 on a 5-1/4-inch refractor, because of the
advantage from the brighter image in the former case.
_Overstating Powers._—It seems to be a fashionable imposition on the
part of opticians to overstate magnifying powers. Eyepieces are usually
advertised at double their true strength. My own 10-inch reflector
was catalogued as having four eyepieces, 100 to 600, but on trial I
found the highest was no more than 330. This custom of exaggerating
powers seems to have long been a privileged deception, and persons
buying telescopes ought to be guarded against it. Dr. Kitchiner says
it originated with the celebrated maker of reflectors, James Short,
and justly condemns it as a practice which should be discontinued.
I suppose it is thought that high powers advertised in connection
with a telescope have an exalted sound and are calculated to attract
the unwary purchaser; but good instruments need no insidious trade
artifices to make them saleable. The practice does not affect observers
of experience, because it is well understood, and they take good
care to test their eyepieces directly they get them. But the case is
different with young and inexperienced amateurs, who naturally enough
accept the words of respectable opticians, only to find, in many cases,
that they have been misleading and a source of considerable annoyance.
_Method of finding the Power._—The magnifying power of a telescope
may be determined by dividing the focal length of the object-glass or
mirror by the focal length of the eye-lens. Thus, if the large glass
has a focus of 70 inches and the eye-lens a focus of one inch, then
the power is 70. If the latter is only 1/4-inch focus, the resulting
power will be 280. But this method is only applicable to single lens
eyepieces. We may, however, resort to several other means of finding
the powers of the compound eyepieces of Huygens or Ramsden. Let the
observer fix a slip of white cardboard, say 1 inch wide, to a door or
post some distance off, and then (with a refractor) view it, while
keeping the disengaged eye open, and note the exact space covered by
the telescopic image of the card as projected on the door seen by the
other eye. The number of inches included in the space alluded to will
represent the linear magnifying power. A brick wall or any surface with
distinct, regularly marked divisions will answer the same purpose, the
number of bricks or divisions covered by the telescopic image of one
of them being equivalent to the power. But it should not be forgotten
that a telescope magnifies slightly less upon a celestial object than
upon a near terrestrial one owing to the shorter focus, and a trifling
allowance will have to be made for this. Another plan may be mentioned.
When the telescope is directed to any fairly bright object or to
the sky, and the observer removes his eye about 10 inches from the
eyepiece, a sharply defined, bright little disk will be perceived in
the eye-lens. If the diameter of this disk is ascertained and the clear
aperture of the object-glass or mirror is divided by it, the quotient
will be the magnifying power. Thus, if the small circle of light is ·2
inch diameter and the effective aperture of the large glass 5 inches,
then the power is 25. If the former is ·02 inch diameter and the
latter 7·5 inches, the power will be 375. The dynamometer is a little
instrument specially designed to facilitate this means of fixing the
magnifying power. It enables the diameter of the small luminous circle
in the eye-lens to be very accurately measured, and this is a most
important factor in deriving the power by this method.
[Illustration: Fig. 16.
BERTHON’S DYNAMOMETER. _HORNE & THORNTHWAITE LONDON._]
_Field of Eyepiece._—Observers often require to know the diameter of
the fields of their eyepieces. Those engaged in sweeping up comets,
nebulæ, or other objects requiring large fields and low powers, find
it quite important to have this information. They may acquire it for
themselves by simple methods. A planet, or star such as δ Orionis, η
or γ Virginis, or η Aquilæ, close to the equator, should be allowed to
run exactly through the centre of the field, and the interval occupied
in its complete transit from ingress to egress noted several times. The
mean result in min. and sec. of time must then be multiplied by 15,
and this will represent the diameter required in min. and sec. of arc
on the equator. A planet or star near the meridian is the best for the
purpose. If the object occupies 1 min. 27 sec. of time in passing from
the E. to the W. limit of the field, then 87 sec. × 15 = 1305″, or 21′
45″. A more accurate method of deriving the angle subtended by the
field is to let a star, say Regulus, pass through the centre, and fix
the time which lapses in its entire passage by a sidereal clock; then
the interval so found × 15 × cosine of the declination of Regulus will
indicate the diameter of the field. Suppose for instance, that the star
named occupies 2 min. 14 sec. = 134 sec. in its passage right across
the whole and central part of the field: then
134 log 2·127105
15 log 1·176091
Dec. of Regulus 12° 30′ log cos 9·989581
————————
1962″ log 3·292777
so that the diameter of the field of the eyepiece must be 32′ 42″,
nearly corresponding with the diameter of the Moon.
