RU2446420C1 - Catadioptric system - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: system includes two hyperbolic mirrors lying on a beam path, the first facing the object space with its concave side, and the second facing the first with its convex side. The first mirror has a centre hole with diameter D₁. D₁≤D₂, where D₂ is the clear aperture of the second mirror. Near the focal plane of the lens there is a lens corrector, all of the working surfaces of which are spherical and the clear aperture D₃ is such that D₃≤D₁. The first mirror has optical power φ1 such that -3.2≤φ1≤-3.0, with eccentricity squared ℮₁ ² such that 1.00≤℮₁ ²≤1.26. The second mirror has optical power φ2 such that -8.0≤φ2≤-7.0, with eccentricity squared ℮₂ ² such that 4.5≤℮₂ ²≤6.5. The distance d_1-2 between the first and second mirrors is such that 0.3f≤d_1-2≤-0.2f, where f is the focal length of the system. The corrector is in form of a meniscus lens whose convex surface faces the second mirror, with optical power φ3 such that -3.4≤φ3≤-2.8, and lies at a distance d_2-3 from the second mirror, which is such that 0.16f≤d_2-3≤0.3f. The thickness h of the meniscus of the corrector is such that 0.004f≤h≤0.006f.

EFFECT: obtaining a high-quality image in a wide spectral range, which is uniform on the field of view using standard manufacturing, assembling and adjustment techniques.

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Description

The present invention relates to the field of optical instrumentation and can be used to create lenses designed to produce high-quality images suitable for use in a wide range of optical spectra.

In optical instrumentation, mirrored lenses are common. They have several advantages, including the relative ease of manufacture, including large-sized products, the ability to work in a wide spectral range. Their disadvantage is the presence of irreparable field aberrations, including coma, astigmatism, image curvature, and distortion. Usually, afocal correctors from refracting optical elements are used to correct them. The corrector allows you to eliminate field aberrations, but complicates the design, manufacturing technology, assembly and alignment, reduces transmission, introduces its own distortion.

Known catadioptric (catadioptric - optical system, including both mirrors and elements with light-refracting surfaces, such as a mirror-lens system. Physical encyclopedic dictionary. M: Soviet Encyclopedia, 1983, p. 245) system [Application WO 2007067701, IPC G02B 17/08, publ. 06/14/2007.], Including a two-mirror lens and a two-lens corrector. The first correction element is located in parallel with the rays in front of the lens and is made with a Central hole. The radiation flux passed through it falls onto a mirror, which is also made with a central hole. The second correction element is located in the Central part of the first correction element in a converging stream of radiation reflected from the first mirror and has a rear reflective surface that performs the functions of the second mirror. The radiation flux from the second correction element is collected in the focal plane. The disadvantage of this system is that its dimensions are limited by the availability of material for the first correction element, which is especially significant in situations when it is made of crystalline materials. Therefore, the system cannot be made large. Accordingly, the spectral range is limited.

Known for our chosen as a prototype catadioptric system [Sky and telescope. Ed. VGSurdin, M .: Fizmatlit, 2009, p.120], including a mirror lens made of two hyperboloid-shaped mirrors with an afocal two-lens corrector. The first (In mirror lenses, the first is considered to be the mirror on which the light initially hits, that is, the first along the rays. N.N. Michelson. Optical telescopes. Theory and construction. M: Nauka, 1976, p. 231.) in the direction of the rays, the mirror is concave in the direction of the space of objects, in the center of it a hole is made, the diameter of which is less than or equal to the diameter of the second along the rays of the mirror, convex side to the first mirror. In the converging radiation flux reflected from the second mirror, there is a two-lens corrector, transparent in the working spectral range. As a corrector material, optical glass was used.

The main disadvantages of the system include a low aperture, the presence of residual field aberrations, of which the most significant field curvature and astigmatism. The presence of four optical surfaces in the corrector implies the necessity of applying antireflection coatings, which introduces additional restrictions on the working spectral range and complicates the manufacturing, assembly, and alignment technology.

We have proposed a high-speed catadioptric system that provides high-quality images in a wide spectral range, uniform in field of view in an angle of ± 30 '. For the system, standard manufacturing, assembly, and alignment technologies are applicable.

