Richard A. Proctor - Half hours with the Telescope
R >>
Richard A. Proctor >> Half hours with the Telescope
Pages:
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 [Illustration: PLATE I. Maps I.-IV.]
HALF-HOURS
WITH
THE TELESCOPE;
BEING A POPULAR GUIDE TO THE USE OF THE TELESCOPE
AS A MEANS OF AMUSEMENT AND INSTRUCTION.
BY
RICHARD A. PROCTOR, B.A., F.R.A.S.,
AUTHOR OF "SATURN AND ITS SYSTEM," ETC.
WITH ILLUSTRATIONS ON STONE AND WOOD.
* * * * *
An undevout astronomer is mad:
True, all things speak a God; but, in the small
Men trace out Him: in great He seizes man.
YOUNG.
* * * * *
NEW YORK:
G.P. PUTNAM'S SONS.
1873.
LONDON:
PRINTED BY WILLIAM CLOWES AND SONS, STAMFORD STREET
AND CHARING CROSS.
PREFACE.
The object which the Author and Publisher of this little work have
proposed to themselves, has been the production, at a moderate price, of
a useful and reliable guide to the amateur telescopist.
Among the celestial phenomena described or figured in this treatise, by
far the larger number may be profitably examined with small telescopes,
and there are none which are beyond the range of a good 3-inch
achromatic.
The work also treats of the construction of telescopes, the nature and
use of star-maps, and other subjects connected with the requirements of
amateur observers.
R.A.P.
_January_, 1868.
CONTENTS.
CHAPTER I. PAGE
A HALF-HOUR ON THE STRUCTURE OF THE TELESCOPE 1
CHAPTER II.
A HALF-HOUR WITH ORION, LEPUS, TAURUS, ETC. 33
CHAPTER III.
A HALF-HOUR WITH LYRA, HERCULES, CORVUS, CRATER, ETC. 47
CHAPTER IV.
A HALF-HOUR WITH BOOTES, SCORPIO, OPHIUCHUS, ETC. 56
CHAPTER V.
A HALF-HOUR WITH ANDROMEDA, CYGNUS, ETC. 66
CHAPTER VI.
HALF-HOURS WITH THE PLANETS 74
CHAPTER VII.
HALF-HOURS WITH THE SUN AND MOON 93
DESCRIPTION OF PLATES.
PLATE I.--_Frontispiece._
This plate presents the aspect of the heavens at the four seasons, dealt
with in Chapters II., III., IV., and V. In each map of this plate the
central point represents the point vertically over the observer's head,
and the circumference represents his horizon. The plan of each map is
such that the direction of a star or constellation, as respects the
compass-points, and its elevation, also, above the horizon, at the given
season, can be at once determined. Two illustrations of the use of the
maps will serve to explain their nature better than any detailed
description. Suppose first, that--at one of the hours named under Map
I.--the observer wishes to find Castor and Pollux:--Turning to Map I. he
sees that these stars lie in the lower left-hand quadrant, and very
nearly towards the point marked S.E.; that is, they are to be looked for
on the sky towards the south-east. Also, it is seen that the two stars
lie about one-fourth of the way from the centre towards the
circumference. Hence, on the sky, the stars will be found about
one-fourth of the way from the zenith towards the horizon: Castor will
be seen immediately above Pollux. Next, suppose that at one of the hours
named the observer wishes to learn what stars are visible towards the
west and north-west:--Turning the map until the portion of the
circumference marked W ... N.W. is lowermost, he sees that in the
direction named the square of Pegasus lies not very high above the
horizon, one diagonal of the square being vertical, the other nearly
horizontal. Above the square is Andromeda, to the right of which lies
Cassiopeia, the stars [beta] and [epsilon] of this constellation lying
directly towards the north-west, while the star [alpha] lies almost
exactly midway between the zenith and the horizon. Above Andromeda, a
little towards the left, lies Perseus, Algol being almost exactly
towards the west and one-third of the way from the zenith towards the
horizon (because one-third of the way from the centre towards the
circumference of the map). Almost exactly in the zenith is the star
[delta] Aurigae.
The four maps are miniatures of Maps I., IV., VII., and X. of my
'Constellation Seasons,' fourth-magnitude stars, however, being omitted.
PLATES II., III., IV., and V., illustrating Chapters II., III., IV., and V.
