Large field of view off-axis four-mirror optical system based on freeform surface

Abstract

The application provides a large-view-field off-axis four-mirror optical system based on free-form surface, which solves the problems of large system volume, difficult assembly and adjustment, difficult correction of wide-view-field astigmatism and field curvature, and limited aberration balance capability of the existing off-axis four-mirror system. The system comprises a primary mirror, a secondary mirror and a third mirror; light emitted by a light source is reflected by the primary mirror, the secondary mirror, the third mirror and the secondary mirror again in sequence and then converges on a focal plane; the primary mirror is an extended polynomial mirror, the secondary mirror is an even aspheric mirror, and the third mirror is a quadratic surface mirror; at least one of the primary mirror, the secondary mirror and the third mirror adopts an XY polynomial free-form surface with a highest order of 9, and the equation of the free-form surface is: wherein z is a sag of the surface, c is a curvature at a vertex of the surface, k is a quadratic surface coefficient, and r is a radial height; C j is an x m y n term coefficient; the superscripts m and n are non-negative integers, and represent orders of x and y respectively.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of space optics, and particularly relates to a large field of view off-axis four-mirror optical system based on free-form surface. BACKGROUND

[0002] In recent years, with the deepening of space remote sensing applications, the requirements for the indicators such as the width, spatial resolution, spectral range, spectral resolution, time resolution and calibration accuracy of hyperspectral remote sensing are higher and higher. The larger the field of view coverage of the instrument, the shorter the revisit period, and the higher the time resolution. Therefore, the large field of view miniaturized high-resolution imaging optical system gradually becomes an urgent demand for space remote sensing. In the design of space optical pre-lens, the optical system mainly has several types such as transmission type, coaxial reflection type and off-axis reflection type. The off-axis reflection type has the characteristics of large field of view and long focal length, and is widely used.

[0003] With the progress of aspheric surface processing technology, the off-axis four-mirror system is also widely used. The optical elements used in the off-axis four-mirror system are usually aspheric surfaces. For example, the patent with the patent name of a large field of view off-axis four-mirror lens device and the publication number of CN107300759A has four aspheric surfaces. The patent with the patent name of deep ultraviolet spectral off-axis four-mirror optical imaging system and the publication number of CN106646839A also uses aspheric surface design. The off-axis four-mirror system usually adopts four-mirror design, and has the problems of large system volume and difficult assembly and adjustment.

[0004] The traditional spherical mirror makes the whole optical system structure complex for large field of view design, is difficult to reduce the volume and mass, and the aberration is difficult to correct, and the corresponding system structure is relatively complex. Compared with the traditional spherical system, the aspheric surface system can correct the primary spherical aberration, coma and astigmatism. However, the surface shape of the aspheric surface has rotational symmetry (axial symmetry), and the curvature radii in the meridian and sagittal directions are not independent of each other, so that the wide field of view astigmatism and field curvature are difficult to correct, and the aberration balance ability is limited, which is difficult to meet the imaging quality requirements. SUMMARY

[0005] In order to solve the technical problems that the optical elements of the existing off-axis four-mirror system adopt four-mirror design, the whole system has the problems of large volume and difficult assembly and adjustment, and the aspheric surface has the problems that the wide field of view astigmatism and field curvature are difficult to correct, the aberration balance ability is limited, and the imaging quality requirements are difficult to meet, the present application provides a large field of view off-axis four-mirror optical system based on free-form surface.

[0006] In order to achieve the above-mentioned purpose, the technical scheme provided by the present application is:

[0007] A large field of view off-axis four-mirror optical system based on free-form surface, which is characterized by comprising a primary mirror, a secondary mirror and a third mirror; light emitted by a light source is reflected by the primary mirror, the secondary mirror, the third mirror and the secondary mirror again in sequence and converges on a focal plane;

[0008] The primary mirror is an extended polynomial mirror, the secondary mirror is an even aspheric mirror and the third mirror is a quadratic mirror.

