Optical systems, telescopes and binoculars

An optical system including at least one lens and erecting mirrors designed and arranged to give an inherent aperture and field greater in one direction than another direction. By using a low power eyepiece lens it is possible to arrange for a user's eye or eyes to be further behind the eyepiece lens so that the user can wear normal spectacles to correct for eye defects. When a pair of the optical systems is used in a pair of binoculars, it is possible for the exit pupils to be in a form of horizontal slots so the systems do not have to have adjustable spacing to allow for a particular user's eye separation.

FIELD OF THE INVENTION 
The present invention relates to optical systems for viewing devices such 
as binoculars and telescopes. 
BACKGROUND OF THE INVENTION 
Optical systems used in binoculars have optical paths folded by means of 
reflective prisms and are arranged to give erect images. The substitution 
of prisms by mirrors has been analysed in a paper "Camera viewfinder using 
tilted concave mirror erecting elements" by Donald DeJager in the SPIE Vol 
237 at page 292 of 1980 but the arrangement was found unsatisfactory due 
to excessive amounts of astigmatism, variation of focus across the field, 
distortion and coma. Arrangements using erecting mirrors are also 
described in U.S. Pat. Nos. 4,598,981, 4,804,258, 4,758,077, 4,221,459 and 
3,897,133. The first two do not incorporate any lenses and solely use 
mirrors which do not have an erecting function. The last three have 
circular fields and apertures with the last one having an annular aperture 
and it is known that annular apertures give poor visual images. 
SUMMARY OF THE INVENTION 
The present invention in one of its aspects aims at providing an approach 
whereby an optical system can use mirrors while giving acceptable 
performance with compact shape. 
One aspect of the present invention provides an optical system for 
telescopes and binoculars having an optical path folded by erecting 
mirrors and comprising at least one lens characterized in that the system 
has an inherent aperture and field greater in a direction transverse to a 
direction in which the path is folded. 
Normally the optical systems would have an objective lens unit and an 
eyepiece lens means with an additional aspect of the present invention 
providing an optical system for telescopes and binoculars comprising an 
objective lens unit and an eyepiece lens unit with an optical path 
there-between folded by erecting mirrors characterized in that the system 
has an aperture and field greater in a direction transverse to a direction 
in which the path is folded. 
In use the greater field and aperture will be horizontal and the smaller 
field vertical. This will be acceptable for most uses such as scanning the 
horizon and permits a very advantageous binocular design to be designed 
with the optical paths folded tightly on themselves possibly in the shape 
of a Z or with the optical path folded so as to cross itself. 
In the optical system the smaller field and aperture may be offset so that 
the optical system may otherwise be centered about a single axis as is the 
case in most optical systems where all the surfaces have their centers of 
curvature lying on a single axis. 
The invention is not limited to the mirrors, objective lens unit and 
eyepiece lens means having spherical surfaces, but these can also be 
aspheric such as parabolic or toroidal. In this event such surfaces do not 
have a single point center of curvature and when used with off-axis 
apertures will tend to have centers of curvature blurred over a volume in 
space centered about two locations and with the volume increasing with 
increased aperture. The Gauss center of curvature or the center of 
curvature of the best-fit sphere can be taken as a representative value 
and will herein be termed the center of curvature. This center of 
curvature will also be used to define a mean radius of curvature. 
It is convenient to define the positions of some of the centers of 
curvature in relation to a viewing axis which is the line joining the 
center of the viewed object to the center of the user's eye through the 
system when the system is being used. 
It has been said that the folded path can be in the nature of a Z. The 
angles contained in the Z are preferably no greater than 30.degree. and 
this can be achieved by offsetting the used areas of the objective lens 
unit, mirrors and eyepiece lens means from the viewing axis possibly 
assisted by having the centers of curvature of the mirrors at distances 
not more than 20% but preferably less than 10% of their radii of curvature 
from the viewing axis and the centers of curvature of the objective lens 
unit and eyepiece lens means displaced by up to about twice the greater of 
said distances. The Z can be slightly skew, that is with the bottom bar of 
the Z not parallel with the top bar, so that the spacing of the objective 
lens units in a pair of binoculars can differ from the spacing of the 
eyepiece means. If the folded path is not Z shape, the optical path could 
still be as tightly folded and skewed. 
