Compact hyperspectral imager optical system and imaging method thereof

Abstract

The invention provides a compact hyperspectral imager optical system and an imaging method thereof, which are used for solving the technical problems that the overall size of a front telescopic assembly in a traditional hyperspectral imager optical system is difficult to compress, the installation and adjustment difficulty is relatively large, or the incident energy is lost and the space coverage capability is insufficient. And the technical problems of low energy utilization rate and difficulty in improving the sensitivity and the signal-to-noise ratio of the hyperspectral imager of the traditional Dyson type dispersion light splitting assembly are solved. The invention provides a compact hyperspectral imager optical system, which comprises an off-axis two-image-reflection telecentric optical path system based on a free-form surface and a Dyson light splitting system based on a curved surface prism, and realizes high regulation and control capability on light through multiple degrees of freedom of a curved surface shape. Compared with the prior art, the high-spectral imaging device has the advantages that the high-spectral imaging device is simple in structure, high image quality, small distortion and good telecentricity are achieved, meanwhile, the curved prism serves as a dispersion light splitting element, the whole spectrum has high light splitting efficiency, and the signal-to-noise ratio and sensitivity of the high-spectral imaging device can be improved.

Description

Technical Field

[0001] This invention relates to hyperspectral imaging systems, and more particularly to a compact hyperspectral imager optical system and its imaging method. Background Technology

[0002] Hyperspectral imaging is a cutting-edge remote sensing technology that integrates spectral analysis and image processing. Based on its sophisticated spectroscopic optical system and unique imaging method, it organically combines image acquisition with spectral detection, enabling the simultaneous capture of both spatial and continuous spectral information of a target. Unlike traditional red-green-blue three-channel imaging or broadband multispectral imaging, hyperspectral imaging "slices" the target across hundreds of continuous and narrow bands, forming a data cube (spatial dimensions X and Y, and spectral dimension λ) containing rich physicochemical characteristics. Because this data cube possesses the characteristic of combining image and spectrum, it can detect and identify observed objects from both geometric and spectral perspectives, thereby achieving the classification, detection, and quantitative characterization of objects in a scene. As a crucial component of remote sensing payloads, hyperspectral imagers have injected powerful vitality into the development of remote sensing technology. They represent one of the most significant breakthroughs in Earth observation achieved by humankind over the past 30 years and will remain a vital technological tool for future Earth science research and deep space exploration. Currently, the application value of hyperspectral imagers in various fields is gradually becoming apparent. They have been widely used in agricultural and forestry surveys, mineral resource exploration, environmental monitoring, disaster prevention and mitigation, medical diagnosis, and scientific research, and have yielded fruitful results.

[0003] Miniaturization of spaceborne remote sensing payloads is a significant trend in aerospace technology development, driven primarily by cost reduction and efficiency improvement. Firstly, miniaturization significantly reduces launch costs. Within the limited payload capacity of launch vehicles, lightweight and compact remote sensing equipment enables multi-satellite deployment in a single launch or coordinated deployment with other payloads, drastically reducing the launch cost per unit payload. Secondly, miniaturization facilitates the construction of distributed satellite constellations. By networking multiple satellites carrying lightweight payloads, the temporal resolution (revisit period) and spatial coverage of Earth observation can be improved, meeting the real-time requirements of disaster monitoring, environmental surveys, and other missions. Furthermore, miniaturization enhances system flexibility and scalability, facilitating rapid technology iteration, adapting to diverse mission needs, and opening up new low-cost, high-efficiency remote sensing service models for commercial spaceflight. Therefore, miniaturization of spaceborne remote sensing payloads is a crucial path to promoting the widespread adoption and commercialization of aerospace remote sensing applications.

[0004] Traditional hyperspectral imager optical systems typically consist of a front-view telescope and a dispersive beam splitter. Common front-view telescopes are often off-axis three-mirror or coaxial two-mirror structures. Off-axis three-mirror telescopes, composed of three off-axis aspherical mirrors, are difficult to compress in size and require complex assembly. Coaxial two-mirror telescopes typically employ a Karl von Schüco system, consisting of two aspherical mirrors. The central obstruction in these systems leads to energy loss, and their relatively small field of view (usually less than 1°) results in insufficient spatial coverage. The dispersive beam splitter typically uses a traditional Dyson optics system. Its multiplexing structure allows for a compact, small, and lightweight design, facilitating the miniaturization of hyperspectral imagers. Furthermore, traditional Dyson optics are closer to coaxial optical systems, exhibiting excellent aberration characteristics and easily achieving large fields of view and large relative apertures, resulting in higher detection sensitivity. Therefore, they demonstrate significant advantages in miniaturized, high-performance spaceborne spectrometers. Traditional Dyson-type dispersive beam splitters typically use a concave grating as the beam splitting element, utilizing the diffraction principle of the grating. However, the diffraction efficiency of the grating usually varies with wavelength. In general designs, the diffraction efficiency is high near the center wavelength and low near the ends within the operating band, which leads to some waste of incident light energy. Furthermore, as a beam splitting element, the grating usually generates multiple diffraction orders, but only one order is effectively utilized by the downstream optical path (e.g., the -1st or +1st order), while the energy of the remaining orders is wasted. Due to these two factors, the energy utilization rate of grating beam splitting is typically in the range of 30%-80%. Due to the requirement for compact structure, miniaturized hyperspectral imager optical systems usually have small optical apertures and inherently weak light collection capabilities. Therefore, using grating beam splitting is not conducive to improving the sensitivity and signal-to-noise ratio of hyperspectral imagers. Therefore, to meet the commercial needs of the aerospace remote sensing field, it is urgent to research a miniaturized hyperspectral imager optical system with high sensitivity, good imaging quality, and a simple and compact structure. Summary of the Invention