Plates II. and IV. contain four star-maps. They not only serve to
indicate the configuration of certain important star-groups, but they
illustrate the construction of maps, such as the observer should make
for himself when he wishes to obtain an accurate knowledge of particular
regions of the sky. They are all made to one scale, and on the conical
projection--the simplest and best of all projections for maps of this
sort. The way in which the meridians and parallels for this projection
are laid down is described in my 'Handbook of the Stars.' With a little
practice a few minutes will suffice for sweeping out the equidistant
circular arcs which mark the parallels and ruling in the straight
meridians.
The dotted line across three of the maps represents a portion of the
horizontal circle midway between the zenith and the horizon at the hour
at which the map is supposed to be used. At other hours, of course, this
line would be differently situated.
Plates III. and V. represent fifty-two of the objects mentioned in the
above-named chapters. As reference is made to these figures in the text,
little comment is here required. It is to be remarked, however, that the
circles, and especially the small circles, do not represent the whole
of the telescope's field of view, only a small portion of it. The object
of these figures is to enable the observer to know what to expect when
he turns his telescope towards a difficult double star. Many of the
objects depicted are very easy doubles: these are given as objects of
reference. The observer having seen the correspondence between an easy
double and its picture, as respects the relation between the line
joining the components and the apparent path of the double across the
telescope's field of view, will know how to interpret the picture of a
difficult double in this respect. And as all the small figures are drawn
to one scale, he will also know how far apart he may expect to find the
components of a difficult double. Thus he will have an exact conception
of the sort of duplicity he is to look for, and this is--_crede
experto_--a great step towards the detection of the star's duplicity.
PLATES VI. and VII., illustrating Chapters VI. and VII.
The views of Mercury, Venus, and Mars in these plates (except the
smaller view of Jupiter in Plate VII.) are supposed to be seen with the
same "power."
The observer must not expect to see the details presented in the views
of Mars with anything like the distinctness I have here given to them.
If he place the plate at a distance of six or seven yards he will see
the views more nearly as Mars is likely to appear in a good three-inch
aperture.
The chart of Mars is a reduction of one I have constructed from views by
Mr. Dawes. I believe that nearly all the features included in the chart
are permanent, though not always visible. I take this opportunity of
noting that the eighteen orthographic pictures of Mars presented with my
shilling chart are to be looked on rather as maps than as representing
telescopic views. They illustrate usefully the varying presentation of
Mars towards the earth. The observer can obtain other such illustrations
for himself by filling in outlines, traced from those given at the foot
of Plate VI., with details from the chart. It is to be noted that Mars
varies in presentation, not only as respects the greater or less opening
out of his equator towards the north or south, but as respects the
apparent slope of his polar axis to the right or left. The four
projections as shown, or inverted, or seen from the back of the plate
(held up to the light) give presentations of Mars towards the sun at
twelve periods of the Martial year,--viz., at the autumnal and vernal
equinoxes, at the two solstices, and at intermediate periods
corresponding to our terrestrial months.
In fact, by means of these projections one might readily form a series
of sun-views of Mars resembling my 'Sun-views of the Earth.'
In the first view of Jupiter it is to be remarked that the three
satellites outside the disc are supposed to be moving in directions
appreciably parallel to the belts on the disc--the upper satellites from
right to left, the lower one from left to right. In general the
satellites, when so near to the disc, are not seen in a straight line,
as the three shown in the figure happen to be. Of the three spots on the
disc, the faintest is a satellite, the neighbouring dark spot its
shadow, the other dark spot the shadow of the satellite close to the
planet's disc.
HALF-HOURS WITH THE TELESCOPE.
CHAPTER I.
A HALF-HOUR ON THE STRUCTURE OF THE TELESCOPE.
There are few instruments which yield more pleasure and instruction than
the Telescope. Even a small telescope--only an inch and a half or two
inches, perhaps, in aperture--will serve to supply profitable amusement
to those who know how to apply its powers. I have often seen with
pleasure the surprise with which the performance even of an opera-glass,
well steadied, and directed towards certain parts of the heavens, has
been witnessed by those who have supposed that nothing but an expensive
and colossal telescope could afford any views of interest. But a
well-constructed achromatic of two or three inches in aperture will not
merely supply amusement and instruction,--it may be made to do useful
work.