In its basic form if the system were generally horizontal, the field seen 
by the objective lens unit would be below the horizon and the final image 
viewed by the user's eye after magnification would appear to be even 
further below the horizon by the amount of the magnification. This in some 
instances may be an advantage but at least the displacement of the image 
can be corrected by further optical elements and/or the discrepancy of the 
direction of the system and the horizon can be corrected by design of a 
housing. 
The mirrors need not be simple mirrors but could for example be more 
complex. For example, at least one mirror could be a so-called Mangin 
mirror comprising a lens with a reflective rear surface which would of 
course alter the effective center of curvature. 
The optical systems according to the present invention lend themselves to 
combination in pairs to form binoculars with the systems generally 
parallel and with the smaller field and aperture in each case vertical, 
that is at right angles to the plane through the two systems forming a 
pair of binoculars. 
Another aspect of the present invention concerns a pair of binoculars 
having exit pupils in the form of horizontal slots. 
If these slots are long enough, say up to twenty but typically eight or 
nine millimeters, they will form linear eye rings so that there will 
generally be no need to adjust the separation of the two systems as in 
conventional binoculars to match the separation of the user's eyes.

DESCRIPTION OF EXEMPLARY EMBODIMENTS 
FIG. 1 illustrates the basic concept of an optical system according to the 
present invention. Light from an object, which may be infinitely distant 
so that the light is in the form of substantially parallel bundles of 
light rays, enters the optical system through an objective lens unit 11 
which may be a single lens and after a distance from that unit impinges on 
a first erecting mirror 12. The unit has a focal length such that it forms 
an image reasonably close to the mirror so as to minimize pupil 
aberrations due to the mirror. The mirror has an effective radius of 
curvature half to twice the said distance. The path of the light is folded 
into a Z-shape with the included angles up to 30.degree.. The light after 
reflection by the mirror 12 is directed onto a similar second erecting 
mirror 14 which redirects the light to be largely parallel to the original 
direction and forms an image to be viewed by an eyepiece lens means 15. 
This means directs the light substantially collimated through an exit 
pupil 16. The image to be viewed by the eyepiece lens means will be in 
many cases straddled by components 15a and 15b of the eyepiece lens means, 
which conveniently is below the mirror 12, in the manner of a Huygens-type 
eyepiece. One or each of the mirrors can be a simple concave element or a 
planar or even convex mirror combined with lenses in the manner of a 
Mangin mirror which largely performs as a concave mirror. To ensure that 
only the desired light reaches the eye, baffles 17a and 17b of opaque 
material can be provided, as can field stops 18a and 18b, in any 
advantageous positions. In this embodiment the centers of curvature of the 
mirrors' and the unit and means' surfaces can all lie on the same axis 13. 
A view on the objective lens unit and the back face of the mirror 14 or on 
the back face of mirror 12 and the eyepiece lens means would be as shown 
in FIG. 1a with the various items being semicircular or truncated as shown 
by chain-dot lines 11a and 14a. In FIG. 2, the field of view of the system 
is as shown in full lines 30 (for comparison the circular field obtained 
with conventional binoculars is shown in broken lines 31). It will be seen 
that the field with the present system is reduced from a wider overall 
field so as to leave a wider field of view in one direction (usually 
horizontal) and a narrower field of view in a direction transverse to said 
one direction. Chain dot line 30a shows the field when the unit mirrors 
and means are as shown with reference to lines 11a and 14a. The wider 
field of view is in a direction transverse to a direction in which the 
optical path is folded. There are two such directions of fold in some 
arrangements, a direction which is generally in the vertical plane 
containing the viewing axis and another direction slightly skew to that 
direction to allow for different spacings of the objective lens units and 
the user's eyes but such directions are substantially the same. The field 
of view with the present system is not centered with the wide overall 
field nor with the conventional field but this may not be significant. 