[0005] The purpose of this invention is to solve the technical problems of the front-mounted telescope component in traditional hyperspectral imager optical systems, such as difficulty in compressing the overall size, high assembly and adjustment difficulty, or insufficient incident energy loss and spatial coverage, as well as the technical problems of low energy utilization and difficulty in improving the sensitivity and signal-to-noise ratio of traditional Dyson-type dispersive beam splitters. The invention provides a compact hyperspectral imager optical system and its imaging method.

[0006] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0007] A compact hyperspectral imager optical system, characterized in that it includes a front telescope assembly, a slit, a dispersive beam-splitting assembly, and an area array detector;

[0008] The front telescope assembly is an off-axis telecentric optical path system based on a freeform surface, which includes an aperture stop, a freeform primary mirror, a freeform secondary mirror and a first planar folding mirror arranged sequentially along the optical path. It is used to make the light emitted by the ground target converge and form an image on the primary image plane after passing through the aperture stop, the freeform primary mirror, the freeform secondary mirror and the first planar folding mirror in sequence.

[0009] The slit is located at the primary image plane and is used to limit the light-transmitting area on the primary image plane to form slit light;

[0010] The dispersive beam splitter is a Dyson beam splitter system based on a curved prism, used for dispersive imaging of the primary image plane. It includes a second planar folding mirror, a positive lens, a negative lens, a curved prism, and a quadrature surface mirror arranged sequentially in the slit light path. The slit light passes through the second planar folding mirror, the positive lens, the negative lens, and the curved prism in sequence before reaching the quadrature surface mirror. After being reflected by the quadrature surface mirror, it passes back through the curved prism, the negative lens, and the positive lens in sequence to form probe light of different wavelengths.

[0011] The area array detector is located on the output light path of different wavelength detection light, and is used to image the detection light at different positions of the area array detector according to different wavelengths.

[0012] Furthermore, the aperture stop is a circular ring structure;

[0013] Both the freeform primary mirror and the freeform secondary mirror are freeform reflecting mirrors.

[0014] Both the first and second planar folding mirrors are planar reflecting mirrors;

[0015] Both the positive and negative lenses are spherical lenses.

[0016] The front and back surfaces of the curved prism are both spherical, and there is an included angle α between the rotational symmetry axes of the front and back surfaces, where 0°<α≤30°;

[0017] The surface shape of the quadric surface reflector is a quadric surface.

[0018] Furthermore, the aperture stop is made of metal;

[0019] The freeform primary mirror, freeform secondary mirror, first plane folding mirror, second plane folding mirror, and quadric surface reflecting mirror are all made of metal or glass.

[0020] The slit is a metal slit or a glass slit;

[0021] The positive lens, negative lens, and curved prism are all made of fused silica.

[0022] Furthermore, the slit is a metal slit, which is a sandwich structure formed by sequentially arranging glass sheets, metal sheets, and glass sheets; the metal sheet has a slit.

[0023] Alternatively, the slit may be a glass slit, which is a structure of two planar glass sheets bonded together; one of the planar glass sheets has a metal film coated on its bonding surface, and the metal film has a slit.

[0024] Furthermore, the surfaces of the freeform primary mirror, the freeform secondary mirror, the first planar folding mirror, the second planar folding mirror, and the quadric surface reflecting mirror are respectively coated with anti-reflection films;

[0025] The front and rear surfaces of the positive lens, negative lens, and curved prism are respectively coated with anti-reflective coatings;

[0026] The surface of the glass sheet or flat glass sheet is coated with an anti-reflective film.

[0027] Furthermore, the surface shapes of both the freeform primary mirror and the freeform secondary mirror are expressed using an 8th-order XY polynomial with non-rotational symmetry, as follows:

[0028]

[0029] Where z(x,y) is the surface elevation of the freeform surface, (x,y) are the coordinates of the freeform surface aperture, c is the curvature of the base quadratic surface, and k is the conic constant of the base quadratic surface. R is the radial and radial coordinates of a freeform surface. m It is the normalized radius, usually taken as 1, C i is the weight coefficient of the i-th XY polynomial, m is the degree of the x term in the XY polynomial, n is the degree of the y term in the XY polynomial, and 1≤m+n≤8.