[0009] At least one of the primary mirror, the secondary mirror and the third mirror adopts a highest 9th order XY polynomial free-form surface, and the highest 9th order XY polynomial free-form surface equation is as follows:

[0010]

[0011] In the formula, z is the sag of the surface, c is the curvature at the vertex of the surface, k is the quadratic surface coefficient, and r is the radial height.

[0012]

[0013] C j is the coefficient of the x m y n term, and the superscripts m and n are non-negative integers, representing the orders of x and y respectively.

[0014] Further, the primary mirror adopts the highest 9th order XY polynomial free-form surface.

[0015] The radii of curvature of the primary mirror, the secondary mirror and the third mirror adopt positive, positive and negative structures respectively.

[0016] Further, the radius of curvature of the primary mirror is 182.05 mm.

[0017] The radius of curvature of the secondary mirror is 256.9 mm.

[0018] The radius of curvature of the third mirror is -153.01 mm.

[0019] Compared with the prior art, the present application has the following advantages:

[0020] 1. The optical system of the present application only adopts three mirrors to realize the requirement of large field of view and small volume imaging, the three mirrors realize four-mirror design, reduce the difficulty of assembly and adjustment, and can reduce the system volume and cost; at least one of the three mirrors adopts the highest 9th order XY polynomial free-form surface, which can correct the off-axis aberration caused by the asymmetry of the meridional plane and the sagittal plane, and improve the imaging quality.

[0021] 2, the third mirror of the application can be used as a diaphragm to limit the size of the on-axis light beam, the focal length of the system is 45mm, the F number is 4, the total length of the system is less than 75mm, and the system field of view angle reaches 60°*30°, and the volume is 74mm*93mm*62mm. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a structural schematic diagram of the large field of view off-axis four-mirror optical system based on freeform surface of the application;

[0023] Figure 2 is an imaging MTF diagram of the large field of view off-axis four-mirror optical system based on freeform surface of the application;

[0024] Figure 3 is a point array diagram of the large field of view off-axis four-mirror optical system based on freeform surface of the application;

[0025] In the drawings, the reference signs are as follows:

[0026] 1-main mirror, 2-secondary mirror, 3-third mirror, 4-focal plane. DETAILED DESCRIPTION

[0027] The content of the application is further described in detail below in combination with the drawings and specific embodiments.

[0028] Compared with the traditional rotationally symmetric spherical surface and aspherical surface, the freeform surface has great design freedom, and the optical surface type can be randomly combined by asymmetric, irregular and complex freeform surfaces, so that the system structure can be simplified to the greatest extent, integration can be realized, and the system performance can be improved to the greatest extent, therefore, the application designs a higher performance freeform imaging system to realize miniaturization and light weight of the wide field of view imaging system.

[0029] In view of the current development demand of large field of view high resolution space optics, as shown in the drawings, the application provides a large field of view off-axis four-mirror optical system based on freeform surface. Figure 1 The application includes a main mirror 1, a secondary mirror 2 and a third mirror 3; the light emitted by the light source is reflected by the main mirror 1, the secondary mirror 2 and the third mirror 3 in turn, reflected by the secondary mirror 2 again, and then converged on the focal plane 4; the third mirror 3 can be used as a diaphragm. The system only uses three mirrors to realize the demand of large field of view and small volume imaging; the three mirrors are coaxial, without eccentricity and inclination.

[0030] The main mirror 1 is an extended polynomial mirror, the secondary mirror 2 is an even aspherical mirror, and the third mirror 3 is a quadratic surface mirror, and the quadratic surface mirror is set as a diaphragm, the field of view is off-axis, and the three mirrors are coaxial, forming an off-axis four-mirror optical system.

[0031] The embodiment is designed by using a right-hand coordinate system, and in the meridional plane, Figure 1 The Z axis is from left to right, and the Y axis is perpendicular to the Z axis. At least one of the primary mirror 1, the secondary mirror 2 and the third mirror 3 is a highest 9th order XY polynomial free surface, and preferably only the primary mirror 1 is a highest 9th order XY polynomial free surface.