FIG. 3 shows the exit pupil 32 available with the present invention 
compared with the exit pupil 33 of conventional optical systems (chain-dot 
line 32a corresponds to lines 11a and 14a). This exit pupil is in the form 
of a slot up to 20 mm long but typically 8 to 9 mm. This yields advantages 
when a pair of optical systems are combined into binoculars when the exit 
pupils can be in line and there would be no need to allow for adjustable 
separation of the optical systems to accommodate different eye spacing of 
various users. The wide field of view and the wide exit pupil being in the 
same orientation does not lead to excessive coma since a user's eye will 
not accept light from the entire pupil but only from part of it. Any coma 
and the resulting anamorphic distortion due to using a non-central viewing 
position can be minimized by optical design. 
FIG. 4 illustrates the effect of making the angle of fold less tight so the 
optical axes of the objective lens unit 11 and the eyepiece lens means are 
not in the same horizontal plane but in spaced apart planes. This permits 
better baffles 17a and 17b and the mirrors, objective lens unit and 
eyepiece lens means to be more than semicircular as shown in FIG. 4a. The 
center 12' of curvature of mirror 12 would in this event be displaced by 
not more than 20% but preferably less than 10% of the radius of curvature 
from the viewing axis and the center 14' of curvature of mirror 14 
similarly displaced. Thus FIG. 4 represents a decentered system wherein 
the optical system does not have a single axis and FIG. 4a shows the 
appearance of this in the same manner as FIG. 1a. The systems as 
illustrated select from the overall view an object field from the lower 
part of that overall view (i.e. the system looks slightly down) so that 
the inverted image projected in the vicinity of the first mirror is wholly 
or mainly above the optical axis of the objective lens unit. This enables 
a vertically compact design which does not require fold angles greater 
than 30.degree.. 
FIG. 5 shows a different optical geometry. The objective lens unit 11 is 
below the viewing axis and the light path crosses itself. 
Optical systems according to the present invention lend themselves to 
combinations in pairs to form binoculars with the two systems parallel and 
with the larger field horizontal as shown in perspective in FIG. 6. 
FIG. 7 shows a suitable housing 51 for the two systems of a pair of 
binoculars. This housing is formed as a flattish box with an upper member 
52 pivoted to a lower member 53 by a hinge 54 so that the two members can 
be pushed together when the binoculars are not in use and then freed by a 
suitable catch (not shown) so that a spring (not shown) can push the 
members ajar for use into the illustrated condition. The upper member 
carries the objective lens units and the mirrors 12 with the lower member 
carrying the mirrors 14 and the eyepiece lens means, the mechanical 
arrangement should be such that the various components would not interfere 
with the two members being pushed together, thus for example in FIG. 1 the 
objective lens unit 11, the baffle 17a and the mirror 14 could not be as 
shown but the unit 11 must be further to the left or right as shown in the 
drawings. The hinge 54 can be used as a mounting or attachment point for a 
carrying strap 55. The upper member has a forward extension or extensions 
56 acting as a lens hood and a rearward extension or extensions 57 acting 
as a brow rest or rests to space the binoculars from the wearer's eyes. 
Miniature periscopes 58 may be provided above each of the user's eyes in 
or near the brow rest or rests to allow the user to see a direct view of 
the object as well as the magnified view which is seen below the direct 
view. The top of the housing can be sloped at least in part so that the 
top surface can be aligned with the distant object or the apparent object 
can be moved by means of a prism but such provision may not be necessary 
and aiming can be done by the periscopes. (The feel of the binoculars may 
not matter.) 