The student of astronomy is often deterred from telescopic observation
by the thought that in a field wherein so many have laboured, with
abilities and means perhaps far surpassing those he may possess, he is
little likely to reap results of any utility. He argues that, since the
planets, stars, and nebulae have been scanned by Herschel and Rosse, with
their gigantic mirrors, and at Pulkova and Greenwich with refractors
whose construction has taxed to the utmost the ingenuity of the
optician and mechanic, it must be utterly useless for an unpractised
observer to direct a telescope of moderate power to the examination of
these objects.
Now, passing over the consideration that a small telescope may afford
its possessor much pleasure of an intellectual and elevated character,
even if he is never able by its means to effect original discoveries,
two arguments may be urged in favour of independent telescopic
observation. In the first place, the student who wishes to appreciate
the facts and theories of astronomy should familiarize himself with the
nature of that instrument to which astronomers have been most largely
indebted. In the second place, some of the most important discoveries in
astronomy have been effected by means of telescopes of moderate power
used skilfully and systematically. One instance may suffice to show what
can be done in this way. The well-known telescopist Goldschmidt (who
commenced astronomical observation at the age of forty-eight, in 1850)
added fourteen asteroids to the solar system, not to speak of important
discoveries of nebulae and variable stars, by means of a telescope only
five feet in focal length, mounted on a movable tripod stand.
The feeling experienced by those who look through a telescope for the
first time,--especially if it is directed upon a planet or nebula--is
commonly one of disappointment. They have been told that such and such
powers will exhibit Jupiter's belts, Saturn's rings, and the
continent-outlines on Mars; yet, though perhaps a higher power is
applied, they fail to detect these appearances, and can hardly believe
that they are perfectly distinct to the practised eye.
The expectations of the beginner are especially liable to
disappointment in one particular. He forms an estimate of the view he is
to obtain of a planet by multiplying the apparent diameter of the planet
by the magnifying power of his telescope, and comparing the result with
the apparent diameter of the sun or moon. Let us suppose, for instance,
that on the day of observation Jupiter's apparent diameter is 45", and
that the telescopic power applied is 40, then in the telescope Jupiter
should appear to have a diameter of 1800", or half a degree, which is
about the same as the moon's apparent diameter. But when the observer
looks through the telescope he obtains a view--interesting, indeed, and
instructive--but very different from what the above calculation would
lead him to expect. He sees a disc apparently much smaller than the
moon's, and not nearly so well-defined in outline; in a line with the
disc's centre there appear three or four minute dots of light, the
satellites of the planet; and, perhaps, if the weather is favourable and
the observer watchful, he will be able to detect faint traces of belts
across the planet's disc.
Yet in such a case the telescope is not in fault. The planet really
appears of the estimated size. In fact, it is often possible to prove
this in a very simple manner. If the observer wait until the planet and
the moon are pretty near together, he will find that it is possible to
view the planet with one eye through the telescope and the moon with the
unaided eye, in such a manner that the two discs may coincide, and thus
their relative apparent dimensions be at once recognised. Nor should the
indistinctness and incompleteness of the view be attributed to
imperfection of the telescope; they are partly due to the nature of the
observation and the low power employed, and partly to the inexperience
of the beginner.
It is to such a beginner that the following pages are specially
addressed, with the hope of affording him aid and encouragement in the
use of one of the most enchanting of scientific instruments,--an
instrument that has created for astronomers a new sense, so to speak, by
which, in the words of the ancient poet:
Subjecere oculis distantia sidera nostris,
AEtheraque ingenio supposuere suo.
In the first place, it is necessary that the beginner should rightly
know what is the nature of the instrument he is to use. And this is the
more necessary because, while it is perfectly easy to obtain such
knowledge without any profound acquaintance with the science of optics,
yet in many popular works on this subject the really important points
are omitted, and even in scientific works such points are too often left
to be gathered from a formula. When the observer has learnt what it is
that his instrument is actually to do for him, he will know how to
estimate its performance, and how to vary the application of its
powers--whether illuminating or magnifying--according to the nature of
the object to be observed.
Let us consider what it is that limits the range of _natural_ vision
applied to distant objects. What causes an object to become invisible as
its distance increases? Two things are necessary that an object should
be visible. It must be _large_ enough to be appreciated by the eye, and
it must _send light_ enough. Thus increase of distance may render an
object invisible, either through diminution of its apparent size, or
through diminution in the quantity of light it sends to the eye, or
through both these causes combined. A telescope, therefore, or (as its
name implies) an instrument to render distant objects visible, must be
both a magnifying and an illuminating instrument.
