KR100277210B1 - Infrared afocal zoom telescope - Google Patents

Abstract

This optical system is a focal zoom telescope optical system for thermal imaging equipment used in the far infrared wavelength band of 7.5μm to 11.0μm, and it can continuously change the magnification from 2.0 to 8.1 times and satisfy the diffraction limit performance at all magnifications. Has performance. The telescope optical system consists of nine lenses in five lens groups, and the zoom objective is designed in the form of a telephoto so that the number of f is small at 1.3 and the length of the equipment is very short compared to the focal length. For the chromatic aberration correction, ZnSe is used for only one material out of the nine lenses, and all the other lenses are made of Ge, and only one surface is formed as an aspheric surface for aberration correction among the total curvature of nine lenses for nine lenses. , All other faces are spherical. In addition, since the focusing lens is disposed behind the zooming lens, even if the focusing lens is adjusted according to the distance to the object, the focusing lens is focused at all magnifications and the aberration fluctuation is small.

Description

Infrared zoom telescope optical system {INFRARED AFOCAL ZOOM TELESCOPE}

The present invention relates to a focal zoom telescope optical system for thermal imaging equipment having a diffraction limit of excellent performance in the far infrared region and capable of continuously changing the magnification.

The zoom objective of the infrared zoom telescope of the conventional 8μm to 12μm wavelength band is generally composed of four lens groups, each of the four lens groups plays a unique role for the zoom lens function. The refractive power arrangement of each lens group is usually designed in one of the forms of +,-,-, + or +,-, +, +, and each lens group can be composed of one or several lenses according to the design.

Conventional infrared zoom telescopes with such 4: 1 or higher zoom ratios have a relatively long overall length of the system. In addition, since the number of lenses is increased, or aspherical or diffractive surfaces must be used for aberration correction, processing of the lens is difficult and manufacturing costs are high. In addition, although the value of the modulation transfer function (MTF) has a good performance value near the center of the field of view, the performance has a disadvantage in that the performance sharply decreases toward the edge.

Accordingly, it is an object of the present invention to provide an afocal zoom telescope optical system with low aberration fluctuations by appropriately selecting material of the optical element and proper shape and arrangement of spherical surfaces in order to reduce aberration fluctuations.

Still another object of the present invention is to provide an infrared focal telescope optical system capable of 2 to 8.1 times magnification conversion and having optical performance of diffraction limit at all magnifications.

1 is a cross-sectional view showing the arrangement of an optical element at 8.1 times the magnification of the zoom telescope optical system according to the present invention.

Fig. 2 is a cross-sectional view showing the arrangement of optical elements at 5 times magnification in the zoom telescope optical system according to the present invention.

3 is a cross-sectional view showing the arrangement of the optical elements at 2.0 times the low magnification in the zoom telescope optical system according to the present invention.

4 shows an on-axis modulation transfer function with respect to the center wavelength (9 μm) of the zoom telescope optical system according to the present invention when the magnification is 8.1 times.

Hereinafter, the configuration and operation of the infrared focus free zoom telescope optical system according to the present invention will be described with reference to the accompanying drawings showing an embodiment.

The optical system according to the present invention is composed of five lens groups and a total of nine lenses, and the zoom objective portion is designed in a telephoto form in which the distance from the front convex surface of the lens system to the image surface is shorter than the focal length. Small at 1.3 and very short in overall length compared to the focal length. The objective part of the telescope is composed of four lens groups to play a zooming role, and the eyepiece is composed of one lens group. Features of the optical system constituting the infrared zoom telescope according to the present invention are as follows.

The first lens group is placed in front of the zoom objective and is a single lens 1 having positive refractive power. As can be seen in the figure, the diameter of the lens is about Φ174mm, the largest of the lenses constituting the telescope, and is fixed in consideration of the size and weight, sealing of equipment, and the like. The lens is also designed to intentionally over-correct spherical aberration to reduce spherical aberration resulting from the use of relatively large apertures at high magnification.

