HAND TELESCOPES AND BINOCULARS
The hand telescope finds comparatively little use in observing
celestial bodies. It is usually quite too small for any except very
limited applications, and cannot be given sufficient power without
being difficult to keep steady except by the aid of a fixed mounting.
Still, for certain work, especially the observation of variable
stars, it finds useful purpose if sufficiently compact and of good
light-gathering power.
There is most decidedly a limit to the magnifying power which can be
given to an instrument held in the hand without making the outfit too
unsteady to be serviceable. Anything beyond 8 to 10 diameters is highly
troublesome, and requires a rudimentary mount or at least steadying the
hand against something in order to observe with comfort.
The longer the instrument the more difficult it is to manage, and
the best results with hand telescopes are to be obtained with short
instruments of relatively large diameter and low power. The ordinary
field glass of Galilean type comes immediately to mind and in fact the
field glass is and has been much used. As ordinarily constructed it is
optically rather crude for astronomical purposes. The objectives are
rarely well figured or accurately centered and a bright star usually
appears as a wobbly flare rather than a point.
Furthermore the field is generally small, and of quite uneven
illumination from centre to periphery, so that great caution has to
be exercised in judging the brightness of a star, according to its
position in the field. The lens diameter possible with a field glass
of ordinary construction is limited by the limited distance between
the eyes, which must be well centered on the eyepieces to obtain clear
vision.
The inter-pupillary distance is generally a scant 2½ inches so that
the clear aperture of one of the objectives of a field glass is rarely
carried up to 2 inches. The best field glasses have each objective
a triple cemented lens, and the concave lenses also triplets, the
arrangement of parts being that shown in Fig. 110. Glasses of this sort
rarely have a magnifying power above 5.
In selecting a field glass with the idea of using it on the sky try it
on a bright star, real or artificial, and if the image with careful
focussing does not pull down to a pretty small and uniform point take
no further interest in the instrument.
[Illustration: FIG. 110.—Optical Parts of Field Glass.]
The advantage of a binocular instrument is popularly much exaggerated.
It gives a somewhat delusive appearance of brilliancy and clearness
which is psychological rather than physical. During the late war a
very careful research was made at the instance of the United States
Government to determine the actual value of a binocular field glass
against a monocular one of exactly the same type, the latter being
cheaper, lighter, and in many respects much handier.
The difference found in point of actual seeing all sorts of objects
under varying conditions of illumination was so small as to be
practically negligible. An increase of less than 5 per cent in
magnifying power enabled one to see with the monocular instrument
everything that could be seen with the binocular, equally well, and it
is altogether probable that in the matter of seeing fine detail the
difference would be even less than in general use, since it is not
altogether easy to get the two sides of a binocular working together
efficiently or to keep them so afterwards.
There has been, therefore, a definite field for monocular hand
telescopes of good quality and moderate power and such are manufactured
by some of the best Continental makers. Such instruments have
sometimes been shortened by building them on the exact principle of
the telephoto lens, which gives a relatively large image with a short
camera extension.
[Illustration: FIG. 111.—Steinheil Shortened Telescope.]
A much shortened telescope, as made by Steinheil for solar photographic
purposes, is shown in Fig. 111. This instrument with a total length
of about 2 feet and a clear aperture of 2⅜ inches gives a solar
image of ½ inch diameter, corresponding to an ordinary glass of
more than double that total length. Quite the same principle has been
applied to terrestrial telescopes by the same maker, giving again an
equivalent focus of about double the length of the whole instrument.
This identical principle has often been used in the so-called Barlow
lens, a negative lens placed between objective and eyepiece and
giving increased magnification with small increase of length; also
photographic enlargers of substantially similar function have found
considerable use.
A highly efficient hand telescope for astronomical purposes might be
constructed along this line, the great shortening of the instrument
making it possible to use somewhat higher powers than the ordinary
without too much loss of steadiness. There is also constructed a
binocular for strictly astronomical use consisting of a pair of small
hand comet-seekers.
One of these little instruments is shown in Fig. 112. It has a
clear diameter of objectives of 1⅜ inch, magnification of 5, and
a brilliant and even field of 7½° aperture. The objectives are
triplets like Fig. 57, already referred to, the oculars achromatic
doublets of the type of Fig. 104_a_.
With the exception of these specialized astronomical field glasses
the most useful and generally available hand instrument is the prism
glass now in very general use. It is based on reversal of the image
by internal total reflection in two prisms having their reflecting
surfaces perpendicular each to the other. The rudiments of the process
lie in the simple reversion prism shown in diagram in Fig. 113.