[Illustration: _Fig. 1._]
Let EF, fig. 1, be an object, not near to AB as in the figure, but so
far off that the bounding lines from A and B would meet at the point
corresponding to the point P. Then if a large convex glass AB (called an
_object-glass_) be interposed between the object and the eye, all those
rays which, proceeding from P, fall on AB, will be caused to converge
nearly to a point _p_. The same is true for every point of the object
EMF, and thus a small image, _emf_, will be formed. This image will not
lie exactly on a flat surface, but will be curved about the point midway
between A and B as a centre. Now if the lens AB is removed, and an eye
is placed at _m_ to view the distant object EMF, those rays only from
each point of the object which fall on the pupil of the eye (whose
diameter is about equal to _mp_ suppose) will serve to render the object
visible. On the other hand, every point of the image _emf_ has received
the whole of the light gathered up by the large glass AB. If then we can
only make this light _available_, it is clear that we shall have
acquired a large increase of _light_ from the distant object. Now it
will be noticed that the light which has converged to _p_, diverges from
_p_ so that an eye, placed that this diverging pencil of rays may fall
upon it, would be too small to receive the whole of the pencil. Or, if
it did receive the whole of this pencil, it clearly could not receive
the whole of the pencils proceeding from other parts of the image _emf_.
_Something_ would be gained, though, even in this case, since it is
clear that an eye thus placed at a distance of ten inches from _emf_
(which is about the average distance of distinct vision) would not only
receive much more light from the image _emf_, than it would from the
object EMF, but see the image much larger than the object. It is in this
way that a simple object-glass forms a telescope, a circumstance we
shall presently have to notice more at length. But we want to gain the
full benefit of the light which has been gathered up for us by our
object-glass. We therefore interpose a small convex glass _ab_ (called
an eye-glass) between the image and the eye, at such a distance from the
image that the divergent pencil of rays is converted into a pencil of
parallel or nearly parallel rays. Call this an emergent pencil. Then all
the emergent pencils now converge to a point on the axial line _m_M
(produced beyond _m_), and an eye suitably placed can take in all of
them at once. Thus the whole, or a large part, of the image is seen at
once. But the image is seen inverted as shown. This is the Telescope, as
it was first discovered, and such an arrangement would now be called a
_simple astronomical Telescope_.
Let us clearly understand what each part of the astronomical telescope
does for us:--
The object-glass AB gives us an illuminated image, the amount of
illumination depending on the size of the object-glass. The eye-glass
enables us to examine the image microscopically.
We may apply eye-glasses of different focal length. It is clear that the
shorter the focal length of _ab_, the nearer must _ab_ be placed to the
image, and the smaller will the emergent pencils be, but the greater the
magnifying power of the eye-glass. If the emergent pencils are severally
larger than the pupil of the eye, light is wasted at the expense of
magnifying power. Therefore the eye-glass should never be of greater
focal length than that which makes the emergent pencils about equal in
diameter to the pupil of the eye. On the other hand, the eye-glass must
not be of such small focal length that the image appears indistinct and
contorted, or dull for want of light.
[Illustration: _Fig. 2._]
Let us compare with the arrangement exhibited in fig. 1 that adopted by
Galileo. Surprise is sometimes expressed that this instrument, which in
the hands of the great Florentine astronomer effected so much, should
now be known as the _non-astronomical Telescope_. I think this will be
readily understood when we compare the two arrangements.
In the Galilean Telescope a small concave eye-glass, _ab_ (fig. 2), is
placed between the object-glass and the image. In fact, no image is
allowed to be formed in this arrangement, but the convergent pencils are
intercepted by the concave eye-glass, and converted into parallel
emergent pencils. Now in fig. 2 the concave eye-glass is so placed as to
receive only a part of the convergent pencil A _p_ B, and this is the
arrangement usually adopted. By using a concave glass of shorter focus,
which would therefore be placed nearer to _m p_, the whole of the
convergent pencil might be received in this as in the former case. But
then the axis of the emergent pencil, instead of returning (as we see it
in fig. 1) _towards_ the axis of the telescope, would depart as much
_from_ that axis. Thus there would be no point on the axis at which the
eye could be so placed as to receive emergent pencils showing any
considerable part of the object. The difference may be compared to that
between looking through the small end of a cone-shaped roll of paper and
looking through the large end; in the former case the eye sees at once
all that is to be seen through the roll (supposed fixed in position), in
the latter the eye may be moved about so as to command the same range of
view, but _at any instant_ sees over a much smaller range.