_Limited Means no Obstacle._—There are many observers who, having
limited means, are apt to consider themselves practically unable to
effect good work. This is a great illusion. There are several branches
of astronomy in which the diligent use of a small instrument may
be turned to excellent account. Perseverance will often compensate
for lack of powerful appliances. Many of the large and expensive
telescopes, now becoming so common, are engaged in work which could
be as well performed with smaller aperture, and when the manifold
advantages of moderate instruments are considered, amateurs may well
cease to deplore the apparent insufficiency of their apparatus. It is,
however, true that refractors have now attained dimensions and a degree
of proficiency never contemplated in former times, and that the modern
ingenuity of art has given birth to innumerable devices to facilitate
the work of those engaged in observation. In many of our best
appointed observatories the arrangements are so very replete with
conveniences, and so sedatory in their influences, that the observer
has every inducement to fall asleep, though we do not find instances
of “nodding” recorded in their annals. Further progress in the same
direction leads us to joyfully anticipate the time when, instead of
standing out in the frost, we may comfortably make our observations
in bed. This will admirably suit all those who, like Bristol people,
are reported to sleep with one eye open! But, to be more serious,
the work of amateurs is much hindered by lack of means to construct
observatories wherein they may conduct researches without suffering
from all the rigours of an unfavourable climate. Many of them have,
like William Herschel a century ago, to pursue their labours under
no canopy but the heavens above, and are exposed to all the trying
severity of frost and keen winds, which keep them shivering for hours
together, and very much awake!
[Illustration: Fig. 17.
Cooke and Son’s Educational Telescope.]
_Observing-Seats._—As to observing-seats, many useful contrivances have
been described from time to time in the ‘Astronomical Register’ and
‘English Mechanic.’ Some of these answer their design admirably, but I
believe a good chair, embodying all the many little requirements of the
observer, yet awaits construction. Those I have seen, while supplying
certain acknowledged wants, are yet deficient in some points which need
provision. With my reflector I find an ordinary step-ladder answers
the purpose very well. It is at once light, simple, and durable, and
enables observations to be secured at any altitude. It may be readily
placed so that the observer can work in a sitting posture, and the
upper shelves, while convenient to lean upon, may be so arranged as to
hold eyepieces, and are to be further utilized when making drawings
at the telescope. I find it possible to obtain very steady views of
celestial objects in this way. Everyone knows that during a critical
observation it is as essential for the observer to be perfectly still
as it is for the instrument to be free from vibration. A person
who stands looking through a telescope feels a desire to ensure a
convenient stability by catching hold of it. The impression is no doubt
correctly conveyed to his mind that he may obtain a better view in this
way; and so he would, were it not for the dancing of the image which
instantly follows the handling of the instrument. For this reason it is
absolutely necessary that no part of the observer touch the telescope
while in use. He must ensure the desired steadiness, which is really
a most important consideration, by other means; and an observer who
provides for this contingency will have taken a useful step in the way
of achieving delicate work.
_Advantage of Equatoreals._—Those who employ equatoreal mounting and
clock-work will manifestly command an advantage in tracing features
on a planet or other object requiring critical scrutiny. Common
stands, though often good make-shifts, require constant application
on the part of the observer, when his undivided attention should be
concentrated on the object. With an alt-azimuth stand nearly one half
the observer’s time is occupied in keeping the object near the middle
of the field. Though good views are obtainable, they are very fugitive.