[Illustration: FIG. 112.—Astronomical Binocular.]
This is nothing more nor less than a right angled glass prism set with
its hypothenuse face parallel and with its sides at 45° to the optical
axis of the instrument. Rays falling upon one of its refracting faces
at an angle of 45° are refracted upon the hypothenuse face, are there
totally reflected and emerge from the second face of the prism parallel
to their original course.
Inspection of Fig. 113 shows that an element like A B perpendicular to
the plane of the hypothenuse face is inverted by the total reflection
so that it takes the position A′ B′. It is equally clear that an
element exactly perpendicular to A B will be reflected from the
hypothenuse face flatwise as it were, and will emerge without its ends
being reversed so that the net effect of this single reflection is to
invert the image without reversing it laterally at the same time.
On the other hand if a second prism be placed behind the first, flat
upon its side, with its hypothenuse face occupying a plane exactly
perpendicular to that of the first prism, the line A′B′ will be
refracted, totally reflected and refracted again out of the prism
without a second inversion, while a line perpendicular to A′B′ will be
refracted endwise on the hypothenuse face of the second prism and will
be inverted as was the line A B at the start.
[Illustration: FIG. 113.—Reversion Prism.]
Consequently two prisms thus placed will completely invert the image,
producing exactly the same effect as the ordinary inverting system Fig.
106. The simple reversion prism is useful as furnishing a means, when
placed over an eye lens, and rotated, of revolving the image on itself,
a procedure occasionally convenient, especially in stellar photometry.
The two prisms together constitute a true inverting system and have
been utilized in that function, but they give a rather small angular
field and have never come into a material amount of use. The exact
effect of this combination, known historically as Dove’s prisms, is
shown plainly in Fig. 114.
The first actual prismatic inverting system was due to M. Porro, who
invented it about the middle of the last century, and later brought it
out commercially under the name of “Lunette à Napoleon Troisiéme,” as a
glass for military purposes.
[Illustration: FIG. 114.—Dove’s Prisms.]
[Illustration: FIG. 115.—Porro’s Prism System.]
The prism system of this striking form of instrument is shown in Fig.
115. It was composed of three right angle prisms _A_, _B_, and _C_. _A_
presented a cathetus face to the objective and _B_ a cathetus face to
the ocular. Obviously a vertical element brought in along _a_ from the
objective would be reflected at the hypothenuse face _b_, to a position
at right angles to the original one, would enter the hypothenuse face
of _C_ and thence after two reflections at _c_ and _d_ flatwise and
without change of direction would emerge, enter the lower cathetus face
of _B_ and by reflection at the hypothenuse face _e_ of _B_ would be
turned another 90° making a complete reversion as regards up and down
at the eye placed at _f_. An element initially at right angles to the
one just considered would enter _A_, be reflected flatwise, in the
faces of _C_ be twice reflected endwise, thereby completely inverting
it, and would again be reflected flatwise from the hypothenuse face
of _C_, thereby effecting, as the path of the rays indicated plainly
shows, a complete inversion of the image. Focussing was very simply
attained by a screw motion affecting the prism _C_ and the whole affair
was in a small flat case, the external appearance and size of which is
indicated in Fig. 116.
[Illustration: FIG. 116.—Lunette à Napoleon Troisiéme.]
[Illustration: FIG. 117.—Porro’s First Form of Prisms.]
From ocular to objective the length was about an inch and a half. It
was of 10 power and took in a field of 45 yards at a distance of 1000
yards. Here for the first time we find a prismatic inverting system of
strictly modern type. And it is interesting to note that if one had
wished to make a binocular “Lunette à Napoleon Troisiéme” he would
inevitably have produced an instrument with enhanced stereoscopic
effect like the modern prism field glass by the mere effort to dodge
the observer’s nose. Somewhat earlier M. Porro had arranged his prisms
in the present conventional form of Fig. 117, where two right angle
prisms have their faces positioned in parallel planes, but turned
around by 90° as in Fig. 114. The ray traced through this conventional
system shows that exactly the same inversion occurs here as in the
original Porro construction, and this form is the one which has been
most commonly used for prismatic inversion and is conveniently known as
Porro’s first form, it actually having been antecedent in principle and
practice to the “Lunette à Napoleon Troisiéme.” The original published
description of Porro’s work, translated from “Cosmos” Vol. 2, p. 222
(1852) et seq. is here annexed as it sets forth the origin of the
modern prism glass in unmistakable terms.