To return to the arrangement actually employed, which is illustrated by
the common opera-glass. We see that the full illuminating power of the
telescope is not brought into play. But this is not the only objection
to the Galilean Telescope. It is obvious that if the part C D of the
object-glass were covered, the point P would not be visible, whereas, in
the astronomical arrangement no other effect is produced on the
visibility of an object, by covering part of the object-glass, than a
small loss of illumination. In other words, the dimensions of the field
of view of a Galilean Telescope depend on the size of the object-glass,
whereas in the astronomical Telescope the field of view is independent
of the size of the object-glass. The difference may be readily tested.
If we direct an opera-glass upon any object, we shall find that any
covering placed over a part of the object-glass _becomes visible_ when
we look through the instrument, interfering therefore _pro tanto_ with
the range of view. A covering similarly placed on any part of the
object-glass of an astronomical telescope does not become visible when
we look through the instrument. The distinction has a very important
bearing on the theory of telescopic vision.
In considering the application of the telescope to practical
observation, the circumstance that in the Galilean Telescope no real
image is formed, is yet more important. A real image admits of
measurement, linear or angular, while to a _virtual_ image (such an
image, for instance, as is formed by a common looking-glass) no such
process can be applied. In simple observation the only noticeable effect
of this difference is that, whereas in the astronomical Telescope a
_stop_ or diaphragm can be inserted in the tube so as to cut off what is
called the _ragged edge_ of the field of view (which includes all the
part not reached by _full pencils of light_ from the object-glass),
there is no means of remedying the corresponding defect in the Galilean
Telescope. It would be a very annoying defect in a telescope intended
for astronomical observation, since in general the edge of the field of
view is not perceptible at night. The unpleasant nature of the defect
may be seen by looking through an opera-glass, and noticing the gradual
fading away of light round the circumference of the field of view.
The properties of reflection as well as of refraction have been enlisted
into the service of the astronomical observer. The formation of an image
by means of a concave mirror is exhibited in fig. 3. As the observer's
head would be placed between the object and the mirror, if the image,
formed as in fig. 3, were to be microscopically examined, various
devices are employed in the construction of reflecting telescopes to
avoid the loss of light which would result--a loss which would be
important even with the largest mirrors yet constructed. Thus, in
Gregory's Telescope, a small mirror, having its concavity towards the
great one, is placed in the axis of the tube and forms an image which is
viewed through an aperture in the middle of the great mirror. A similar
plan is adopted in Cassegrain's Telescope, a small convex mirror
replacing the concave one. In Newton's Telescope a small inclined-plane
reflector is used, which sends the pencil of light off at right-angles
to the axis of the tube. In Herschel's Telescope the great mirror is
inclined so that the image is formed at a slight distance from the axis
of the telescope. In the two first cases the object is viewed in the
usual or direct way, the image being erect in Gregory's and inverted in
Cassegrain's. In the third the observer looks through the side of the
telescope, seeing an inverted image of the object. In the last the
observer sees the object inverted, but not altered as respects right and
left. The last-mentioned method of viewing objects is the only one in
which the observer's back is turned towards the object, yet this method
is called the _front view_--apparently _quasi lucus a non lucendo_.
[Illustration: _Fig. 3._]
It appears, then, that in all astronomical Telescopes, reflecting or
refracting, a _real image_ of an object is submitted to microscopical
examination.
Of this fact the possessor of a telescope may easily assure himself;
for if the eye-glass be removed, and a small screen be placed at the
focus of the object-glass, there will appear upon the screen a small
picture of any object towards which the tube is turned. But the image
may be viewed in another way which requires to be noticed. If the eye,
placed at a distance of five or six inches from the image, be directed
down the tube, the image will be seen as before; in fact, just as a
single convex lens of short focus is the simplest microscope, so a
simple convex lens of long focus is the simplest telescope.[1] But a
singular circumstance will immediately attract the observer's notice. A
real picture, or the image formed on the screen as in the former case,
can be viewed at varying distances; but when we view the image directly,
it will be found that for distinct vision the eye must be placed almost
exactly at a fixed distance from the image. This peculiarity is more
important than it might be thought at first sight. In fact, it is
essential that the observer who would rightly apply the powers of his
telescope, or fairly test its performance, should understand in what
respect an image formed by an object-glass or object-mirror differs from
a real object.
Pages:
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8