Just as the delicate features are being impressed on the retina they
are lost in the ill-defined margin of the field, or from the necessity
of suddenly shifting the object back. A succession of hurried views of
this kind, during which the observer is frantically endeavouring to
grasp details which only require a steady view to be well displayed,
are often tantalizing and seldom satisfactory in their issues. This is
especially the case when a single lens and high powers are used, and
if the night is windy the difficulty is intensified. It is, therefore,
evident that a clock-driven telescope possesses marked advantages in
delicate work on faint objects, because the prolonged view better
enables the eye to gather in the details which are all but lost in
the elusive glimpses afforded by inferior means. Still we must not
forget that rough appliances do not present an effectual barrier to
success. The very finest definition comes only in momentary glimpses.
The sharply-cut outlines of planetary configuration cannot steadily
be held for long together. Only now and then the image acquires the
distinctness of an engraving, when the air and the focus of eye and
telescope severally combine to produce a perfect picture. Observers,
therefore, whose instruments are simply, though perhaps substantially
mounted in handy fashion, must profit by these moments of fine seeing,
and, when drawing, will find it expedient to fill in, little by little,
the delicate forms which reach the eye. This will take much time owing
to the drawbacks alluded to, but the outcome will more than justify its
expenditure, and the observer will gain patience and perseverance which
will prove a useful experience in the future.
Lenses out of centre or misplaced are, like other defects, calculated
to give rise to errors as numerous as they are various. But the most
striking of these apparently belong to a period when telescopes were
far less perfect and popular than at the present day. Indeed, it
is surprising that so very few false or imaginary discoveries are
announced when we consider the vast array of instruments that are
now employed. It is true we occasionally hear that a comet has been
discovered close to Jupiter, that several companions have been seen to
Polaris, or that some other extraordinary “find” has been effected, but
the age is dead when such announcements were accepted without suitable
investigation. The satellite of Venus has long since ceased to exist.
The active volcanoes on the Moon have become extinct. Even Vulcan will
have to be set aside, and, like many another sensation which caused
quite a _furóre_ in its day, must soon be altogether expunged from the
category of “suspects.”
_Test-objects._ Opticians sometimes advertise lists of
objects—generally double stars—which may be seen with their
instruments, but it does not appear to be sufficiently understood that
the character of a telescope is dependent in a great degree upon the
ability of the observer, who can either make or mar it, according to
the skill he displays in its management. Some men will undoubtedly
see more with 5 inches of aperture than others will with 10. Certain
observers appear to excel in detecting delicate planetary markings,
while others possess special aptitude for glimpsing minute objects such
as faint satellites, or _comites_ to double stars, and the explanation
seems to be that partly by experience and partly by differences in the
sensitiveness of vision, exceptional powers are sometimes acquired in
each of these departments. The various test-objects which have been
given by reliable authorities, though representing average attainments,
are not applicable to the abnormal powers of vision possessed by
certain observers. In fact, the capacity of a telescope cannot be
correctly assigned and its powers circumscribed by arbitrary rules,
because, as already stated, the character of the observer himself
becomes a most important factor in this relation. Climatic influences
have also considerable weight, though less so than the personal
variations referred to, for one man will succeed, where another meets
with utter failure. This is unquestionably due to differences in
eyesight, method, and experience. But whatever the primary causes may
be, everyone knows they induce widely discordant results, and occasion
many of the contradictions which become the subjects of controversy.
And, as a rule, amateurs should avoid controversy, because it very
rarely clears up a contested point. There is argument and reiteration,
but no mutual understanding or settlement of the question at issue.
It wastes time, and often destroys that good feeling which should
subsist amongst astronomers of every class and nationality. In cases
where an important principle is involved, and discussion promises to
throw light upon it, the circumstances are quite different. But paltry
quibblings, fault-finding, or the constant expression of negative
views, peculiar to sceptics, should be abandoned, as hindering rather
than accelerating the progress of science. Let observers continually
exercise care and discretion and satisfy themselves in every legitimate
way as to the accuracy of their results, and they may fearlessly give
them expression and overcome any objections made to their acceptance.
They should accord one another an equal desire for the promotion of
truth. Competition and rivalry in good spirit increase enthusiasm,
but there is little occasion for the bitterness and spleen sometimes
exhibited in scientific journals. There are some men whose reputations
do not rest upon good or original work performed by themselves, but
rather upon the alacrity with which they discover grievances and upon
the care they will bestow in exposing trifling errors in the writings
of their not-infallible contemporaries. Such critics would earn a more
honourable title to regard were they to devote their time to some
better method of serving the cause of science.
