Optical: systems and elements – Lens – Selective magnification by exchanging or adding a lens...
Reexamination Certificate
1999-04-27
2001-04-03
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
Selective magnification by exchanging or adding a lens...
C359S672000, C359S793000, C359S794000, C359S795000, C359S827000
Reexamination Certificate
active
06212016
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an objective lens system of a telescope, used (for example) in an astronomical telescope and the like, which exhibits optimum optical performance both in an observing mode through the eyepiece, and in a photographing mode to photograph a target object.
2. Description of the Related Art
An optical system, for example, an objective lens system for an astronomical telescope, including a combination of a positive lens element and a negative lens element made of glass materials having different dispersion values, has been widely known and used. An objective lens system of a telescope in general has a long focal length, and it is therefore necessary to correct axial chromatic aberration. In particular, it is essential to correct the secondary spectrum of axial chromatic aberrations at a higher magnification in the observing mode through the eyepiece. In order to correct the above-mentioned secondary spectrum of axial chromatic aberration, there have recently been many attempts to employ fluorite and low dispersion special glass to form a positive lens element. However, these prior arts have caused an increase in cost because a positive lens element in the first lens group having a large-diameter has to be made of an expensive glass material such as fluorite or low dispersion special glass and the like.
On the other hand, along with an increase in popularity of astronomical photography, an objective lens system of an astronomical telescope has been required to have a wider angle-of-view. In connection with this trend, an objective lens system disclosed in Japanese Unexamined Patent Publication No. Sho-57-93309 should be noted as an example. According to this patent publication, by attaching a detachable field-curvature correcting optical system to an objective lens system of an astronomical telescope in which axial chromatic aberrations are sufficiently corrected, the astronomical telescope can be used in both the observing mode through the eyepiece and the photographing mode to photograph a target object. More specifically, a wider angle-of-view required for the photographing mode is attained by positioning a detachable field-curvature correcting lens element in the vicinity of the image plane of the first lens group in which axial chromatic aberrations are sufficiently corrected by the combination of a negative lens element and a positive lens element.
However, the above-explained example has also caused an increase in cost because the field-curvature correcting lens element has to additionally be attached to the first lens group after the axial chromatic aberrations have been sufficiently corrected.
SUMMARY OF THE INVENTION
The object of the present invention is to provide, at low cost, an objective lens system of a telescope in which axial chromatic aberration is sufficiently corrected in an observing mode through the eyepiece, and field curvature attributable to a wider angle-of-view is corrected by a field-curvature correcting lens element in a photographing mode to photograph a target object.
In order to achieve the above-mentioned object, according to the present invention, there is provided a telescopic objective lens system comprises a positive first lens group and a second lens group, in this order from the object. The second lens group comprises a first sub-lens group and a second sub-lens group, wherein the first sub-lens group is combined with the first lens group to be used for correcting axial chromatic aberration, the second sub-lens group is combined with the first lens group to be used for correcting field curvature; and wherein the combination of the first lens group and the first sub-lens group, and the combination of the first lens group and the second sub-lens group can be alternatively selected.
As a mechanical structure, the first lens group is formed as a first lens unit, the first sub-lens group is formed as a first sub-lens unit, and the second sub-lens group is formed as a second sub-lens unit, wherein detachable coupling mechanisms are preferably provided on the image-side of the first lens unit, and on the object-side of both the first sub-lens unit and the second sub-lens unit. Furthermore, it is preferable to constitute each of the first lens group, the first sub lens group and the second sub-lens group by a single positive lens element and a single negative lens element.
Each of the first lens group and the first sub-lens group of the telescopic objective lens system according to the present invention is composed of a positive lens element and a negative lens element which are made of glass materials having different dispersion values, and the telescopic objective lens system satisfies the following conditions:
&ngr;
I+
<80 (1)
&ngr;
II+
>70 (2)
1.1<
f
I
/f<
2.0 (3)
wherein
&ngr;
I+
designates the Abbe number of the glass material forming the positive lens element in the first lens group;
&ngr;
II+
designates the Abbe number of the glass material forming the positive lens element in the second lens group;
f
I
designates the focal length of the first lens group;
f designates the focal length of the entire lens system.
The first lens group and the second sub-lens group of the telescopic objective lens system according to the present invention is composed of a positive lens element and a negative lens element which are made of glass materials having different dispersion values, and the telescopic objective lens system satisfies the following conditions:
n
II+
>1.55 (4)
n
II−
II+
<1.0 (5)
wherein
n
II+
designates the refractive index, with respect to the d line, of the positive lens element in the second lens group;
n
II−
designates the refractive index, with respect to the d line, of the negative lens element in the second lens group.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 10-118560 (filed on Apr. 28, 1998) which is expressly incorporated herein by reference in its entirety.
REFERENCES:
patent: 4206971 (1980-06-01), Hamanishi et al.
patent: 4239340 (1980-12-01), Ogino
patent: 4394071 (1983-07-01), Yamada
patent: 4653870 (1987-03-01), Imaizumi et al.
patent: 4768868 (1988-09-01), Wakamiya et al.
patent: 4830473 (1989-05-01), Kudo
patent: 5082360 (1992-01-01), Sato et al.
patent: 5589976 (1996-12-01), Kanai et al.
patent: 5729390 (1998-03-01), Abe
patent: 57-93309 (1982-06-01), None
Asahi Kogako Kogyo Kabushiki Kaisha
Epps Georgia
Greenblum & Bernstein P.L.C.
Lester Evelyn A.
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