[0030] Furthermore, the aperture stop has a diameter of 45 mm;

[0031] The freeform primary mirror has a circular structure with a diameter of 51 mm, a base surface curvature radius of 2185.710 mm, a conic coefficient of 0, and an inclination of 18.440° in the X direction.

[0032] The freeform secondary mirror has a circular structure with a diameter of 51 mm, a base surface radius of curvature of 263.889 mm, a conic coefficient of 0, and an inclination of -15.202° in the X direction.

[0033] The first planar folding mirror has a circular structure with a diameter of 21 mm. Both its front and rear surfaces are planar, and its tilt in the X direction is 47.8°.

[0034] The slit is a circular structure with a diameter of 8mm. The slit opening is symmetrical about the center of the quartz glass and has a size of 3.5345mm × 0.0065mm.

[0035] The first planar folding mirror has a circular structure with a diameter of 16mm, and both its front and rear surfaces are planar, with an inclination of 39.5° in the X direction;

[0036] The positive lens has a circular structure with a diameter of 52mm. Both the front and rear surfaces are spherical, with a radius of curvature of -100.531mm on the front surface and -41.871mm on the rear surface.

[0037] The negative lens has a circular structure with a diameter of 57mm. Both the front and rear surfaces are spherical, with a radius of curvature of -108.951mm on the front surface and -321.484mm on the rear surface.

[0038] The curved prism has a circular structure with a diameter of 73 mm. Both the front and rear surfaces are spherical. The radius of curvature of the front surface is -215.654 mm, the radius of curvature of the rear surface is -185.636 mm, the inclination of the front surface in the X direction is -6.373°, and the inclination of the rear surface in the X direction is 5.630°.

[0039] The quadric surface reflector has a circular structure with a diameter of 73 mm, a radius of curvature of -222.781 mm, a conic coefficient of 0.0903, an eccentricity of 1.286 mm in the Y direction, and a tilt of -3.427° in the X direction.

[0040] Furthermore, the distance between the aperture stop and the freeform primary mirror is 70 mm;

[0041] The distance between the freeform primary mirror and the freeform secondary mirror is 76.649 mm;

[0042] The distance between the freeform secondary mirror and the first planar folding mirror is 98.463 mm;

[0043] The distance between the first planar folding mirror and the slit is 35.5 mm;

[0044] The distance between the slit and the second planar folding mirror is 27.375 mm;

[0045] The distance between the first planar folding mirror and the positive lens is 15mm;

[0046] The interval between the positive lens and the negative lens is 83.570 mm;

[0047] The distance between the negative lens and the curved prism is 68.689 mm;

[0048] The distance between the curved prism and the quadrature reflector is 9.047 mm;

[0049] The distance between the positive lens and the area array detector is 35.369 mm.

[0050] Furthermore, the center thickness of the freeform primary mirror is 10mm;

[0051] The center thickness of the freeform secondary mirror is 10mm;

[0052] The center thickness of the first planar folding mirror is 5mm;

[0053] The center thickness of the first planar folding mirror is 5mm;

[0054] The center thickness of the positive lens is 7.009 mm;

[0055] The center thickness of the negative lens is 5.300 mm;

[0056] The thickness of the curved prism along the optical axis is 14.516 mm;

[0057] The center thickness of the quadric surface reflector is 10 mm.

[0058] In addition, the present invention also provides an imaging method based on the above-mentioned compact hyperspectral imager optical system, which is characterized by including the following steps:

[0059] Step 1: Align the optical system of the compact hyperspectral imager with any position of the ground target and acquire an image of the target at that position using the area array detector;

[0060] Step 2: Move the compact hyperspectral imager optical system to make it sweep along a fixed path for the ground target, so that the array detector can obtain target images of the ground target at multiple different locations in the space.

[0061] Step 3: The images of ground objects obtained by the area array detector at multiple different locations in the space are stitched together to obtain a data cube that combines the ground object map and the target map.

[0062] The advantages of this invention compared to the prior art are as follows:

[0063] 1. This invention provides a compact hyperspectral imager optical system. The front telescope component is designed as an off-axis telecentric optical path system based on a freeform surface and two mirrors. Its structure is simple, compact, and easy to assemble and adjust. Furthermore, the freeform surface shape offers considerable design freedom and strong control over light, simultaneously producing high image quality, low distortion, and good telecentricity, thus meeting the requirements of a hyperspectral imager for its front telescope component. Simultaneously, the Dyson spectral splitting system based on curved prisms uses curved prisms as dispersive beam splitting elements, exhibiting high splitting efficiency across the entire spectrum. This also helps improve the signal-to-noise ratio and sensitivity of the hyperspectral imager, which is particularly advantageous for small-aperture optical systems. The application of the freeform surface-based off-axis telecentric optical path system and the curved prism-based Dyson spectral splitting system provides a novel optical system form for the research of compact, high-resolution hyperspectral imagers.

