CN102866487A - Coaxial four-reflector ultra-low distortion optical system - Google Patents

Abstract

The invention relates to a coaxial four-mirror ultra-low distortion optical system, comprising primary mirrors, secondary mirrors, three mirrors, four mirrors, plane mirrors and a receiving image surface, wherein the primary mirrors, secondary mirrors, three mirrors, four mirrors and plane mirrors The optical axis is on the same straight line, the plane mirror is located between the third mirror and the fourth mirror, the primary mirror and the secondary mirror form a classic RC system, and form a real image, the first real image is relayed by the three mirrors and the four mirrors and then passed through the plane mirror Turn to the receiving image plane; the primary mirror, secondary mirror and three mirrors constitute a coaxial TMA system and undertake most of the optical power, and the four mirrors undertake a small optical power in the imaging of the optical system, accounting for 10% of the total optical power. 10-20%; the aperture diaphragm of the optical system is located on the main mirror, and the fourth mirror is placed at the exit pupil of the system. The optical system of the present invention can realize high image quality, ultra-low distortion and high stability, and can greatly reduce the processing difficulty of a large-caliber primary mirror, and is suitable for a high-precision space-borne three-dimensional surveying and mapping camera.

Description

Coaxial four-mirror ultra-low distortion optical system

technical field

The invention belongs to the technical field of aerospace optical remote sensors, and relates to an ultra-low distortion high-precision three-dimensional surveying and mapping imaging optical system suitable for satellite-borne space to the ground, in particular to a coaxial four-mirror ultra-low distortion optical system.

Background technique

With the continuous development of space optical remote sensing technology, the requirements for the optical system of surveying and mapping cameras are becoming increasingly stringent.

The main task of surveying and mapping satellites is to carry imaging sensors that meet the requirements of stereo photography to perform stereo photography on the earth's surface, obtain multi-dimensional image data of ground objects, use surveying and mapping processing technology to process image data, and accurately measure landforms, shape, size, attributes and Spatial position information, however, the distortion of the optical system determines the geometric positioning accuracy of the image data, which directly affects the final mapping accuracy of the image.

At present, many countries in the world have successfully launched stereoscopic mapping cameras, among which the more representative ones include the CCD stereoscopic mapping camera on the IKONOS-2 of the United States, the three-line array CCD mapping camera MEOSS of Germany, and the stereoscopic mapping camera on the ALOS satellite developed by Japan. Panchromatic Remote Sensing Instrument for Surveying and Mapping (PRISM). Among them, the CCD stereoscopic mapping camera on the IKONOS-2 in the United States has a focal length of 10m and a ground resolution of 1m. It adopts a coaxial three-mirror astigmatism (TMA) optical system structure, and the primary, secondary and third mirrors are all aspherical designs; Germany’s single-lens three-line array CCD surveying and mapping camera (MEOSS) has a focal length of only 61.6mm, adopts a transmission structure, and has a ground pixel resolution of only 52m×80m; the stereoscopic surveying and mapping camera on the Japanese ALOS satellite uses an optical system that is off-axis Three-mirror anastigmatism (TMA) system structure, the focal length is 2m, and the pixel resolution is 2.5m.

The optical system types of stereoscopic mapping cameras that have been successfully launched so far include transmission system, off-axis reflection three-mirror astigmatism elimination system and coaxial reflection three-mirror astigmatism elimination system. For high-resolution stereo mapping cameras with long focal lengths, the transmissive system cannot be adopted due to the limitation of material size and its properties. Therefore, the optical system forms used in high-resolution stereoscopic mapping are currently mainly limited to off-axis reflective three-mirror astigmatism elimination systems and coaxial reflection three-mirror astigmatism elimination systems. Among them, the processing of the mirror of the off-axis reflective system is extremely difficult, and traditional methods cannot be used for mirror processing, testing, and adjustment. It is difficult to control the tangential meridional distortion of the optical design, the system engineering is difficult, and the temperature control accuracy is difficult to guarantee. Especially not suitable for the optics of agile stereo mapping cameras. Although the coaxial reflective three-mirror astigmatism elimination system has good system engineering and is easy to achieve high-precision temperature control, it cannot achieve a large field of view and the system distortion is difficult to eliminate. The optical system distortion of the three-dimensional surveying and mapping cameras that have been successfully operated in orbit and adopt the coaxial reflection three-mirror astigmatism elimination structure is greater than one percent. It is difficult to meet the development needs of future surveying and mapping cameras.