_Cheapness and increasing number of Telescopes._—A marked feature
of optical instruments is their increasing cheapness. Little more
than half a century ago Tulley charged £315 for a 10-inch Newtonian
reflector. At the present time Calver asks £50 for an instrument of the
same aperture, and sometimes one may be picked up, second-hand, for
half of that amount. Not only have telescopes become cheaper, but they
have greatly improved in performance since silvered glass superseded
the metallic speculum. Hence we find moderately-powerful instruments
in the hands of a very large number of observers. Astronomical
publications have proportionately increased, so that amateurs of to-day
can boast of facilities, both of making and recording observations,
which were scarcely dreamt of a century ago. It must be admitted,
however, that the results hardly do justice to the means available.
Such an enormous number of telescopes are variously employed that one
cannot avoid a feeling of surprise at the comparative rarity of new
discoveries, and, indeed, of published observations generally. It is
certain that the majority of existing telescopes are either lying idle
or applied in such a desultory fashion as to virtually negative the
value of the results. Others, again, are indiscriminately employed
upon every diversity of object without special aim or method, and with
a mere desire to satisfy curiosity. Now it is to be greatly deplored
that so much observing strength is either latent or misdirected.
The circumstances obviously demand that an earnest effort should
be made to utilize and attract it into suitable channels. To do
this effectually, the value of collective effort should be forcibly
explained, the interest and enthusiasm of observers must be aroused in
a permanent manner, and they must be banded together according to their
choice of subjects. An effort in this direction has been made by the
Liverpool Astronomical Society, and the results have proved distinctly
favourable; a considerable amount of useful work has been effected in
several branches and it forms the subject of some valuable reports
which have been annually published in the ‘Journal.’
_Utility of Stops._—There are a good many details connected with
observation which, though advice may be tendered in a general way,
are best left to the discrimination of observers, who will very
soon discover their influences by practical trial and treat them
accordingly. The employment of stops or diaphragms to contract the
aperture of telescopes is a question on which a diversity of opinion
has been expressed. It is often found, on nights of indifferent seeing,
that the whole aperture, especially of a faulty instrument, gives
bad images, and that, by reducing it, definition becomes immensely
improved. But Mr. Burnham, the double star observer, records his
opinion that a good glass needs no contraction, and that the whole
aperture shows more than a part unless there is defective figuring
at the outer zone of the lens, which will be cut off by the stop and
its performance thereby greatly improved. He seems to think that
a glass requiring contraction is essentially defective, but this
is totally opposed to the conclusions of other observers. It is
almost universally admitted that, on bad nights, the advantages of a
large aperture are neutralized by unsteady definition, and that, by
reducing the diameter, the character of the images is enhanced. As
regards instruments of moderate calibre the necessity is less urgent.
With my 10-inch reflector I rarely, if ever, employ stops, for by
reducing the aperture to 8 inches the gain in definition does not
sufficiently repay for the serious loss of light. But in the case of
large telescopes the conservation of light is not so important, and
a 14-inch or 16-inch stop may be frequently employed on an 18-inch
glass with striking advantage. The theory that only defective lenses
improve with contraction is fallacious, for in certain cases where
stops are regularly employed it is found that, under circumstances
of really good seeing, the whole aperture gives images which are as
nearly perfect as possible. It is clear from this that the fault
lies with the atmosphere, and that under bad conditions it becomes
imperative to limit its interference consistently with the retention of
sufficient light to distinguish the object well. In large reflectors,
particularly, the undulations of the air are very active in destroying
definition, and the fact will be patent enough to anyone who compares
the images given in widely different apertures. The hard, cleanly cut
disks shown by a small speculum or object-glass offer an attractive
contrast to the flaring, indefinite forms often seen in big telescopes.
_Cleaning Lenses._—As to wiping objectives or mirrors, this should be
performed not more often than absolute necessity requires; and in any
case the touches should be delicate and made with materials of very
soft texture. The owner of a good objective should never take the
handkerchief out of his pocket and, in order to remove a little dust
or dew, rub the glass until the offensive deposit is thought to be
removed. Yet this is sometimes done, though frequent repetition of such
a process must ultimately ruin the best telescope notwithstanding the
hardness of the crown glass forming the outer lens of the objective.