[0064] 2. The front telescope assembly of the present invention contains only two freeform mirrors and one planar folding mirror. The freeform primary mirrors in the two freeform mirrors have no central obstruction and no energy loss, resulting in a high overall energy transmittance, which is beneficial to further improve the signal-to-noise ratio and sensitivity of the hyperspectral imager.

[0065] 3. The dispersive beam splitter of the present invention adopts a Dyson beam splitter system based on curved prisms, which can perform optical path multiplexing. It includes three transmissive optical elements and one reflective optical element, and has the characteristics of simple optical path, high transmittance and compact structure.

[0066] 4. The compact hyperspectral imager optical system provided by this invention can achieve an instantaneous field of view of 48.15 μrad in the spectral range of 400 nm-1000 nm, a linear field of view of 2°, an F number of 3, and a pixel size of 6.5 μm. The system transfer function is better than 0.62 at the Nyquist frequency of 77 lp / mm, achieving a spectral resolution of no more than 7.5 nm, a spectral smile distortion of no more than 0.055 pixels, and a spectral keystone distortion of no more than 0.016 pixels.

[0067] 5. The imaging method provided by the present invention is based on the above-mentioned compact hyperspectral imager optical system. It is simple and convenient to operate and can quickly obtain a data cube that combines ground object maps and spectra. Attached Figure Description

[0068] Figure 1 This is a schematic diagram of an embodiment of a compact hyperspectral imager optical system according to the present invention;

[0069] Figure 2 This is a matrix point array diagram according to an embodiment of the present invention;

[0070] Figure 3The following are design modulation transfer function curves for embodiments of the present invention in different frequency bands;

[0071] Figure 4 This is a schematic diagram of the full-spectrum spectral resolution of an embodiment of the present invention;

[0072] Figure 5 This is a schematic diagram of spectral bending according to an embodiment of the present invention;

[0073] Figure 6 This is a schematic diagram of spectral distortion according to an embodiment of the present invention.

[0074] The specific reference numerals in the attached figures are as follows:

[0075] 1-Aperture stop; 2-Freeform primary mirror; 3-Freeform secondary mirror; 4-First plane folding mirror; 5-Slit; 6-Second plane folding mirror; 7-Positive lens; 8-Negative lens; 9-Curved prism; 10-Duboid reflector; 11-Area array detector. Detailed Implementation

[0076] To make the advantages and features of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0077] like Figure 1 As shown, a compact hyperspectral imager optical system is designed based on the principle of a pushbroom hyperspectral imager, including a front telescope assembly, a slit 5, a dispersive beam splitter assembly, and an area array detector 11.

[0078] The front-mounted telescope assembly employs an off-axis telecentric optical path based on freeform surfaces and two mirrors. Specifically, it includes an aperture stop 1, a freeform primary mirror 2, a freeform secondary mirror 3, and a first planar folding mirror 4, arranged sequentially along the incident light path. The aperture stop 1 is a circular ring structure, positioned at the very front of the entire optical system, serving as a physical stop in the optical path to limit the light-passing aperture. The freeform primary mirror 2 and the freeform secondary mirror 3 are both freeform mirrors, and the first planar folding mirror 4 is a planar mirror. In this embodiment, the aperture stop 1 is made of metal, while the freeform primary mirror 2, the freeform secondary mirror 3, and the first planar folding mirror 4 are all made of glass. Light emitted from ground objects passes sequentially through the aperture stop 1, the freeform primary mirror 2, the freeform secondary mirror 3, and the first planar folding mirror 4 before converging to form an image on the primary image plane. The front-mounted telescope module adopts an off-axis telecentric optical path based on freeform surfaces, achieving advantages such as no chromatic aberration, no central obstruction, excellent image quality, and high transmittance.

[0079] When setting up the optical elements of the front telescope assembly, a certain distance is set between the upper edge of the aperture stop 1 and the reflected light from the freeform primary mirror 2, so that the upper edge of the aperture stop 1 does not block the light; a certain distance is set between the upper edge of the freeform primary mirror 2 and the reflected light from the freeform secondary mirror 3, so that the upper edge of the freeform primary mirror 2 does not block the light; a certain distance is set between the lower edge of the freeform secondary mirror 3 and the transmitted light from the aperture stop 1, so that the lower edge of the freeform secondary mirror 3 does not block the light.