It has been said that the field as shown in FIG. 2 is centered below the 
center seen with conventional binoculars. This can be corrected as shown 
in FIG. 8 by using a pair of plane mirrors 19 and 20 arranged generally as 
a periscope as shown within the eyepiece lens means so as to allow the 
component 15b of the eyepiece lens means to be tilted. The spacing between 
the components 15a and 15b of the eyepiece lens means can be increased to 
accommodate these mirrors and the longer optical path produced if the 
mirror 14 projects an image further to the right. 
FIG. 9 shows a modification of FIG. 8 in which the direct view or periscope 
facility mentioned in relation to FIG. 7 is provided within the eyepiece 
lens means by inserting a compensating lens 21 between lenses 15b and 
either rotating the mirror 20 or inserting a new mirror 22. The mirror 20 
or mirror 22 would direct the optical path up to a permanent mirror 23 so 
forming a periscope. The component 15b with the lens 21 in place would 
have negligible optical effect so the direct view would not be magnified 
but the system would be readily switched to a magnified view by a 
mechanism (not shown) for inserting the lens 21 and either rotating mirror 
20 or inserting mirror 22. 
FIG. 10 shows an alternative to the housing shown in FIG. 7 in which two 
housing parts 25 and 26 can be extended telescopically from a shorter (as 
shown) non-use condition to a longer in-use condition. 
In relation to FIG. 7 mention has been made of a brow rest. The design of 
the optical system according to the present invention wherein the use of 
erecting mirrors which tend to introduce negative field curvature 
offsetting the positive field curvature normally present enables the power 
of optical surfaces in the eyepiece lens means to be reduced and for the 
user's eye to be further behind the eyepiece lens means. This enables the 
user to use his own spectacles which will correct for the user's optical 
faults such as long-sightedness, short-sightedness and astigmatism freeing 
the optical system according to the present invention from the need to be 
adjustable to cope with these. The main requirement is then to focus for 
near and distant objects and this could be done by arranging mirrors 12 
and 14, eyepiece lens means and/or objective lens unit, or the mirrors 19 
and 20, to be displaceable a small distance (less than 3 or 4 millimeters 
possibly) by a spring loaded action or actions. In binoculars the focusing 
action can be achieved by having both systems adjustable as one although 
separate focusing is possible. 
The optical systems can be fitted in the housing as illustrated in FIGS. 1 
to 5 or upside down. 
The optical performance can be optimized by the complexity and quality of 
the mirrors, objective lens unit and eyepiece lens means. In general 
spherical aberration and longitudinal color will be controlled at the 
objective lens unit and the mirror 14, lateral color and astigmatism 
within the eyepiece lens means, curvature of the field largely by 
balancing the effects of the mirror reflections and refractions in the 
unit and the means, and coma by arranging that the light deviates less at 
larger apertures and more at smaller apertures. The mirrors, unit and 
means can have more components than shown and may use complex forms such 
as doublet or triplet lenses. 
In each system the limited vertical field is obtained by the shapes of the 
mirror, objective lens unit, eyepiece lens means and/or the stops. Instead 
of stops, field lenses could be used. 
FIGS. 11 and 12 are views similar to FIG. 1 giving respectively a low and a 
high power system except that in FIG. 11 the mirror 12 is a Mangin mirror 
with the lens part of the mirror being that part of the eyepiece lens 
component 15a (which is cut away in FIG. 1) above the viewing axis and 
which part is silvered at 59 to form the mirror and in FIG. 12 the mirror 
12 is similarly formed but on an auxiliary lens 60 disposed between the 
mirror and the eyepiece means. These figures are referenced to give the 
various surfaces referred to in the accompanying tables given below. 
