William F. Denning.

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nature confront the observer. The comparatively tranquil and sharply
definite images seen in small instruments disappear, and in their
places forms are presented much more brilliant and expansive, it is
true, but involved in glare and subject to constant agitation, which
serve to obliterate most of the details. The observer becomes conscious
that what he has gained in light has been lost in definition. At
times—perhaps on one occasion in fifty—this experience is different;
the atmosphere has apparently assumed a state of quiescence, and
objects are seen in a great telescope with the same clearness of detail
as in smaller ones. It is then the observer fully realizes that his
instrument, though generally ineffective, is not itself in fault, and
that it would do valuable work were the normal condition of the air
suitable to the exercise of its capacity.

Those who have effected discoveries with large instruments have done
so in spite of the impediment alluded to. Relying mainly upon great
illuminating power, bad or indifferent definition has been tolerated;
and they have succeeded in detecting minute satellites, faint nebulæ,
clusters, and small companions to double stars. Telescopes of great
aperture are at home in this kind of work. But when we come to consider
discoveries on the surfaces of the Sun, Moon, and planets, the case
is entirely different; the diligent use of small appliances appears
to have left little for the larger constructions to do. There are
some thousands of drawings of the objects named, made by observers
employing telescopes from 3 up to 72 inches in diameter; and a careful
inspection shows that the smaller instruments have not been outdone in
this interesting field of observation. In point of fact they rather
appear to have had the advantage, and the reason of this is perhaps
sufficiently palpable. The details on a bright planetary object are apt
to become obliterated in the glare of a large instrument. Even with a
small telescope objects like Venus and Jupiter are best seen at about
the time of sunset, and before their excessive brilliancy on the dark
sky is enabled to act prejudicially in effacing the delicate markings.
Probably this is one of the causes which, in combination with the
undulations of the atmosphere, have restricted the discoveries of large
instruments chiefly to faint satellites, stars, and nebulæ.

Prof. Young ascribes many of the successes of small instruments to
exceptional cuteness of vision on the part of certain observers, and to
the fact that such instruments are so very numerous and so diligently
used that it is fair to conclude they must reap the main harvest of
discoveries. We must remember that for every observer working with an
aperture of 18 inches and more, there are more than a hundred employing
objectives or specula of from 5 to 12 inches; hence we may expect some
notable instances of keen sight amongst the latter. The success of men
like Dawes and others, who outstrip their contemporaries, and with
small glasses achieve phenomenal results, is to be ascribed partly
to good vision and partly to that natural aptitude and pertinacity
uniformly characteristic of the best observers. These circumstances go
far to explain the unproductiveness of large telescopes: in the race
for distinction they are often distanced by their more numerous and
agile competitors.

The objections which applied to the large reflecting instruments of
Herschel, Lassell, and Rosse scarcely operate with the same force
in regard to the great refractors of the present day, and for these
reasons:—Refractors are somewhat less sensitive to atmospheric
disturbances than reflectors. The modern instruments are mounted in
much improved style, and placed in localities selected for their
reception. In fact, all that the optician’s art can do to perfect such
appliances has been done, and Nature herself has been consulted as to
essentials; for we find the most powerful refractor of all erected on
the summit of Mount Hamilton, where the skies are clear and Urania ever
smiles invitingly.

Some observers who have obtained experience both with large and
small telescopes aver that, even on a bright planet, they can see
more, and often see it much better, with the larger glasses. But we
rarely, if ever, find them saying they can discern anything which is
absolutely beyond the reach of small instruments. It would be much
more satisfactory evidence of the super-excellence of the former if
definite features could be detected which are quite beyond the reach
of telescopes of inferior size; but we seldom meet with experiences of
this kind, and the inference is obvious.

There is undoubtedly a certain aperture which combines in itself
sufficient light-grasping power with excellent definition. It takes a
position midway between great illuminating power and bad definition on
the one hand, and feeble illuminating power and sharp definition on the
other. Such an aperture must form the best working instrument in an
average situation upon ordinary nights and ordinary objects. M. Wolf
fixes this aperture at about 15 inches, and he is probably near the
truth.

The quaint Dr. Kitchener, who, early in the present century, made
a number of trials with fifty-one telescopes, entertained a very
poor opinion of big instruments. In his book on ‘Telescopes,’ he
says:—“Immense telescopes are only about as useful as the enormous
spectacles suspended over the doors of opticians.” ... “Astronomical
amateurs should rather seek for _perfect_ instruments than _large_
ones. What good can a great deal of bad light do?”

We shall be in a better position a few years hence to estimate the
value of great telescopes; for the principal instruments of this class
have only been completed a short time. Judging from the statements
of some of the observers, who are men of the utmost probity and
ability, certain of the large instruments are capable of work far in
advance of anything hitherto done. Definition, they say, is excellent,
notwithstanding the great increase of aperture. The old stumbling-block
appears, therefore, to have been removed, and astronomy is to be
congratulated on the acquirement of such vastly improved implements
of research. Even should the large telescopes continue to prove
disappointing in certain branches, they may certainly be expected to
maintain their advantage in others. They will always be valuable as
a corrective to smaller and handier instruments. For special lines
of work in which very small or very faint objects are concerned,
considerable light-grasping power is absolutely required; and it is
chiefly in these departments that large instruments may be further
expected to augment our knowledge. In photographic and spectroscopic
work they also have a special value, which late researches have brought
prominently to the fore.

The telescopes of the future will probably surpass in dimensions
those of our own day. The University of Los Angelos, in California,
propose to erect a 42-inch refractor on the summit of Wilson’s Peak
of the Sierra Madre mountains, which is 6000 feet high and about 25
miles from Los Angelos. In reference to this contemplated extension
of size, it may be opportune to mention that large objectives do not
transmit light proportionately with their increased diameter, owing to
greater thickness of the lenses, which increases the absorption. The
Washington objective of 25·8-inch aperture is 2·87 inches in thickness,
and more than half the light which falls upon it is lost by absorption.
On the other hand, specula, with every enlargement of aperture, give
proportionately more light-grasping power, and their diameters might
be greatly increased but for the mechanical obstacles in the way of
their construction. Mr. Ranyard expresses the opinion that “with the
refractor we are fast approaching the practical limit of size.” After
referring to the Washington object-glass as above, he says:—“If we
double the thickness, more than three quarters of the light would be
absorbed and less than one quarter would be transmitted. The greatest
loss of light is only for the centre of the object-glass; but in all
parts the absorption is quadrupled for a lens of double aperture.”
If, therefore, future years see any great development in the sizes of
telescopes, it will probably be in connection with reflectors; for the
loss of light by absorption in the thick lenses of large refractors
must ultimately determine their limits. Mr. Calver says:—“The light of
reflectors exceeding 18 inches in diameter is certainly greater than
that of refractors of equal size, and for anything like 3 feet very
much greater.” He nearly obtained the order for a monster reflector for
the Lick Observatory, the Americans admitting that the reflector must
be the instrument of the future for power and light because there were
practically no limits to its size. But the reflector has not been much
used in America, and therefore is little known. For this reason the
authorities decided to erect a large refractor, and they appear to have
been justified in their selection, for the 36-inch objective has proved
excellent.


FOOTNOTES:

[5] Such objects show considerable glare in a very large instrument.
The advent of Jupiter into the field of the 6-foot has been compared to
the brightness of a coach-lamp. The outer satellite of Mars was seen
twice with this instrument in 1877, “but the glare of the planet was
found too strong to allow of good measures being taken.”




CHAPTER III.

_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



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 4 of 32)