Contents of the invention

The purpose of the present invention is to overcome the above-mentioned deficiencies of the prior art, and provide a coaxial four-mirror ultra-low distortion optical system, which is suitable for high-precision space-borne stereo surveying and mapping cameras, can achieve high image quality, ultra-low distortion and high stability, and can greatly Reduce the processing difficulty of the large aperture primary mirror.

Above-mentioned purpose of the present invention is mainly achieved through the following technical solutions:

Coaxial four-mirror ultra-low distortion optical system, including primary mirror, secondary mirror, three mirrors, four mirrors, plane mirror and receiving image plane, wherein the optical axes of primary mirror, secondary mirror, three mirrors, four mirrors and plane mirror are at On the same line, the plane mirror is located between the third mirror and the fourth mirror. The primary mirror and the secondary mirror form a classic R-C system and form a real image. The first real image is relayed by the three mirrors and the four mirrors and passes through the plane mirror (5) Turn to the receiving image plane; the primary mirror, secondary mirror and three mirrors constitute a coaxial TMA system and bear most of the focal power, and the four mirrors bear a small focal power in the imaging of the optical system, and the focal power it bears accounts for 10-20% of the total focal power; the aperture diaphragm of the optical system is located on the main mirror, and the four mirrors are placed at the position of the exit pupil of the system.

In the above-mentioned coaxial four-mirror ultra-low distortion optical system, the materials of the primary mirror, the secondary mirror, the third mirror and the fourth mirror are silicon carbide, glass ceramics or fused silica.

In the above-mentioned coaxial four-mirror ultra-low distortion optical system, the reflective surfaces of the primary mirror, the secondary mirror, the third mirror and the fourth mirror are coated with a metal high-reflectivity reflective film made of aluminum or silver.

In the above-mentioned coaxial four-mirror ultra-low distortion optical system, the receiving image plane is the receiving plane of a linear array CCD or TDICCD detector.

In the above-mentioned coaxial four-mirror ultra-low distortion optical system, the quadratic coefficients of the primary mirror and the third mirror are -1.5 to 0, and the quadratic coefficients of the four mirrors are greater than -10.

In the above-mentioned coaxial four-mirror ultra-low distortion optical system, the plane mirror is located between the third mirror and the fourth mirror, and is 280-370 mm away from the fourth mirror.

In the above-mentioned coaxial four-mirror ultra-low distortion optical system, the primary mirror, secondary mirror, three mirrors, and four mirrors are all aspheric mirrors, and the primary mirror is approximately parabolic, and the four mirrors are small mirrors with large quadratic coefficients. Aspheric hyperboloid.

Compared with the prior art, the present invention has the following beneficial effects:

(1) The present invention introduces four mirrors with small focal power and large aspheric coefficients on the basis of the coaxial TMA optical system. The four mirrors are located near the system exit pupil, and the pupil aberration of the system is corrected by the aspheric coefficients of the four mirrors, so The system achieves ultra-low distortion, the distortion value is only three parts per million, and the volume is about 1/2 of the off-axis reflection system of the same index. The advantages of the optical system are obvious.

(2) In the present invention, due to the adoption of four coaxial aspheric reflectors and one planar reflector, the optical-mechanical structure is compact, so that the structural stability of the system is higher, the moment of inertia is smaller, and it is easy to realize high-precision temperature and pointing control, and the overall structure is more compact, which is very beneficial for space-borne long focal length and high-resolution stereo mapping cameras.

(3) In the present invention, since the entrance pupil of the system is located on the primary mirror, the four mirrors are positioned at the exit pupil position through optical path folding, and the aspherical coefficients of the four mirrors with smaller apertures are used to correct the pupil aberration of the system. The primary mirror can use a parabolic mirror , which can greatly reduce the asphericity requirements of the large-aperture primary mirror and reduce the difficulty of processing the primary mirror; while greatly saving costs, it can also reduce the development cycle of the camera.