_Cosmos, Vol. 2_, p. 222.—“We have wished for some time to make known
to our readers the precious advantages of the “longue-vue cornet” or
télémetre of M. Porro. Ordinary spyglasses or terrestrial telescopes
of small dimensions are at least 30 or 40 cm. long when extended to
give distinct vision of distant objects. The length is considerably
reduced by substituting for a fixed tube multiple tubes sliding into
each other. But the drawing out which this substitution necessitates is
a somewhat grave inconvenience; one cannot point the telescope without
arranging it and losing time.
For a long time we have wished it were possible to have the power
of viewing distant objects, with telescopes very short and without
draw. M. Porro’s “longue-vue cornet” seems to us to solve completely
this difficult and important problem. Its construction rests upon an
exceedingly ingenious artifice which literally folds triply the axis
of the telescope and the luminous ray so that by this fact alone the
length of the instrument is reduced by two-thirds.
Let us try to give an idea of this construction: Behind the telescope
objective M. Porro places a rectangular isosceles prism of which the
hypothenuse is perpendicular to the optic axis. The luminous rays
from the object fall upon the rectangular faces of this prism, are
twice totally reflected, and return upon themselves parallel to their
original direction: half way to the point where they would form the
image of the object, they are arrested by a second prism entirely
similar to the first, which returns them to their original direction
and sends them to the eyepiece through which we observe the real image.
If the rectangular faces of the second prism were parallel to the
faces of the first, this real image would be inverted—the telescope
would be an astronomical and not a terrestrial telescope. But M. Porro
being an optician eminently dextrous, well divined that to effect the
reinversion it sufficed to place the rectangular faces of the second
prism perpendicular to the corresponding faces of the first by turning
them a quarter revolution upon themselves.
In effect, a quarter revolution of a reflecting surface is a half
revolution for the image, and a half revolution of the image evidently
carries the bottom to the top and the right to the left, effecting a
complete inversion. As the image is thus _redressed_ independently of
the eyepiece one can evidently view it with a simple two-lens ocular
which decreases still further the length of the telescope so that it
is finally reduced to about a quarter of that of a telescope of equal
magnifying power, field and clearness.
The new telescope is then a true pocket telescope even with a
magnifying power of 10 or 15. Its dimensions in length and bulk are
those of a field glass usually magnifying only 4 to 6 times. The more
draws, the more bother,—it here suffices to turn a little thumbscrew to
find in an instant the point of sharpest vision.
In brilliancy necessarily cut down a little, not by the double total
reflection, which as is well known does not lose light, but by the
quadruple passage across the substance of the two prisms, the cornet
in sharpness and amplification of the images can compare with the
best hunting telescopes of the celebrated optician Ploessl of Vienna.
M. Porro has constructed upon the same principles a marine telescope
only 15 c.m. long with an objective of 40 m.m. aperture which replaces
an ordinary marine glass 70 c.m. long. He has done still better,—a
telescope only 30 c.m. long carries a 60 m.m. objective and can be made
by turns a day and a night glass, by substituting by a simple movement
of the hand and without dismounting anything, one ocular for the other.
Its brilliancy and magnification of a dozen times with the night
ocular, of twenty-five times with the day ocular, permits observing
without difficulty the eclipses of the satellites of Jupiter.
This is evidently immense progress. One of the most illustrious
of German physicists, M. Dove of Berlin, gave in 1851 the name of
reversion prism to the combination of two prisms placed normally one
behind the other so that their corresponding faces were perpendicular.
He presented this disposition as an important new discovery made by
himself. He doubtless did not know that M. Porro, who deserves all the
honor of this charming application, had realized it long before him.”
A little later M. Porro produced what is commonly referred to as
Porro’s second form, which is derived directly from annexing _A_ Fig.
115 to the corresponding half of _C_ as a single prism, the other half
of _B_ being similarly annexed to the prism _C_, thus forming two
sphenoid prisms, such as are shown in Fig. 118 which may be mounted
separately or may have their faces cemented together to save loss of
light by reflections. The sphenoid prisms have had the reputation of
being much more difficult to construct than the plain right angled
prisms of the other forms shown. In point of fact they are not
particularly difficult to make and the best inverting eye pieces for
telescopes are now constructed with sphenoid prisms like those just
described.
[Illustration: FIG. 118.—Porro’s Second Form.]
[Illustration: FIG. 119.—Clark Prismatic Eyepiece.]