It will not bear such “rough and ready” usage and in time must show
some ugly scratches which will greatly affect its value though they may
not seriously detract from its practical utility. Good tools deserve
better treatment. When the glass really wants cleaning, remove it from
the tube and sweep its whole surface gently with a dry camel’s-hair
brush, or when this is not at hand get a piece of linen and “flick”
off the dust particles. Then wipe the lens, as soon as these have been
dislodged, with an old silk, or soft cambric handkerchief; fine chamois
leather is also a good material, and soft tissue paper, aided by the
breath, has been recommended. But whatever substance may be adopted it
must be perfectly clean and free from dust. When not in use it should
be corked up in a wide-necked bottle where it will be safe from contact
with foreign particles. In the case of mirrors there is an obvious need
that, when being repolished, the material used should be perfectly dry
and that the mirror also should be in the same state. It is unnecessary
to say here that in no case must the silver film be touched when it is
clouded over with moisture. This must first be allowed to evaporate in
a free current of air or before a fire; the former is to be preferred.
A suitable polishing-pad may be made with a square piece of washleather
or chamois in which cotton-wool is placed and then tied into a bag.
This may be dipped into a little of the finest rouge, and its
employment will often restore a bright surface to the mirror. But the
latter should be left “severely alone” unless there is urgent occasion
to repolish it, as every application of the rouged pad wears the film
and may take off minute parts of it, especially when dust has not been
altogether excluded. The precarious nature of the silvered surface
undoubtedly constitutes the greatest disadvantage of modern reflectors.
The polish on the old metallic mirrors was far more durable. Some of
Short’s, figured 150 years ago, still exist and are apparently as
bright as when they were turned out of the workshop! I have a 4-inch
Gregorian by Watson which must be quite a century old, and both large
and small specula seem to have retained their pristine condition.
With regard to the duration of the silver-on-glass films, much of
course depends upon the care and means taken to preserve them. Calver
says that sometimes the deposit does not last so long as expected,
though he has known the same films in use for ten years. A mirror
that looks badly tarnished and fit for nothing will often perform
wonderfully well. With my 10-inch in a sadly deteriorated state
I have obtained views of the Moon, Venus, and Jupiter that could
hardly be surpassed. The moderate reflection from a tarnished mirror
evidently improves the image of a bright object by eliminating the
glare and allowing the fainter details to be readily seen. When not
in use a tight-fitting cap should always be placed over the mirror,
and if a pad of cotton wadding of the same diameter is made to
inlay this cap it tends to preserve the film by absorbing much of
the moisture that otherwise condenses on its surface. The ‘Hints on
Reflecting-Telescopes,’ by W. H. Thornthwaite and by G. Calver, and the
‘Plea for Reflectors,’ by J. Browning, may be instructively consulted
by all those who use this form of instrument. The latter work is now,
however, out of print, and Mr. Browning tells me that he has quite
relinquished the manufacture of reflecting-telescopes. Mr. G. With
of Hereford, who formerly supplied the mirrors for his instruments,
has recently disposed of his reserve stock and entered an entirely
different sphere of labour. In the publications above alluded to
amateurs will find a large amount of practical information on the value
and treatment of glass mirrors.