(4) Due to the existence of the intermediate image in the present invention, a field of view stop and an inner light shield can be arranged at the position of the intermediate image, thereby effectively eliminating stray light outside the field of view and reducing the requirement for the length of the outer light shield.

(5) The present invention further optimizes the design of the surface shape, structure and materials of the primary mirror, secondary mirror, third mirror and four mirrors, greatly improves the image quality and imaging stability of the optical system, and realizes the ultra-high performance of the optical system. low distortion.

(6) The optical system of the present invention has the advantages of low distortion, compact optical-mechanical structure, small volume, light weight, and high stability of internal azimuth elements. Through the reasonable layout of multi-line array cameras, a higher accuracy of the terrain and landform of the target scene can be achieved. The stable mapping function of the scale is especially suitable for space-borne high-precision stereoscopic mapping cameras.

Description of drawings

Fig. 1 is the structural diagram of optical system of the present invention;

Fig. 2 is the MTF curve diagram of the ultra-low distortion optical system based on the improved coaxial TMA in the embodiment of the present invention;

Fig. 3 is a distortion grid diagram of an optical system in an embodiment of the present invention.

Detailed ways

Below in conjunction with accompanying drawing and specific embodiment the present invention is described in further detail:

In the embodiment of the present invention, the working spectrum is 0.5-0.9 μm, the entrance pupil diameter is 625 mm, the focal length of the optical system is 10 m, the full field of view is 1.5°, and the total length of the system is 1158 mm.

As shown in Figure 1 is the structural diagram of the optical system of the present invention, it can be seen from the figure that the optical system of the present invention includes: primary mirror 1, secondary mirror 2, three mirrors 3, four mirrors 4, plane mirror 5 and receiving image surface 6. The optical axes of the primary mirror 1, the secondary mirror 2, the third mirror 3, the fourth mirror 4 and the plane mirror 5 are on the same straight line and are coaxial. The primary mirror 1 and the secondary mirror 2 constitute a classic R-C system, and form a primary real image 7, which is relayed by the three mirrors 3 and four mirrors 4 and turned to the receiving image surface 6 by the plane mirror 5. Wherein the flat reflector 5 is located at the upper position between the third mirror 3 and the fourth mirror 4, and the distance between the third mirror 4 and the fourth mirror 4 is 280-370mm, and its function is to compress the optical path, the purpose is to reduce the structural size of the optical system and realize the image Reasonable layout of the surface, the distance between the plane mirror 5 and the four mirrors 4 in this embodiment is 334mm.

The receiving image surface 6 is the receiving surface of a linear array CCD or TDICCD detector. The primary mirror 1, secondary mirror 2, third mirror 3, and four-mirror 4-surface are all aspherical, of which the primary mirror 1-surface is approximately a paraboloid, and the four-mirror 4-surface is a small aspheric hyperboloid with a large quadratic coefficient , the primary mirror 1, the secondary mirror 2, the third mirror 3, and the fourth mirror 4 are made of silicon carbide, or glass-ceramics, or fused silica. And the reflective surfaces of the primary mirror 1, the secondary mirror 2, the third mirror 3 and the fourth mirror 4 are coated with a metal high-reflectivity reflective film made of aluminum or silver.

The aperture diaphragm of the optical system of the present invention is located on the main mirror 1, and the four mirrors 4 are placed at the exit pupil of the system. The mirror 4 undertakes relatively small optical power in the imaging of the optical system, and the assumed optical power accounts for 10-20% of the total optical power. The quadratic term coefficients of the primary mirror 1 and the third mirror 3 are -1.5 to 0, and the quadratic term coefficients of the fourth mirror 4 are greater than -10.

Because the larger the aspheric coefficient is, the more difficult it is to process the surface shape. Therefore, the present invention places the entrance pupil of the system on the main mirror 1, and makes the four mirrors 4 on the exit pupil through optical path folding. The advantage of doing this is to utilize The aspherical coefficient of the small-aperture four-mirror 4 corrects the pupil aberration of the system, so that the primary mirror can use a parabolic mirror with an aspherical quadratic coefficient of -1, which greatly reduces the processing difficulty of the primary mirror and introduces more optimization The degree of freedom makes the image quality of the optical system good, so as to achieve ultra-low distortion while ensuring high imaging quality. Figure 2 is the MTF curve diagram of the ultra-low distortion optical system based on the improved coaxial TMA in the example of the present invention, the MTF curve is shown in Figure 2, it can be seen that the MTF curve coincides with the diffraction limit.

