About New Kinds of Telescopes, Especially for Handheld Use.
Ueber neue Arten von Fernrohren, insbesondere fuer den Hangebrauch.
Lecture held at the session of the
Society for Advancement of Industrial Activity on January 7, 1895
by Dr. S. Czapski,
Scientific Colleague of the Optical Laboratory of Carl Zeiss in Jena
Continuation, Number 4; from the 'Central-Zeitung fuer Optik und Mechanik'
(Journal for Optics and Mechanics), Berlin, 15 Feb., 1896
Translation by Ilse Roberts and Peter Abrahams
From these two simplest types of image reversing prism systems, many
other forms can be derived that are useful for other purposes. The prisms
can be separated from each other, and parts can be removed and shifted
along the optical axis while retaining orientation. This division of a
prism system into two or three separated parts is the way to make the prism
useful for purposes besides image reversing, and to gain other practical
advantages for the construction of hand held telescopes and binoculars.
The first advantage is a significant shortening of the distance between
objective and ocular, in other words a reduction in the length of the
telescope. This can be done in many ways. Fig. 8 and 9 show two
arrangements for this purpose. In fig 8, the two prisms of fig 6 are
separated in the direction of the axis so that a ray has to travel the
distance between them three times. Fig. 9 shows the middle part of the
prism from fig. 7, built as a right angled isosceles prism. It is
perpendicular to the axis and separated from the other two prisms, so that
the light travels the distance between the two pairs twice. In fig. 8, the
length of the tube can be reduced to a third of the objective's focal
length. For fig. 9, which needs three separated prisms, there is almost no
lower limit to tube length in the direction of viewing, if length in the
direction perpendicular to this is extended to nearly half the objective's
focal length.
The tube length of the prism telescope, in comparison with the
terrestrial or the astronomical (image reversing) telescope with the same
objective and ocular, is shorter even when the prism system is separated
into parts, as in fig 6 and 7. The distance between the objective and the
last real image in its focal length, (the tube length in a narrow sense),
becomes smaller than the focal length of the objective, because a piece of
that distance, (almost twice the length of the short side of the prism), is
deflected to the side. Prism telescopes have a tube length smaller than the
focal length of the objective, in contrast to all common telescopes with a
positive ocular or to an otherwise equal astronomical telescope.
Thus the goal stipulated on page 14 can be reached. An astronomical
telescope of 4, 6, or 8 power already has reasonable and handy dimensions.
By combining it with one of these prism systems, it is transformed into a
terrestrial (image erecting) telescope of smaller length, and the proper
prism system will greatly reduce the length. It can surpass a Dutch
telescope of equal magnification wihile being half or a third as long, and
as handy as could be desired. The f.o.v. of such instruments is entirely
equal or superior in size and illumination to the terrestrial telescopes.
An extension of the apparent [scheinbaren] f.o.v. by about 35 degrees is
attainable for any magnification. In contrast to the best Dutch
telescopes, the objective and subjective f.o.v. at 4 power is 2.5 times as
large, at 6 power about 3 times, and at 8 power about 3.3 times as large
(therefore, in area of observation, six to 10 times as large, likewise that
of the terrestrial telescope.)
Finally, in dioptric terms the telescope is an astronomical one
(Keplerian). In regards to intensity of light, in principle it acts
according to the norms for the terrestrial and astronomical telescope. The
external reasons which almost forced the choice of such a low intensity of
light for the terrestrial telescope are here completely eliminated (see
page 13). For example, a 25mm objective with 5, 10 or 20 power, hardly
changes the length of the instrument, and at most, the smaller
magnification requires larger prisms near the ocular, so that the f.o.v.
will not be restricted. Furthermore, progress beyond all earlier designs
is found in the choice of magnification for the given dimensions of length
and objective diameter. The power used is determined by optical
considerations, not external design.
The ability to change powers with a single telescope is an option, and
some of these designs require this. Instead of an ocular that is firmly
connected to the body, several oculars can be attached via a revolving
turret and held firmly in position by a detent. All of the oculars must be
able to focus on the real image created by the objective. A magnification
that requires strong illumination can be used; or one suitable for dim
light, which would provide a wider f.o.v. for an overview of the terrain.