_Opera-Glass._—A very useful adjunct, and often a really valuable one
to the astronomical amateur, is the Opera-Glass, or rather the larger
form of this instrument generally known as the Field-Glass. Of certain
objects it gives views which cannot be surpassed, and it is especially
useful in observations of variable stars and large comets. Whenever the
horizon is being scanned for a glimpse of the fugitive Mercury, or when
it is desired to have a very early peep at the narrow crescent of the
young Moon, or to pick up Venus at midday, or Jupiter before sunset,
all one has to do is to sweep over the region where the object is
situated, when it is pretty sure to be caught, and the unaided eye will
probably reach it soon afterwards. The opera-glass has the dignity of
being the first telescope invented, for even its binocular form is not
new; it is virtually the same pattern of instrument that was introduced
at Middleburg in 1609, though its compound object-glasses are of more
modern date. Anyone who entertains any doubts as to the efficacy of the
opera-glass or has had little experience in its use will do well to
look at the Pleiades and compare the splendid aspect of that cluster,
as it is there presented, with the view obtained by the naked eye, and
he will acknowledge at once that it constitutes a tool without which
the observer’s equipment is by no means perfect. The object-glasses
should have diameters of 2 or 2½ inches, and the magnifying power lie
between 4 and 6. There is a large field of view and the images are very
bright. The observer is enabled to enjoy the luxury of using both his
eyes, and when he directs the instrument upon a terrestrial landscape
he will be gratified that it does not turn the world upside down! It is
not surprising that an appliance, with recommendations so significant,
is coming more into favour every day, and for those branches suitable
to its means it is doing much useful work. A volume has been recently
published dealing expressly with the use of the opera-glass in
Astronomy; and in the ‘Journal of the L.A.S.’ vol. vii. p. 120, there
is an excellent paper by Major Markwick on the same subject. This
instrument will never, of course, by the nature of its construction,
be comparable to a modern telescope in regard to power, for Galilei,
when he augmented his magnifiers to 30, appears to have practically
exhausted the resources of this appliance. But in all those departments
requiring an expansive field and little power with a brilliant and
distinct image, the larger form of opera-glass is a great desideratum,
and its portability is not one of the least of its advantages.
_Dewing of Mirrors._—The disposition of mirrors to become clouded over
upon rises of temperature is a point meriting comment. When permanently
left in a telescope, fully exposed out of doors, the speculum undergoes
daily transitions. The heat generated in the interior of the tube by
the sun’s action causes a thick film of moisture to form upon the
silvered surface of the mirror, which remains in this state for a
considerable time, though the moisture evaporates before the evening.
The flat is similarly affected, and the result of these frequent
changes is that the coating of silver becomes impaired and presents
a crackly appearance all over the surface. Sometimes when a marked
increase of temperature occurs towards evening the speculum is rendered
totally unserviceable until it has been submitted to what Dr. Kitchiner
terms a process of “roasting.” The vapour will soon disappear when
the mirror is brought indoors and placed before a fire; but it is not
till some time after it has been remounted in the tube that it will
perform satisfactorily. Those who keep their mirrors in more equable
temperatures will not experience these inconveniences, which may also
in some measure be obviated by regularly placing a tight-fitting cap,
inlaid with cotton-wool, over the speculum at the conclusion of work.
This also protects the silver from the yellow sulphurous deposit which
soon collects upon it if used in a town. All sudden variations of
temperature act prejudicially on the performance of specula, and their
best work is only accomplished when free from such disturbing elements.
I have rarely found the flat to become dewed in a natural way during
the progress of observation. If on a cold night the observer puts his
hand upon its supports in order to alter its adjustment it instantly
becomes dewed, or if he stands looking down the tube it is almost sure
to be similarly affected; but in the ordinary course of work the flat
is little liable to become dewed in sensible degree. With refractors
dew-caps are very necessary, though they do not always prevent the
deposition of moisture on the object-glass, and this occasions frequent
wiping or drying, which in either case is very objectionable.
_Celestial Globe._—This forms another extremely useful addendum to
the appliances of the amateur. It enables a great many problems to be
solved in a very simple manner, and helps the young student to a lucid
comprehension of the apparent motions and positions of the fixed stars.
With ‘Keith on the Globes’ as a reference-book he may soon acquire
the method of determining the times of rising, southing, and setting
of any celestial object the place of which is known. He can also
readily find the height (altitude) and bearing (azimuth) at any time.