In the embodiment of the present invention, the main mirror 1, the secondary mirror 2, and the third mirror 3 bear most of the optical power of the system, and the fourth mirror 4 is an aspheric surface with a small optical power but a large quadratic coefficient (its radius of curvature R= 3500mm, the quadratic term coefficient is -15), the quadratic term coefficient of primary mirror 1 is -1 in the present embodiment, the quadratic term coefficient of secondary mirror 2 is -2, and the quadratic term coefficient of third mirror 3 is -1 . The present invention utilizes the quadratic term coefficient of the four mirrors 4 to correct the distortion of the optical system, so that the entire optical system realizes ultra-low distortion, and its distortion grid diagram of the optical system is shown in Figure 3. less than three parts per million. The characteristics of this ultra-low distortion optical system are very beneficial for improving the mapping accuracy of surveying and mapping satellites.

The optical system of the present invention has the advantages of low distortion, compact optical-mechanical structure, small volume, light weight, and high stability of internal azimuth elements. Through the reasonable layout of multi-line array cameras, the stability of a relatively high scale of the terrain and landform of the target scene can be realized. The surveying and mapping function is especially suitable for the optical system of the spaceborne high-precision stereoscopic mapping camera.

The above description is only the best specific implementation mode of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of changes or modifications within the technical scope disclosed in the present invention. Replacement should be covered within the protection scope of the present invention.

The content that is not described in detail in the specification of the present invention belongs to the well-known technology of those skilled in the art.

Claims (7)

1. coaxial four surpass in reverse low distortion optical system, it is characterized in that: comprise primary mirror (1), secondary mirror (2), three mirrors (3), four mirrors (4), plane mirror (5) and reception image planes (6), primary mirror (1) wherein, secondary mirror (2), three mirrors (3), the optical axis of four mirrors (4) and plane mirror (5) is on same straight line, plane mirror (5) is positioned between three mirrors (3) and four mirrors (4), primary mirror (1) and the classical R-C of secondary mirror (2) formation system, and forming a real image (7), a real image (7) is through three mirrors (3), four mirrors (4) relay imaging is also turned back by plane mirror (5) and is located to receiving image planes (6); Primary mirror (1), secondary mirror (2) and three mirrors (3) consist of coaxial TMA system and bear most focal powers, and four mirrors (4) are born less focal power in optical system imaging, and the focal power of bearing accounts for the 10-20% of total focal power; The optical system aperture diaphragm is positioned on the primary mirror (1), and four mirrors (4) place place, system exit pupil position.

2. according to claim 1 coaxial four surpass in reverse low distortion optical system, it is characterized in that: the material of described primary mirror (1), secondary mirror (2), three mirrors (3) and four mirrors (4) is silit, devitrified glass or fused quartz.

3. according to claim 1 coaxial four surpass in reverse low distortion optical system, it is characterized in that: the reflecting surface of described primary mirror (1), secondary mirror (2), three mirrors (3) and four mirrors (4) is coated with the metal high reflectance reflectance coating of aluminium or ag material.

4. according to claim 1 coaxial four surpass in reverse low distortion optical system, it is characterized in that: described reception image planes (6) are line array CCD or TDICCD detector receiving plane.

5. according to claim 1 coaxial four surpass in reverse low distortion optical system, it is characterized in that: the quadratic term coefficient of described primary mirror (1) and three mirrors (3) is that the quadratic term coefficient of-1.5～0, four mirrors (4) is greater than-10.

6. according to claim 1 coaxial four surpass in reverse low distortion optical system, it is characterized in that: described plane mirror (5) is positioned between three mirrors (3) and four mirrors (4), and distance four mirrors (4) 280-370mm position.

7. according to claim 1 coaxial four surpass in reverse low distortion optical system, it is characterized in that: described primary mirror (1), secondary mirror (2), three mirrors (3), four mirrors (4) face type are non-spherical reflector, wherein primary mirror (1) face type is approximate parabolic, and four mirrors (4) face type is the little aspherical degree hyperboloid with large quadratic term coefficient.

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