The second lens group is composed of single lenses 2 and 3 having two negative refractive powers. The main role of this group of lenses is to move along the optical axis and change the magnification of the telescope, where the relative distances of lenses 2 and 3 are fixed. This lens group is designed so that the spherical aberration is under-corrected at high magnification, thereby compensating for over corrected spherical aberration of the first lens group. In addition, for the chromatic aberration correction of the entire system, the material of the lens 2 uses ZnSe having a larger color dispersion than other lenses, and the remaining eight lenses are all made of Ge material. Since the optical system used in the infrared region uses a wider wavelength band than visible light, the correction of chromatic aberration is an important problem. Therefore, in this infrared zoom telescope, ZnSe lens with large dispersion coefficient is used for lens 2 with negative refractive power for chromatic aberration correction, and Ge lens is used for the rest. Here, the ZnSe lens for chromatic aberration correction is not necessarily limited to lens 2, but it is most effective for the chromatic aberration correction to use for lens 2.

The third lens group is a single lens having positive refractive power and, like the second lens group, moves along the optical axis and compensates for the movement of the focal plane according to the change in magnification.

The fourth lens group is composed of two single lenses 5 and 6 having positive refractive power. This lens group acts as a focusing lens that focuses as the distance to the object changes, and the relative distance between 5 and 6 is fixed. That is, since the focusing lens is disposed behind the zooming lens, the focusing lens is focused at all magnifications and the aberration fluctuation is small even when the focusing lens is adjusted. In addition, the infrared zoom telescope is designed to aspherically design the object-side surface of the lens 5 in order to correct not only normal spherical aberration but also relatively large axle caused by large telephoto ratio. ) Are all spherical. In this case, the aspherical lens is not necessarily limited to lens 5, but in order to effectively correct the axle and spherical aberration described above, it is necessary to apply the aspherical surface to a surface far from the stop or pupil of the optical system. In this respect, it is most effective to form the object-side surface of the lens 5 as an aspherical surface.

The fifth lens group is composed of three single lenses 7, 8 and 9, and collimates the image formed on the focal plane by the zoom objective to serve as an eyepiece of a focal-free telescope system.

As described above, although the infrared zoom telescope according to the present invention has a caliber of 174 mm, the overall length of the telescope is small and compact in and around 309 mm, and the optical performance of the telescope is also shown in the diffraction limit as shown in the multi-wavelength modulation transfer function of FIG. You will have close performance.

In addition, by incorporating ZnSe with a large dispersion coefficient, one of many lenses corrects chromatic aberration, and by simply correcting spherical aberration and axle of optical system by using only one lens surface as an aspherical surface, it is inexpensive, simple and has excellent optical performance. Can provide a telescope

Claims (5)

An infrared zoom telescope optical system for thermal imaging equipment used in a far infrared wavelength band of 7.5 μm to 11.0 μm, the optical system comprising: a fixed first lens group having positive refractive power and allowing to form an image of an object; A second lens group moving along the optical axis of the optical system to change the magnification of the telescope with negative refractive power; A third lens group moving along the optical axis to have a positive refractive power and to store the shop of the optical system at a fixed position when the magnification of the telescope changes with the movement of the second lens group; A fourth lens group having positive refractive power and moving along the optical axis to adjust a focus according to a change in distance to an object; And a fifth lens group fixed to collimate the image formed on the focal plane by the first to fourth lens groups, wherein the magnification of the optical system is moved by moving the second lens group and the third lens group. Infrared zoom telescope optics, characterized in that can vary continuously from 2.0 to 8.1. The method of claim 1, wherein the first lens group is composed of one block lens, the second lens group is composed of two or less concave lenses, and the third lens group is composed of one convex lens. And the fourth lens group is composed of two or less convex lenses, and the fifth lens group is composed of three or fewer convex lenses. 2. The method according to claim 1, wherein the zoom objectives of the first to fourth lens groups are configured in the form of a telephoto, so that the number of F is small as 1.3, and even the maximum magnification of the optical system is very short compared to the focal length. Infrared zoom telescope optical system. The infrared zoom telescope optical system according to claim 2, wherein the lens on the object side of the two lenses of the second lens group is made of ZnSe, and all other lenses are made of Ge. The infrared zoom telescope optical system according to claim 2, wherein the object-side curved surface of the lens on the object side of the fourth lens group is formed as an aspherical surface for aberration correction, and all the remaining lens curved surfaces are spherical surfaces.

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