The lower power could quickly be changed to the higher, to study the
details of a previously noticed object, and a triple ocular revolver
provides an additional medium power. The lower powers require a larger
prism and substantially increase the price, especially compared to a
relatively weakly illuminated, strongly magnifying glass.
Without affecting these properties, a prism system can achieve another
advantage by making one or more of the four required prism elements
effective as lenses. One or both of the shorter planes of a reflecting
prism can be replaced by convex or concave spherical surfaces. These must
be centered, and are reflected at the hypotenuse by 90 degrees. The glass
used in the prism is determined by the chromatic and spherical correction
of the lens.
An isosceles right angled prism can have its short side planes replaced
by spherical surfaces ab & ac, whose centerpoints lie on the lines AB & BC
which are 45 degrees to the hypotenuse (fig. 10). According to the laws of
optics, this prism acts as a simple lens with the same spherical profile,
the same glass, and a thickness on the axis AC of mn=mB+Bn (also adding the
reflection).
Such a biconvex lens-prism L (fig. 11), made of crown glass can be made
achromatic and spherically corrected by cementing to it a meniscus or
planoconcave lens l of flint glass. Likewise, an achromatic objective can
be created by cementing a lens-prism of flint, with the curvatures of a
[Zertrennungslinse=separation lens?], to a biconvex or planoconvex crown
glass lens.
In both cases an objective is created in which the axis is reflected by
90 degrees.
Two prisms from an image erecting system can be replaced with lens-
prisms, cemented or separated, acting as an achromatic objective in which
the axis is twice reflected by 90 degrees. Fig. 12 shows a biconvex prism
L1 made of crown glass, cemented to a convex-concave prism L2 made of
flint. The whole prism is equivalent to the depicted double-lens below,
whose total thickness is m'q'=mn+np+pq.
The four prism systems of fig. 4 & 5 can thereby have their front
prisms or pair of prisms transformed into the objective of a telescope.
Likewise, the rearmost element(s) of these prism systems can be used to
form an ocular by using lens-prisms. It is possible to construct
telescopes with an erect image and a positive ocular with no more
refracting air-glass surfaces than are unavoidable in the astronomical
telescope.
This economy of elements is restrained by the dimensions designed for
the telescope, and by the purpose for which it is to be used. The
calculation of the lens-prisms is part of the standard methods of practical
optics, whether for the objective or the ocular.
The useful and effective separation of the glass body as shown in fig 6
and 7, which served to shorten the telescope, can now be used to gain other
essential advantages, for these and other purposes, needing only the
devices used for image reversal. The exiting ray is parallel to the
entering one in the described prism system, so that the direction of view
is not changed by the prism, but the ray is displaced to the side. For the
system in fig. 7, the measure of this displacement of the exiting ray from
the entering ray is equal to the length of the short side of a prism, if
all prisms are equal in size. For the system in fig. 6, the displacement
is equal to the square root of 2 times the length of the short side. These
are the minimum values for the displacement of the ray axis in such an
image reversal.
This displacement can be increased without enlarging the prisms above
their minimum size, by separating the prisms in a different way than used
for shortening the telescope. Fig 13 and 14 show the simplest sequence of
the four prisms of the two basic prism systems suitable for displacing the
ray from the axis of the objective. In these sequences either the first or
the last prism is separated from the three following or preceding prisms,
and shifted while retaining orientation angle. The system of fig. 7 can
also be separated between the second and third reflections, so that the
system becomes two groups of two prisms each, fig. 15.
In theory, there is no limit to this displacement of entering ray
against exiting ray. These systems, like those described earlier, can be
arranged in any sequence or combination in relation to the lenses of the
telescope. Each prism can be placed in front of or behind the next lens,
or one or more prisms can be placed outside of the telescope if aligned
with the axis of the objective. This allows a lengthening of the
telescope, limited by practical considerations of intended purpose, effects
on the design of the total instrument, and availability of raw materials
for larger objectives and prisms.
As discussed, it makes no difference which of the two separated prisms
from the system is installed near (in front or back of) the objective or
which is near the ocular. For practical economical reasons one will
generally put a single separated prism at the point where the largest prism
is needed, so that the majority of the prisms can be smaller. Thus, in
telescopes with less magnification, where often the objective is smaller
than the ocular, the separated prism is placed near the ocular and the rest
of the prism system placed near (in front or back of) the objective. This
decision often depends on other considerations, which eventually must be
addressed.
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