[0080] Slit 5 is located on the primary image plane and is used to limit the light-passing area on the primary image plane, forming slit light. The slit length is one of the main factors determining the field of view of the hyperspectral imager, the slit width is one of the main factors determining the spectral resolution, and the slit area is one of the main factors determining the signal-to-noise ratio of the hyperspectral imager. The slit also suppresses stray light. In this embodiment, slit 5 is a glass slit, which is a structure of two planar glass sheets bonded together. In this embodiment, the planar glass sheets are fused silica. An anti-reflection coating is deposited on the surface of the planar glass sheets, and a metal film is deposited on the bonding surface of one of the planar glass sheets. The slit is formed on the metal film using micro-nano processing. The bonding structure protects the slit from dust contamination. In other embodiments of the invention, slit 5 can also be a metal slit, which uses a sandwich structure formed by sequentially arranging glass sheets, metal sheets, and more glass sheets. The glass sheets are coated with an anti-reflection coating, and the metal sheets have slits formed using mechanical processing. The glass sheets on both sides protect the slit, preventing dust contamination and thus affecting light transmission.

[0081] The dispersive beam splitter is a Dyson spectral system based on a curved prism, used for dispersive imaging of the primary image plane. It includes a second planar folding mirror 6, a positive lens 7, a negative lens 8, a curved prism 9, and a quadratic surface mirror 10, sequentially arranged along the slit optical path. The positive lens 7 and negative lens 8 are both spherical lenses. The front and rear surfaces of the curved prism 9 are both spherical, and there is an angle α between the rotational symmetry axes of the front and rear surfaces, where 0° < α ≤ 30°. The quadratic surface mirror 10 has a quadratic surface shape. In this embodiment, the positive lens 7, negative lens 8, and curved prism 9 are all made of fused silica, while the second planar folding mirror 6 and the quadratic surface mirror 10 are both made of glass. The slit light passes sequentially through the second planar folding mirror 6, the positive lens 7, the negative lens 8, and the curved prism 9 before reaching the quadrature surface mirror 10. After being reflected by the quadrature surface mirror 10, it passes in the opposite direction sequentially through the curved prism 9, the negative lens 8, and the positive lens 7. Due to the setting of the curved prism 9, slit light of different wavelengths will be separated to form probe light of different wavelengths.

[0082] The area array detector 11 is located on the output optical path of different wavelength probe light, and is used to image the probe light at different positions on the area array detector 11 according to different wavelengths. The field of view direction of the imager optical system is denoted as the X direction, and the dispersion direction as the Y direction. The spatial dimension of the field of view information is distributed along the X direction of the area array detector 11, and the spectral dimension is distributed along the Y direction of the area array detector 11. Combined with the on-orbit pushbroom operation mode, it can realize the detection of ground targets and the acquisition of three-dimensional data cube information integrating image and spectrum. The dispersion beam splitter adopts the form of "single prism dispersion + optical path multiplexing", which has the characteristics of simple optical path, high transmittance, and compact structure.

[0083] The light emitted from the ground target is first imaged at the position of slit 5, i.e., the primary image plane position, by an off-axis telecentric optical path system based on freeform surfaces (front-mounted telescope assembly). Then, it undergoes dispersive beam splitting by a Dyson beam splitting system based on curved prisms and is refocused onto the focal plane of the area array detector 11. The non-rotationally symmetric freeform primary mirror 2 and freeform secondary mirror 3 used in the front-mounted telescope assembly of this invention have strong light control and aberration correction capabilities, achieving good image quality and telecentricity on the primary image plane with low distortion, which is crucial for hyperspectral imaging. The Dyson beam splitting system based on curved prisms still uses a multiplexed optical path structure, but the beam splitting element is changed from a concave grating to a curved prism 9. The beam splitting principle of the curved prism 9 is the same as that of a triangular prism, i.e., different wavelengths of light have different refractive indices in the medium, resulting in different deflection angles. This dispersive spectral method has a significant advantage in energy utilization of a single spectral channel, with a spectral efficiency of nearly 100% across the entire spectrum (with only a small amount of Fresnel reflection at the medium interface and absorption of light by the medium material). This is of great significance for improving the signal-to-noise ratio of small-aperture optical systems.

[0084] To achieve better reflection and transmission effects, this embodiment coats anti-reflection films on the surfaces of the freeform primary mirror 2, the freeform secondary mirror 3, the first planar folding mirror 4, the second planar folding mirror 6, and the quadratic reflector 10, respectively. Anti-reflection films are coated on the front and back surfaces of the positive lens 7, the negative lens 8, and the curved prism 9, respectively. An anti-reflection film is also coated on the surface of the planar glass plate of the slit 5.

[0085] It is worth noting that in other embodiments of the present invention, the freeform primary mirror 2, the freeform secondary mirror 3, the first planar folding mirror 4, the second planar folding mirror 6, and the quadratic surface reflecting mirror 10 may also be made of other materials such as metal, which are not limited here.