This technical result was obtained by us when in a catadioptric system comprising a mirror lens of two mirrors located along the rays, the optical surfaces of which are hyperbolic, the first is turned with the concave side to the space of objects, the second is turned to the first convex side, the first mirror is made with a central hole , the diameter D ₁ of which is selected from the condition D ₁ ≤D _2, where D ₂ - light diameter of the second mirror, on the optical axis of the lens, near its focal plane located manufacture of a material transparent in the working spectral range, the corrector lens, all the working surface of which spherical, and light diameter D ₃ is selected from the condition D ₃ ≤D _1, new is the fact that the first mirror has an optical lens effect φ1, selected from conditions -3 , 0≤φ1≤-3.2, with the square of the eccentricity e ₁ ² selected from the condition 1.00≤e ₁ ² ≤1.26, the second mirror has an optical power φ2 selected from the condition -8.0≤φ2≤-7 0, with the square of the eccentricity e ₂ ^2, selected from the conditions 4,5≤e _{February ¹} ≤6,5, distance d _1-2 between the first and second mirrors is chosen from the condition Ia -0,2f≤d _1-2 ≤-0,3f, where f - focal length of the optical system, an equalizer is made in the shape of the meniscus facing the convex surface of the second mirror with an optical power φ3, selected from conditions -2,8≤ φ3≤-3.4, located at a distance of d _2-3 from the second mirror selected from the condition 0.16f≤d _2-3 ≤0.3f, while the thickness h of the corrector meniscus is selected from the condition 0.004f≤h≤0.006f .

The focal length is usually given in mm.

Approaches to the mutual coordination of the characteristics of optical elements: their relative position on the basis of the selected optical scheme and focal length are known [1, 2, 3].

The authors are not aware of high-aperture catadioptric systems using a single-element lens corrector with optical power, which provides high-quality images over the entire field of view in the transparency range of the corrector material, including those made of crystalline optical materials.

The figure shows the optical scheme of the proposed system, where the first along the rays of the mirror is 1, the second along the rays of the mirror is 2, the corrector is 3;

F 'is the focal plane of the optical system;

d _1-2 is the distance from the first mirror to the second;

d _2-3 is the distance from the second mirror to the corrector;

h is the thickness of the corrector meniscus.

→ - the course of the rays.

The lens works as follows.

Optical radiation coming from a distant object, reflected successively from mirrors 1, 2 and refracted in the lens corrector 3, is focused in the image plane F 'of the optical system. After successive reflection from the first and second mirrors, an axisymmetric converging radiation flux is formed. Traditionally, when constructing optical imaging systems, a mirror assembly is used as an image-building assembly. The correction unit performs the auxiliary function of correcting the residual field aberrations of the image.

We have proposed a different approach to solving the problem. In the cathodioptric system we proposed, the image is built simultaneously by two nodes, both mirror and corrective lens. The specially selected optical forces of both mirrors φ1, φ2, the distances between them d _1-2, and the values of the squared eccentricity e ₁ ² , e ₂ ² only correct spherical aberration, distortion, and partially - coma. For the first time, we have chosen a single-element, specially designed lens corrector, with optical power, which together with a special-purpose mirror unit, on the one hand takes part in image construction, and on the other hand corrects residual field aberrations: coma, astigmatism and image curvature, which are completely fix the traditional approach fails. Note that the intrinsic chromatic aberrations and distortion introduced by such a single-element lens corrector are small. In particular, for a lens corrector made of barium fluoride in the spectral range Δλ = 0.4-0.8 μm, the residual longitudinal chromatism introduced by the corrector is 8 μm. The residual chromaticity of the increase for the angle of view up to ± 30 'is 16 μm, the residual distortion does not exceed 1%. Effective correction of all the above aberrations made it possible to increase the light diameters with a minimum length of the optical system, which in turn increased the aperture ratio and the relative aperture of the lens with a significantly expanded angle of view, which was previously unattainable. Our single-element lens corrector allows us to do without antireflective optical coatings, which ensures its increased resistance to external influences and a high transmittance of the lens in a wide operating spectral range, which, for example, for barium fluoride is 0.2-12.0 μm.

The geometric characteristics of the lens, normalized to a focal length of 1, with a relative aperture of 1: 3 are shown in table 1.

Table 1 Optical element number in the figure Optical power φ Gaps between optical elements, mm Relative light diameter one φ1 = -3.103 d _1-2 = -0.233 1,0 2 φ2 = -7.420 d _2-3 = 0.217 0.3 3 φ3 = -3.118 - 0,087

An example of a specific implementation.