TABLE 1 
______________________________________ 
4X magnification version 
Radius of Axial 
Surface 
Curvature Separation 
Refractive 
V-value 
Number (mm) (mm) Index (constringence) 
______________________________________ 
61 77.691.sup.(A) 
9.72 1.49176 
57.45 
62 -573.515 
107.10 AIR 
63 248.631 
16.00 1.65713 
49.55 
64 -36.314 
3.00 1.71300 
53.83 
65 -236.263.sup.(B) 
(-)3.00 1.71300 
53.83 
66 -36.314 
(-)16.00 1.65713 
49.55 
67 248.631 
(-)110.10 AIR 
68 74.719 
(-)5.00 1.72825 
28.41 
69 105.464.sup.(B) 
5.00 1.72825 
28.41 
70 74.719 
110.10 AIR 
71 248.631 
16.00 1.65713 
49.55 
72 -36.314 
3.00 1.71300 
53.83 
73 -236.263 
60.00 AIR 
74 -129.938 
20.00 1.49176 
57.45 
75 -44.461.sup.(A) 
0.25 AIR 
76 30.359.sup.(A) 
20.00 1.49176 
57.45 
77 -141.178 
______________________________________ 
.sup.(A) Aspheric (conic) surfaces 
Surface No 61 Asphericity -1.00229 
Surface No 75 Asphericity -0.04036 
Surface No 76 Asphericity -1.17224 
.sup.(B) Reflecting surfaces 
This design provides a magnification of .times.4 with a horizontal field of 
view up to 18.degree., equivalent to 72.degree. in the image. The eye 
relief for the dimensions given in the table is about 25 mm and the eye 
`ring` has a horizontal dimension up to 15 mm. The Z-fold angles are 
approximately 12.degree.. Focusing may be obtained by movement of the 
central doublet. 
TABLE 2 
______________________________________ 
10X magnification version 
Radius of Axial 
Surface 
Curvature Separation 
Refractive 
V-value 
Number (mm) (mm) Index (constringence) 
______________________________________ 
81 105.820.sup.(A) 
9.72 1.49176 
57.45 
82 -219.202 
105.00 AIR 
83 107.666 
10.00 1.67269 
32.21 
84 -39.190 
3.00 1.71300 
53.83 
85 -471.032.sup.(B) 
(-)3.00 1.71300 
53.83 
86 -39.190 
(-)10.00 1.67269 
32.21 
87 107.666 
(-)108.00 AIR 
88 73.975 
(-)5.00 1.72824 
28.41 
89 99.256.sup.(B) 
5.00 1.72824 
28.41 
90 73.975 
108.00 AIR 
91 107.666 
10.00 1.67269 
32.21 
92 -39.190 
3.00 1.71300 
53.83 
93 -471.032 
3.00 1.51680 
64.17 
94 20.627 
80.00 AIR 
95 56.398.sup.(A) 
16.00 1.49176 
57.45 
96 -117.855 
0. 25 AIR 
97 56.398.sup.(A) 
16.00 1.49176 
57.45 
98 -117.855 
0.25 AIR 
99 37.638 
12.00 1.49176 
57.45 
100 350.631 
______________________________________ 
.sup.(A) Aspheric surfaces 
Surface No 81 
Conic -2.24327 A6 -2.022 .times. 10.sup.-11 
Surface No 95 
Conic -6.24301 
Surface No 97 
Conic -6.24301 
.sup.(B) Reflective surfaces 
This design provides a magnification of .times.10 with a horizontal field 
of view up to 8.degree., equivalent to 80.degree. in the image. The eye 
relief for the dimensions given is about 26 mm. The Z-fold angles are 
approximately 14.degree. and 7.degree.. 
Embodiments of the invention can provide a high quality viewing instrument 
with greater ease and comfort of use. 
In this specification and the appended claims an erecting mirror is one of 
a pair of mirrors, the second of which projects an image which is inverted 
with respect to the image received by the pair of mirrors. Normally each 
of the mirrors would have optical power effectively in the same manner as 
a concave mirror. 
While in the above description, the full available horizontal and vertical 
apertures and fields with be used, it would be possible to cut these down 
from the inherent apertures and fields available to give circular or other 
apertures and fields.