William F. Denning.

Telescopic Work for Starlight Evenings online

. (page 5 of 32)
Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 5 of 32)
Font size
QR-code for this ebook


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,



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 5 of 32)