[0086] This invention combines the slit 5 of an off-axis telecentric optical path system (front-mounted telescope component) based on a freeform surface and a Dyson spectral system (dispersion spectral component) based on a curved prism, resulting in a "W"-shaped overall optical path layout. This significantly improves optical performance while achieving excellent structural compactness, enabling the miniaturization of a hyperspectral imager. This structure is not only suitable for miniaturized remote sensing payloads, but also allows for the mounting of the hyperspectral imager of this invention with limited space resources in the field of airborne remote sensing. It can be used on multi-rotor UAVs, fixed-wing UAVs, and small fixed-wing aircraft for applications such as tree species classification, hyperspectral mineral exploration, geological hazard investigation, fine mineral identification, hyperspectral prospecting, mineral mapping, and volcano monitoring.

[0087] In this embodiment, the aperture stop 1 is a metal ring with a diameter of 45 mm. The distance between the aperture stop 1 and the freeform primary mirror 2 is 70 mm.

[0088] The surface shapes of both the freeform primary mirror 2 and the freeform secondary mirror 3 are expressed using an 8th-order XY polynomial with non-rotational symmetry, including only even-power terms of x, symmetric about the YOZ plane, and totaling 24 terms. The expression for the XY polynomial is as follows:

[0089]

[0090] Where z(x,y) is the surface elevation of the freeform surface, (x,y) are the coordinates of the freeform surface aperture, c is the curvature of the base quadratic surface, and k is the conic constant of the base quadratic surface. R is the radial and radial coordinates of a freeform surface. m It is the normalized radius, usually taken as 1, C i is the weight coefficient of the i-th XY polynomial, m is the degree of the x term in the XY polynomial, n is the degree of the y term in the XY polynomial, and 1≤m+n≤8.

[0091] The coefficients of the XY polynomial are shown in the table below:

[0092]

[0093] In this embodiment, the freeform primary mirror 2 is a circular structure made of glass with a diameter of 51 mm, a center thickness of 10 mm, a base surface radius of curvature of 2185.710 mm, a conic coefficient of 0, and an inclination of 18.440° in the X direction. The distance between the freeform primary mirror 2 and the freeform secondary mirror 3 is 76.649 mm.

[0094] The freeform secondary mirror 3 is a circular structure made of glass with a diameter of 51 mm, a center thickness of 10 mm, a base surface radius of curvature of 263.889 mm, a conic coefficient of 0, and an inclination of -15.202° in the X direction. The distance between the freeform secondary mirror 3 and the first planar folding mirror 4 is 98.463 mm.

[0095] Both the first planar folding mirror 4 and the second planar folding mirror 6 are planar reflectors, which play a role in folding the light path and compressing its volume. In this embodiment, the first planar folding mirror 4 is a circular structure made of glass with a diameter of 21 mm, both front and rear surfaces are planar, and the thickness is 5 mm. The tilt in the X direction is 47.8°. The distance between the first planar folding mirror 4 and the slit 5 is 35.5 mm.

[0096] The slit 5 is a circular structure with a diameter of 8 mm, composed of two 1.5 mm thick pieces of fused silica bonded together. Both the front and back surfaces of the fused silica pieces are planar. The bonded surface of the fused silica closest to the first planar folding mirror 4 is coated with a 200 nm-500 nm thick chromium layer, with a rectangular light-transmitting hole at the center of the chromium layer. The slit opening is symmetrically placed about the center of the fused silica glass and has dimensions of 3.5345 mm × 0.0065 mm. The distance between the slit 5 and the second planar folding mirror 6 is 27.375 mm.

[0097] The first planar folding mirror 6 has a circular structure with a diameter of 16 mm. Both its front and rear surfaces are planar, and its thickness is 5 mm. Its tilt in the X direction is 39.5°. The distance between the first planar folding mirror 6 and the positive lens 7 is 15 mm.

[0098] The positive lens 7 has a circular structure with a diameter of 52mm. Both its front and rear surfaces are spherical, with a radius of curvature of -100.531mm on the front surface and -41.871mm on the rear surface. It has a thickness of 7.009mm and is made of fused silica. The distance between the positive lens 7 and the negative lens 8 is 83.570mm.

[0099] The negative lens 8 is a circular structure with a diameter of 57 mm. Both its front and rear surfaces are spherical, with a radius of curvature of -108.951 mm for the front surface and -321.484 mm for the rear surface. It has a thickness of 5.300 mm and is made of fused silica. The distance between the negative lens 8 and the curved prism 9 is 68.689 mm.

[0100] The curved prism 9 has a circular structure with a diameter of 73 mm. Both its front and rear surfaces are spherical, with a radius of curvature of -215.654 mm for the front surface and -185.636 mm for the rear surface. The tilt of the front surface in the X direction is -6.373°, and the tilt of the rear surface in the X direction is 5.630°. Its thickness along the optical axis is 14.516 mm, and it is made of fused silica. The distance between the curved prism 9 and the quadric surface mirror 10 is 9.047 mm.