This particular arrangement lends itself very readily to a fairly
compact and symmetrical mounting, as is well shown in Fig. 119 which
is the terrestrial prismatic eyepiece as constructed by the Alvan
Clark corporation for application to various astronomical telescopes
of their manufacture. A glance at the cut shows the compactness of
the arrangement, which actually shortens the linear distance between
objective and ocular by the amount of the path of the ray through the
prisms instead of lengthening the distance as in the common terrestrial
eyepiece.
The field moreover is much larger than that attainable by a
construction like Fig. 110, extending to something over 40°, and there
is no strong tendency for the illumination or definition to fall off
near the edge of the field.
In the practical construction of prism field glasses the two right
angled prisms are usually separated by a moderate space as in Porro’s
original instruments so as to shorten the actual length of the prism
telescope by folding the ray upon itself as in Fig. 120, which is a
typical modern binocular of this class.
[Illustration: FIG. 120.—Section of Prism Binocular.]
The path of the rays is plainly shown and the manner in which the
prisms fold up the total focal length of the objective is quite
obvious. The added stereoscopic effect obtained by the arrangement of
the two sides of the instrument is practically a very material gain.
It gives admirable modelling of the visible field, a perception of
distance which is at least very noticeable and a certain power of
penetration, as through a mass of underbrush, which results from the
objectives to a certain extent seeing around small objects so that
one or the other of them gives an image of something beyond. For near
objects there is some exaggeration of stereoscopic effect but on the
whole for terrestrial use the net gain is decidedly in evidence.
A well made prism binocular is an extremely useful instrument for
observation of the heavens, provided the objectives are of fair size,
and the prisms big enough to receive the whole beam from the objective,
and well executed enough to give a thoroughly good image with a flat
field.
The weak points of the prism glass are great loss of light through
reflection at the usual 10 air-glass surfaces and the general presence
of annoying ghosts of bright objects in the field. Most such binoculars
have Kellner eyepieces which are peculiarly bad, as we have seen, with
respect to reflected images, and present the plane surface of the last
prism to the plane front of the field lens. Recently some constructors
have utilized the orthoscopic eyepiece, Figure 105_a_, as a substitute
with great advantage in the matter of reflections.
The loss of light in the prism glass is really a serious matter,
between reflection at the surfaces and absorption in the thick masses
of glass necessary in the prisms. If of any size the transmitted light
is not much over one-half of that received, very seldom above 60%.
If the instrument is properly designed the apparent field is in the
neighborhood of 45°, substantially flat and fairly evenly illuminated.
Warning should here be given however that many binoculars are on the
market in which the field is far from flat and equally far from being
uniform.
In many instances the prisms are too small to take the whole bundle of
rays from the objective back to the image plane without cutting down
the marginal light considerably. Even when the field is apparently
quite flat this fault of uneven illumination may exist, and in a glass
for astronomical uses it is highly objectionable.
Before picking out a binocular for a study of the sky make very careful
trial of the field with respect to flatness and clean definition of
objects up to the very edge. Then judge as accurately as you may of the
uniformity of illumination, if possible by observation on two stars
about the radius of the field apart. It should be possible to observe
them in any part of the field without detectable change in their
apparent brilliancy.
If the objectives are easily removable unscrew one of them to obtain a
clear idea as to the actual size of the prisms.[19] Look out, too, for
ghosts of bright stars.
[19] There are binoculars on the market which are to outward
appearance prism glasses, but which are really ordinary opera glasses
mounted with intent to deceive, sometimes bearing a slight variation
on the name of some well known maker.
The objectives of prism glasses usually run from ¾ inch to 1½ inch
in diameter, and the powers from 6 to 12. The bigger the objectives the
better, provided the prisms are of ample size, while higher power than
6 or 8 is generally unnecessary and disadvantageous. Occasional glasses
of magnifying power 12 to 20 or more are to be found but such powers
are inconveniently great for an instrument used without support. Do not
forget that a first class monocular prism glass is extremely convenient
and satisfactory in use, to say nothing of being considerably less in
price than the instrument for two eyes. A monocular prism glass, by
the way, makes an admirable finder when fitted with coarse cross lines
in the eyepiece. It is particularly well suited to small telescopes
without circles.
[Illustration: FIG. 121.—Binocular with Extreme Stereoscopic Effect.]