Based on the selected spectral range, the required angle of view, the size of the photodetector and image quality, the initial requirements for the focal length, relative aperture, overall dimensions of the lens and the amount of residual aberration are determined. Based on the known dependencies between the optical characteristics and the shape of the optical elements, a principle optical scheme of the optical system is selected and optical calculation is performed. The characteristics of a high-speed mirror lens with a single-element lens corrector made of barium fluoride, in which the entrance pupil is located on the surface of the first mirror, suitable for operation in the spectral range from 0.4 to 12.0 μm, for the diameter of the entrance pupil of 600 mm, are given in tables 2, 3, 4 and 5. The choice of material of the lens corrector of the lens is due to the need for the required transmission and correction of chromatic aberration in the working spectral range, as well as the availability of a single crystal with bubbled dimensions.

table 2 Focal length mm 1800 Relative hole 1: 3 Angular field of view, hail one Diagonal linear field of the image, mm 31.8 Screening coefficient of the first mirror by the second 0.3 Mirror Reflective Material aluminum The rear segment from the lens corrector lens to the focal plane, mm 72.04

The overall characteristics of the optical system are shown in Table 3. The hyperbola equation for the surface of the first mirror is: F (z, y) = y ² + 2320z-0.11z ² = 0, where y is the position of the point along the abscissa axis, z is the position along the ordinate axis. The radius at the top of the surface is R ₀ = -1160 mm. The squared eccentricity e ₁ ² = 1.11. The hyperbola equation for the surface of the second mirror is F (z, y) = y ² + 970.23z-4.53z ² = 0. The radius at the top of the surface is R ₀ = -485.15 mm. The square of the eccentricity e ₂ ² = 5.53.

Table 3 The name of the optical element Serial number
surface Light diameter mm Air gaps, mm Mirror one 600 di.2 = - 420.0 Mirror 2 180 d2.3 = 390.4 Meniscus 3 51.5 meniscus thickness h = 8.8 four 45.5

The screening coefficient of the first mirror by the second is 0.3, which ensures acceptable transmission losses of incident radiation and high image contrast. The energy distribution in the scattering circles over the field for the visible spectral range from 0.4 to 0.8 μm for various viewing angles ω in comparison with theoretically possible determined by diffraction limits is shown in Table 4.

Table 4 Energy concentration Diameter of scattering circles, mm limit ω = 0 ° 00 ' ω = ± 0 ° 15 ' ω = ± 0 ° 30 ' 80% 0.006 0.009 0.014 0.027 83.8% 0.007 0.011 0.015 0.031 90% 0.009 0.016 0.018 0.043

The energy distribution in the scattering circles over the field for the “far” spectral range from 8.0 to 12.0 μm is shown in Table 5.

Table 5 Energy concentration Diameter of scattering circles, mm limit ω = 0 ° 00 ' ω = ± 0 ° 15 ' ω = ± 0 ° 30 ' 80% 0.090 0.090 0.092 0.096 83.8% 0.098 0.098 0.100 0.106 90% 0.132 0.134 0.134 0.159

The advantages of the claimed catadioptric system are high image quality comparable with diffraction, its uniformity across the field in the visible, "average" and "far" IR spectral ranges at high aperture and a significant field of view angle, the ability to manufacture large products, the availability of standard manufacturing techniques, assembly and alignment techniques.

Literature

1. V.N. Churilovsky. Theory of chromatism and third-order aberrations. L .: Mechanical engineering, 1968, p. 291-304.

2. G. G. Slyusarev. Calculation of optical systems. L .: Mechanical engineering, 1975, p. 322-360.

3. N.N. Michelson. Optical telescopes. Theory and construction. M .: Nauka, 1976, p. 343-359.

Claims (1)

A catadioptric system, comprising a mirror lens of two mirrors located along the rays, the optical surfaces of which are hyperbolic, the first is turned with the concave side to the space of objects, the second is facing the first convex side, the first mirror is made with a central hole, the diameter D _{1 of} which is selected from condition D ₁ ≤D _2, where D ₂ - light diameter of the second mirror, on the optical axis of the lens near its focal plane is made of material transparent in the working spectral Range, the corrector lens, all of which spherical seating surfaces, and the effective diameter D ₃ is selected from the condition D ₁ ≤D _3, characterized in that the first mirror has an optical lens effect φ1, selected from conditions -3,2≤φ1≤-3, 0, with the squared eccentricity e ₁ ² selected from the condition 1.00≤e ₁ ² ≤1.26, the second mirror has the optical power φ2 selected from the condition -8.0≤φ2≤-7.0, with the squared eccentricity e ₂ ² selected from the condition 4.5≤e ₂ ² ≤6.5, the distance d _1-2 between the first and second mirrors is selected from the condition -0.3f≤d _1-2 ≤ -0.2f, where f is the focal optical distance Coy system corrector is made in the form of a meniscus facing a convex surface of the second mirror with an optical power φ3, selected from conditions -3,4≤φ3≤-2,8, located at a distance d _2-3 of the second mirror, is selected from the condition 0.16f≤d _2-3 ≤0.3f, and the corrector meniscus thickness h is selected from the condition 0.004f≤h≤0.006f.