[0101] The quadric surface reflector 10 has a circular structure with a diameter of 73 mm, a center thickness of 10 mm, a radius of curvature of -222.781 mm, a conic coefficient of 0.0903, an eccentricity of 1.286 mm in the Y direction, and a tilt of -3.427° in the X direction.

[0102] After the light reaches the quadric surface mirror 10, it returns along the original path, passing through the curved prism 9, the negative lens 8 and the positive lens 7 in sequence, and finally reaches the array detector 11. The distance between the positive lens 7 and the array detector 11 is 35.369 mm.

[0103] The imaging method based on the above-mentioned compact hyperspectral imager optical system specifically includes the following steps:

[0104] Step 1: Align the optical system of the compact hyperspectral imager described above with any position of the ground target, and acquire the target image at that position through the area array detector 11.

[0105] Step 2: Move the compact hyperspectral imager optical system to make it sweep along a fixed path to the ground target, so that the area array detector 11 can obtain target images of the ground target at multiple different locations in the space.

[0106] Step 3: The images of ground objects obtained by the area array detector 11 at multiple different locations in the space are stitched together to obtain a data cube that combines ground object images and atlases.

[0107] The compact hyperspectral imager optical system of this embodiment has an F-number of 3, an entrance pupil diameter of 45 mm, a field of view of 2°, and an operating wavelength of 400 nm–1000 nm. The detector pixel size is 6.5 μm, with a pixel scale of 2048 × 1152, and the actual effective number of pixels used is 544 (field of view) × 209 (dispersion direction). The ground resolution at an orbital altitude of 500 km is 24.07 m, and the instantaneous field of view is 48.15 μrad. The distance from the quadric mirror 10 to the array detector 11 along the z-axis is less than 235 mm; the distance from the freeform primary mirror 2 to the curved prism 9 along the y-axis is less than 161 mm. The system length along the x-axis (i.e., the diameters of the curved prism 9 and the quadric mirror 10) is less than 73 mm. The overall spatial dimensions of the optical system are less than 73 mm (X-axis) × 161 mm (Y-axis) × 235 mm (Z-axis).

[0108] The matrix point diagram in this embodiment is as follows: Figure 2 As shown, the diameter of the geometric blur spot across the entire field of view and the entire spectrum is smaller than or slightly larger than the Airy disk diameter. The maximum RMS diameter is 2.628 μm, which does not exceed the pixel size of 6.5 μm, indicating good imaging quality.

[0109] The modulation transfer function curves for different frequency bands in this embodiment are shown below. Figure 3 As shown, the optical transfer function across the entire field of view and across the entire wavelength is better than 0.62@77lp / mm.

[0110] This embodiment achieves 209 spectral bands (400nm~1000nm) in the visible and near-infrared bands, corresponding to 209 effective pixels in the spectral direction, with an average spectral resolution of 2.87nm, and all bands less than 7.5nm. The full-spectrum spectral resolution distribution of this embodiment is as follows: Figure 4 As shown.

[0111] The spectral bending (smile distortion) in this embodiment is as follows: Figure 5 As shown, the maximum spectral curvature across the entire spectrum is less than 0.055 pixels.

[0112] The spectral distortion (keystone distortion) in this embodiment is as follows: Figure 6 As shown, the maximum spectral distortion across the entire spectrum is less than 0.016 pixels.

[0113] If the optical system of the compact hyperspectral imager in this embodiment uses fused silica and the structural material uses 4J32 alloy with a low coefficient of thermal expansion, then theoretically, the characteristics of calorimetry can be achieved.

[0114] At 0°C, the maximum RMS diameter of the full-field-of-view, full-spectrum geometric blur spot in this embodiment is 2.586 μm, which does not exceed the pixel size of 6.5 μm.

[0115] At 40°C, the maximum RMS diameter of the full field of view and full spectrum geometric blur spot in this embodiment is 2.674 μm, which does not exceed the pixel size of 6.5 μm.

[0116] This shows that by using the same optical material and matching structural materials with similar coefficients of thermal expansion, calorimetry can be achieved over a wide temperature range. The optical performance remains almost unchanged compared to the results at room temperature (20°C). The hyperspectral imager can be used over a wide temperature range, thus avoiding the hassle of focusing. Therefore, the requirements for on-orbit thermal control can be relaxed, saving onboard resources.

[0117] The above description is only used to illustrate the technical solutions of the present invention, and is not intended to limit them. For those skilled in the art, modifications can be made to the specific technical solutions described in the above embodiments, or equivalent substitutions can be made to some of the technical features. However, these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions protected by the present invention.

