William F. Denning.

Telescopic Work for Starlight Evenings online

. (page 6 of 32)
Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 6 of 32)
Font size
QR-code for this ebook


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.




CHAPTER IV.

_NOTES ON TELESCOPIC WORK._

Preparation.—Working-Lists.—Wind.—Vision.—Records.—Drawing.—Friendly
Indulgences.—Open-Air Observing.—Method.—Perseverance.—Definition in
Towns.—Photography.—Publications.—Past and Future.—Attractions of
Telescopic Work.


_Preparation._—An observer in commencing work in any department of
astronomy will find it a very great assistance to his progress if he
carefully reads and digests all that has been previously effected in
the same line. He will see many of the chief difficulties and their
remedies explained. He will further learn the best methods and be in
the position of a man who has already gained considerable experience.
If he enter upon a research of which he has acquired no foreknowledge
he will be merely groping in the dark, and must encounter many
obstacles which, though they may not effectually turn him from his
purpose, will at least involve a considerable expenditure of time and
labour. On the other hand, a person who relies upon guidance from prior
experimentalists will probably make rapid headway. He will be fortified
to meet contingencies and to avoid complications as they arise. He
will be better enabled to discriminate as to the most eligible means
and will confidently endeavour to push them to the furthest extent.
By adopting existing instructions for his direction and familiarizing
himself with the latest information from the best authorities he will
in a great measure ensure his own success or at least bring it within
measurable] distance. The want of this foreknowledge has often been
the main cause of failure, and it has sometimes led to misconceptions
and imaginary discoveries; for after much thought and labour a man
will overcome an impediment or achieve an end in a way for which he
claims credit, only to find that he has been anticipated years before
and that had he consulted past records, his difficulties would have
been avoided and he might have pressed much nearer the goal. Too much
importance cannot be attached to the acquisition of foreknowledge of
the character referred to, though we do not mean that former methods
or results are to be implicitly trusted. Let every observer judge for
himself to a certain extent and let him follow original plans whenever
he regards them as feasible; let him test preceding results whenever
he doubts their accuracy. We recommend past experiences as a guide,
not as an infallible precept. It would be as much a mistake to follow
the old groove with a sort of credulous infatuation as it would be to
enter upon it in utter ignorance of theoretical knowledge. An observer
should take the direction of his labours from previous workers, but be
prepared to diverge from acknowledged rules should he feel justified in
doing so from his new experiences.

[Illustration: Fig. 18.

Refracting-Telescope on a German Equatoreal.]

_Working-Lists._—Full advantage should be taken of good observing
weather. Sir John Herschel most aptly said that no time occupied in the
preparation of working-lists is ill-spent. In our climate the value
of this maxim cannot be overrated. If the 100 hours of exceptionally
good seeing, available in the course of the year, are to be profitably
employed, we must be continually prepared with a scheme of systematic
work. The observer should compile lists of objects it is intended to
examine, and their places must be marked upon the globe or chart so
as to avoid all troublesome references during the actual progress of
observation. If he has to consult ephemerides and otherwise withdraw
attention from the telescope he loses valuable time: moreover the
positions hurriedly assigned in such cases are frequently wrong and
entail duplicate references, involving additional waste of time; all
this may be avoided by careful preparation beforehand. If he has a
series of double or variable stars to observe he must tabulate their
places in convenient order so as to facilitate the work. If he intend
hunting up nebulæ or telescopic comets he must carefully mark their
positions relatively to adjoining stars. In the case of selenographical
objects or planetary markings he may equally prepare himself by
previous study. Adopting these precautions, objects may be readily
identified and the work expedited. When no such preparation is made
much confusion and loss of time result. On a cloudy, wet day observers
often consider it unnecessary to make such provision and they are taken
at a great disadvantage when the sky suddenly clears. A good observer,
like a good general, ought to provide, by the proper disposition of his
means, against any emergency. In stormy weather valuable observations
are often permissible if the observer is prompt, for the definition is
occasionally suitable under such circumstances. The most tantalizing
weather of all is that experienced during an anti-cyclone in winter.
For a week or two the barometer is very steady at a high reading, the
air is calm, and the sky is obscured with an impenetrable mass of
clouds.

_Wind._—The influence of wind on definition has been much discussed
in its various aspects, but it is scarcely feasible to lay down
definite rules on the subject. The east wind is rarely favourable to
good seeing, but the law is far from absolute. We must remember that
several distinct currents sometimes prevail, and the air strata at
various elevations are of different degrees of humidity and therefore
exercise different effects upon telescopic definition. A mere surface
breeze from the east may underlie an extensive and moist current from
the south-west, and telescopic definition may prove very fair under
the combination. Calm nights when there is a little haze and fog,
making the stars look somewhat dim, frequently afford wonderfully
good seeing. As a rule, when the stars are sparkling and brilliant,
the definition is bad; planetary disks are unsteady and the details
obliterated in glare. But this is not always so. I have sometimes
found in windy weather after storms from the west quarter, when the
air has become very transparent, that exceptionally sharp views may be
obtained; but unfortunately they are not without drawbacks, for the
telescope vibrates violently with every gust of wind and the images
cannot be held long enough for anything satisfactory to be seen. The
tenuous patches of white cirrous cloud which float at high altitudes
will often improve definition in a surprising manner, especially on the
Moon and planets. Of course this does not apply to nebulæ or comets,
which are objects of totally different character and essentially
require a _dark_ night rather than good definition before they may be
seen under the best conditions. As a rule, a steady, humid atmosphere
is highly conducive to good seeing, and it is rather improved than
impaired by a little fog or thin, white cloud. Some unique effects



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 6 of 32)