[0032] The off-axis four-mirror optical system designed in the embodiment is symmetrical in the X direction, and only the even power terms of the X terms are selected for optimization of the primary mirror 1, and the odd power terms are 0. At the same time, the first order term of Y in the polynomial has an impact on the eccentricity and tilt of the surface, and the value of the first order term of Y is set to 0 in the design, so that the primary mirror 1 adopts a highest 9th order XY polynomial free surface equation:

[0033]

[0034] In the formula, z is the surface height, c is the surface curvature, k is the quadratic surface coefficient, and r is the radial height.

[0035]

[0036] C j is x m y n the coefficient of the term; the superscripts m and n are non-negative integers, representing the orders of x and y, respectively.

[0037] The curvature radii of the lenses in the off-axis four-mirror optical system of the embodiment are in a positive-positive-negative structure, and the specific parameters are shown in Table 1.

[0038] Table 1 Specific parameters of the curvature radii of the lenses in the off-axis four-mirror optical system of the embodiment

[0039] Lens Radius of curvature / mm 1 182.05 2 256.9 3 -153.01

[0040] The optical system of the embodiment sets the quadratic surface mirror as a stop to limit the size of the on-axis light beam; adopts a field off-axis mode to make the light obliquely incident on the primary mirror 1, and makes the system form four reflections through two reflections on the even-order aspheric mirror; the focal length of the system is 45 mm, the F number is 4, the total length of the system is less than 75 mm; and the field of view angle of the system reaches 60°x30°, and the volume is 74mmx93mmx62mm.

[0041] Figure 2 The imaging MTF diagram of the optical system of the embodiment is shown in FIG. 6, and at a cutoff frequency of 34 lp / mm, the central field of view MTF is greater than 0.85, the edge field of view MTF is greater than 0.73, and the imaging contrast is good.

[0042] Figure 3The figure is the point array diagram of the imaging system of the embodiment, the system detector pixel size is 15 μm, the RMS radius is less than 4 μm, and the aberration correction is good.

[0043] The optical system of the embodiment can be applied to a front-mounted telephoto objective lens which needs wide field of view, miniaturization and light weight design, and can cover the visible to near-infrared working waveband of 0.4-0.95 μm.

[0044] The above only describes the preferred embodiments of the application, and does not limit the technical solutions of the application thereto, and any deformation made by the person skilled in the art on the basis of the main technical concept of the application belongs to the technical category to be protected by the application.

Claims

1. A large field of view off-axis four-mirror optical system based on freeform surface, characterized in that: The light emitted by the light source is reflected by the primary mirror (1), the secondary mirror (2) and the third mirror (3) in turn, and then is reflected by the secondary mirror (2) again before converging on the focal plane (4); The primary mirror (1) is an extended polynomial mirror, the secondary mirror (2) is an even aspheric mirror, and the third mirror (3) is a quadratic surface mirror; At least one of the primary mirror (1), the secondary mirror (2) and the third mirror (3) is an XY polynomial free surface with a highest order of 9, and the equation of the XY polynomial free surface is as follows: In the equation, z is the height of the surface, c is the curvature at the vertex of the surface, k is the quadratic surface coefficient, and r is the radial height; C j For x m y n Term coefficient; superscripts m and n are non-negative integers representing the order of x and y, respectively.

2. The free-form surface based large field of view off-axis four-mirror optical system according to claim 1, wherein: The primary mirror (1) is an XY polynomial free surface with a highest order of 9; The radii of curvature of the primary mirror (1), the secondary mirror (2) and the third mirror (3) are positive, positive and negative respectively.

3. The freeform surface based large field of view off-axis four-mirror optical system of claim 2, wherein: The radius of curvature of the primary mirror (1) is 182.05 mm; The radius of curvature of the secondary mirror (2) is 256.9 mm; The radius of curvature of the third mirror (3) is -153.01 mm.

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