The distance in degrees between any two stars or between a star and
the Moon, a planet, or a comet may be found at a glance by laying the
quadrant of altitude on the pair of objects and reading off the number
of degrees separating them. If a new comet has been discovered, its
position should be marked in pencil upon the globe; and the observer,
after having noted its exact place relatively to neighbouring stars,
may proceed to identify the object with his telescope. If a large
meteor is seen, its apparent path amongst the constellations should be
projected on the globe and the points, in R.A. and Dec., of beginning
and ending of the flight read off and entered in a book. In many other
practical branches of astronomy this instrument will prove highly
serviceable, and is far preferable to a star-atlas. But the latter
is the most useful to the beginner who is just learning the names of
the stars and the configuration of the chief groups, because on the
globe the positions are all reversed east and west. The surface of the
globe represents the entire star-sphere reduced to a common distance
from the earth, and as seen from outside that sphere. The observer,
therefore, must imagine his eye to be situated in the centre of the
globe, if he would see the stars in the same relative places as he
sees them in the heavens. The reversion of the star-positions to which
we have been alluding is very confusing at first, and no doubt it
provokes mistakes, but a little experience will practically remove this
objection. The one great recommendation to a star-atlas is that it
displays the stars in the natural positions in which they are discerned
by the eye, thus enabling the student to become readily acquainted with
them, whereas the celestial globe affords no such facility. But in
other respects the latter possesses some valuable functions, and the
amateur who devotes some of his leisure to mastering the really useful
problems will attain a knowledge that will be of great benefit to him
in after years. A globe of 12-inches diameter will be large enough
for many purposes, but one of 18-inches will be the most effective
size. It should be mounted on a tall stand with single body and tripod
base. The stands, fitted with three parallel legs, in which the globe
is supported in the middle by weak connections from them, are not
nearly so durable. I have used several 18-inch globes mounted in this
manner, and the supports have quite given way under the pressure of
constant use; but this is impossible with the strong single body, which
is capable of withstanding any strain. Globes are frequently to be
obtained second-hand, and at trifling cost; but the observer must allow
for precession if he uses an old article. Many of the stars will be
1° or 2° east of the positions in which they are marked on the globe;
and it will be necessary to remember this if the appliance is to be
employed for exact results.
_Observatories._—Massive and lofty buildings have long gone out of
fashion, and lighter, drier structures have properly supplanted them.
Instruments of size are generally placed on or near the ground and
solidly supported to ensure stability, while the other erections are
made consistent with the necessity for pretty equable temperature and
freedom from damp. Amateurs will ordinarily find that a simple wooden
enclosure for the telescope, with suitable arrangements for opening
the top in any direction, is sufficient for their purpose and very
inexpensive. Some observers have, indeed, secured the desired shelter
for themselves and their telescopes by means of a canvas tent provided
with ready means for obtaining sky-room. Berthon has given a good
description of an amateur’s observing-hut in ‘The English Mechanic’ for
October 13th and 20th, 1871; and Chambers supplies some information
about amateur observatories in ‘Nature’ for November 19th, 1885[8].
Mr. Thornthwaite’s. ‘Hints on Telescopes’ may be usefully consulted
for details of the Romsey Observatory, which, like the Berthon model,
seems peculiarly adapted to the necessities of the amateur. The great
requirements in such structures are that they should be dry and not
obstruct any region of the firmament. They should also be large enough
to allow the observer perfect freedom in his movements and during the
progress of his observations. They are then decided advantages, and
will materially add to that comfort and convenience without which it
is rarely possible to accomplish really good work. When an observatory
is to be dispensed with it becomes necessary to erect a small wooden
house near the instrument, especially if placed at the far end of a
garden, in which the observer may keep certain appliances, such as a
lantern, celestial globe, step-ladder or observing-seat, oil, &c. Here
also he may record his seeings, complete his sketches, and consult
his working-list, star-charts, and ephemerides. A shelter of this
sort, apart from its practical helpfulness, avoids any necessity for
the observer to go in and out of doors, up and down stairs, &c., to
the annoyance of the rest of his family, who, on a frosty night, are
decidedly not of an astronomic turn, and vastly prefer house-warming to
stargazing!
FOOTNOTES:
[6] My 10-inch reflector by With-Browning was persistently used for
four years without being resilvered or once getting out of adjustment.
[7] In this and future references to reflectors the Newtonian form is
alluded to. The direct-vision reflectors of Gregory and Cassegrain
have gone out of use, and the present popularity of Newtonians may be
regarded as a case of the “survival of the fittest.”
[8] Chambers’s ‘Descriptive Astronomy,’ 4th ed. vol. ii., also contains
some useful references and diagrams.