Numerous modifications of Porro’s inverting prisms have been made
adapting them to different specific purposes. Of these a single
familiar example will suffice as showing the way in which the Porro
prism system can be treated by mere rearrangement of the prismatic
elements. In Fig. 121 is shown a special Zeiss binocular capable of
extreme stereoscopic effect. It is formed of two Porro prism telescopes
with the rays brought into the objectives at right angles to the axis
of the instrument by a right angled prism external to the objective.
The apertures of these prisms appear pointing forward in the cut. As
shown they are in a position of maximum stereoscopic effect.
Being hinged the tubes can be swung up from the horizontal position,
in which latter the objectives are separated by something like eight
times the interocular distance. The stereoscopic effect with the tubes
horizontal is of course greatly exaggerated so that it enables one to
form a fair judgment as to the relative position of somewhat distant
objects, a feature useful in locating shell bursts.
The optical structure of one of the pair of telescopes is shown in Fig.
122 in which the course of the entering ray can be traced through the
exterior prism of the objective and the remainder of the reversing
train and thence through the eyepiece. This prism erecting system is
obviously derived from the “Lunette à Napoleon Troisiéme” by bringing
down the prism _B_ upon the corresponding half _A_ and cementing it
thereto, meanwhile placing the objective immediately under _A_.
One occasionally meets prismatic inverting systems differing
considerably from the Porro forms. Perhaps the best known of these is
the so called roof prism due to Prof. Abbé, Fig. 123, and occasionally
useful in that the entering and emerging rays lie in the same straight
line, thus forming a direct vision system. Looking at it as we did at
the Porro system a vertical element in front of the prism is reversed
in reflection from the two surfaces a and b, while a corresponding
horizontal element is reflected flatwise so far as these are concerned,
but is turned end for end by reflection at the roof surfaces c and d,
thus giving complete inversion.
In practice the prism is made as shown, in three parts, two of them
right angled prisms, the third containing the roof surfaces. The
extreme precision required in figuring the roof forms a considerable
obstacle to the production of such prisms in quantity and while
they have found convenient use in certain special instruments like
gunsights, where direct vision is useful, they are not extensively
employed for general purposes, although both monocular and binocular
instruments have been constructed by their aid.
[Illustration: FIG. 122.—Path of Ray in Fig. 121.]
[Illustration: FIG. 123.—Abbé Roof Prism.]
One other variety of prism involving the roof principle has found some
application in field glasses manufactured by the firm of Hensoldt. The
prism form used is shown in Fig. 124. This like other forms of roof
prism is less easy to make than the conventional Porro type. Numerous
inverting and laterally reflecting prisms are in use for specific
purposes. Some of them are highly ingenious and remarkably well adapted
for their use, but hardly can be said to form a material portion of
telescope practice. They belong rather to the technique of special
instruments like gunsights and periscopes, while some of them have been
devised chiefly as ingenious substitutes for the simpler Porro forms.
Most prism telescopes both monocular and binocular are generally made
on one or the other of the Porro forms. This is particularly true
of the large binoculars which are occasionally constructed. Porro’s
second form with the sphenoid prisms seems to be best adapted to cases
where shortening of the instrument is not a paramount consideration.
For example, some Zeiss short focus telescopes are regularly made in
binocular form, and supplied with inverting systems composed of two
sphenoid prisms, and with oculars constructed on the exact principle
of the triple nose-piece of a microscope, so that three powers are
immediately available to the observer.
[Illustration: FIG. 124.—Hensoldt Prism.]
Still less commonly binocular telescopes of considerable aperture
are constructed, primarily for astronomical use, being provided with
prismatic inversion for terrestrial employment, but more particularly
in order to gain by the lateral displacement of a Porro system the
space necessary for two objectives of considerable size. As we have
already seen, the practical diameter of objectives in a binocular
is limited to a trifle over 2 inches unless space is so gained. The
largest prismatic binocular as yet constructed is one made years ago
by the Clarks, of 6¼ inches objective aperture and 92¼ inches
focal length. So big and powerful an instrument obviously would give
admirable binocular views of the heavens and so accurately was it
constructed that the reports of its performance were exceedingly good.
The same firm has made a good many similar binoculars of 3 inch and
above, of which a typical example of 4 inch aperture and 60 inch focal
length is shown in Fig. 125. In this case the erecting systems were
of Porro’s first form, and were provided with Kellner oculars of very
wide field. These binoculars constructions in instruments of such size,
however well made and agreeable for terrestrial observation, hardly
justify the expense for purely astronomical use.
[Illustration: FIG. 125.—Clark 4″ Binocular Telescope.]