Improvements in Lenses for Terrestrial Telescopes.
Year: 1896

page_text: Date of Application, 3rd Feb., 1895-Accepted, 18th Apr.. 1896
COMPLETE SPECIFICATION.
Improvements in Lenses for Terrestrial Telescopes.
°
1, FRIEDRICH RITTER VON VOIGTLÄNDER of 2 Monumentstrasse, Brunswick,
Germany, Manufacturer, do hereby declare the nature of this invention and in
what manner the same is to be performed to be particularly described and
ascertained in and by the following statement:-
Heretofore a large angle aperture of the objective in a terrestrial telescope could
not be fully utilized while applying ordinary Fraunhofer oculars, as otherwise
indistinct pictures were obtained because of the aberrations in the inverting system.
This defect was owing to the fact that the inverting system was composed of two
separate single lenses at a fixed distance apart. Between these members a diaphagm
or shade had to be arranged for the purpose of cutting off the passage through the
second element in the inverting system, the one towards the eye-lens all of those
rays of the bundle or pencil of rays entering the objective, which suffered insufficient
spherical correction by the inverting system. This shade had a great defect. The
brightness of the field of view of the eye-piece became diminished by it so that
the diameter of the cone of rays issuing from the last lens of the eye piece became
so far diminished as to become less than the mean diameter of the pupil of the
human eye. Hence the images of these telescopes became dark especially by
confined or subdued light, for in a good telescope the diameter of its cone of rays
issuing from the last lens of the eye piece must be equal to the mean diameter of
the pupil of the eye..
In order to remedy this defect, and specially to increase the effective opening of
the telescope the inverting system has been made of two separate achromatic lenses.
I may name Kellners as an example of these telescopes. In it the first member of
the inverting system and the second member of the eye lens system are achromatically
corrected.
But this telescope has the following defects :-
1. For proportions of aperture which are greater than f: 10, the spherical
correction is insufficient so that the image in the middle is not absolutely defined
and towards the edge is blurred.
2. The number of outer lens surfaces which is 8, that is 4 for the two members
of the inverting system, and 4 for the two members of the eye lens system,
involves a great loss of light hence causes a diminution of the brightness of the
field of view.
3. It is in practice very difficult to centre with sufficient accuracy the two
parts of the inverting system relatively to each other and relatively to the eye
system.
All these defects are obviated by the inverting system forming the object of this
invention. This system is characterized by the fact that it consists of one system
of lenses cemented together, which must be composed of at least three parts and be
so corrected that as a matter of fact a large angle aperture of the objective is
utilized. The proportion of aperture may rise whereby the telescope is rendered
considerably shorter and handier. Further advantages are as follows:
1. The number of the reflecting lens surfaces in the one-member-inverting-
system are reduced from four to two, and the loss of light by reflection of the outer
surfaces therefore becomes reduced, and the brightness from that cause alone
correspondingly increased so that the strength of the light is thereby raised
one-fifth.

page_text: 2. The image becomes more perfectly defined both in the middle and at the edge
"because the spherical aberration on and next to the axis is more perfectly done
-away with in the case of broad bundles or pencils of rays.
3. The manufacture is considerably easier, and notably the whole system can be
very exactly centered.
On the accompanying drawing is shown as an example an inverting system
composed according to this invention and consisting of three lenses, a biconvex
middle one L³ and two convexo-concave dispersing or divergent lenses L and L'
enclosing it.
The system shown which is calculated for a telescope objective of 8 cm. focal
-distance and 18 mm. opening comprises by a diameter of 10 mm. the following
-elements,
Radii.
Το
== 14.5
T: = 10-25
7₁ = 10.25
78 = 29.5
Thickness of glass.
d₁ = 2.1
d₂
= 5.3
d₁ = 1-4
L₁ and I are of flint glass and L, of crown glass; the index of refraction
for L, and L is μD = 1-61880, and for L, the p = 1-51708.
The example chosen is for an objective consisting of three parts viz. a biconvex
a nearly plano-concave, and a nearly plano-convex lens cemented together.
The new reverse system may also be composed of more than three lenses.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed I declare that what I
claim is
An inverting system for terrestrial telescopes consisting of one system of lenses
cemented together which system must be composed of at least three parts and is so
corrected that a large angle of aperture of the objective is practically utilized,
substantially as set forth.
Dated this Third day of February 1896.
JENSEN & SON,
77 Chancery Lane, London, W.C., Patent Agents.