Claims

1. A compact hyperspectral imager optical system, characterized in that: It includes a front-mounted telescope assembly, a slit (5), a dispersive beam-splitting assembly, and an area array detector (11); The front telescope assembly is an off-axis telecentric optical path system based on a freeform surface, which includes an aperture stop (1), a freeform primary mirror (2), a freeform secondary mirror (3), and a first planar folding mirror (4) arranged sequentially along the optical path. It is used to make the light emitted by the ground target converge and form an image on the primary image plane after passing through the aperture stop (1), the freeform primary mirror (2), the freeform secondary mirror (3), and the first planar folding mirror (4) in sequence. The slit (5) is located at the primary image plane and is used to limit the light-transmitting area on the primary image plane to form slit light; The dispersive beam splitter is a Dyson beam splitter system based on a curved prism, used for dispersive imaging of a primary image plane. It includes a second planar folding mirror (6), a positive lens (7), a negative lens (8), a curved prism (9), and a secondary curved surface mirror (10) arranged sequentially on the slit light path. The slit light passes through the second planar folding mirror (6), the positive lens (7), the negative lens (8), and the curved prism (9) in sequence before reaching the secondary curved surface mirror (10). After being reflected by the secondary curved surface mirror (10), the light then passes back through the curved prism (9), the negative lens (8), and the positive lens (7) in sequence to form probe light of different wavelengths. The area array detector (11) is located on the output light path of different wavelength detection light, and is used to image the detection light at different positions of the area array detector (11) according to different wavelengths.

2. The compact hyperspectral imager optical system according to claim 1, characterized in that: The aperture stop (1) has a circular ring structure; The freeform primary mirror (2) and the freeform secondary mirror (3) are both freeform reflecting mirrors; Both the first planar folding mirror (4) and the second planar folding mirror (6) are planar reflecting mirrors; Both the positive lens (7) and the negative lens (8) are spherical lenses; The front and back surfaces of the curved prism (9) are both spherical, and there is an included angle α between the rotational symmetry axes of the front and back surfaces, where 0° < α ≤ 30°. The surface shape of the quadric surface reflector (10) is a quadric surface.

3. The compact hyperspectral imager optical system according to claim 2, characterized in that: The aperture stop (1) is made of metal. The freeform primary mirror (2), freeform secondary mirror (3), first planar folding mirror (4), second planar folding mirror (6) and quadric surface reflecting mirror (10) are all made of metal or glass. The slit (5) is a metal slit or a glass slit; The positive lens (7), negative lens (8) and curved prism (9) are all made of fused silica.

4. The compact hyperspectral imager optical system according to claim 3, characterized in that: The slit (5) is a metal slit, which is a sandwich structure formed by sequentially arranging a glass sheet, a metal sheet, and another glass sheet; the metal sheet has a slit. Alternatively, the slit (5) is a glass slit, which is a structure of two planar glass sheets bonded together; a metal film is coated on the bonding surface of one of the planar glass sheets, and the metal film has a slit.

5. The compact hyperspectral imager optical system according to claim 4, characterized in that: The surfaces of the freeform primary mirror (2), the freeform secondary mirror (3), the first planar folding mirror (4), the second planar folding mirror (6), and the quadric surface reflecting mirror (10) are respectively coated with anti-reflection films; The front and rear surfaces of the positive lens (7), negative lens (8), and curved prism (9) are respectively coated with anti-reflective coatings; The surface of the glass sheet or flat glass sheet is coated with an anti-reflective film.

6. The compact hyperspectral imager optical system according to claim 5, characterized in that: The surface shapes of the freeform primary mirror (2) and the freeform secondary mirror (3) are both expressed using an 8th-order XY polynomial with non-rotational symmetry, and their expressions are as follows: Where z(x,y) is the surface elevation of the freeform surface, (x,y) are the coordinates of the freeform surface aperture, c is the curvature of the base quadratic surface, and k is the conic constant of the base quadratic surface. R is the radial and radial coordinates of a freeform surface. m It is the normalized radius, usually taken as 1, C i is the weight coefficient of the i-th XY polynomial, m is the degree of the x term in the XY polynomial, n is the degree of the y term in the XY polynomial, and 1≤m+n≤8.

7. The compact hyperspectral imager optical system according to claim 6, characterized in that: The aperture stop (1) has a diameter of 45 mm; The freeform primary mirror (2) has a circular structure with a diameter of 51 mm, a center thickness of 10 mm, a base surface curvature radius of 2185.710 mm, a conic coefficient of 0, and an inclination of 18.440° in the X direction. The freeform secondary mirror (3) has a circular structure with a diameter of 51 mm, a center thickness of 10 mm, a base surface curvature radius of 263.889 mm, a conic coefficient of 0, and an inclination of -15.202° in the X direction. The first planar folding mirror (4) has a circular structure with a diameter of 21 mm. Both the front and rear surfaces are planar, with a thickness of 5 mm, and an inclination of 47.8° in the X direction. The slit (5) is a circular structure with a diameter of 8 mm. The slit opening is symmetrical about the center of the quartz glass and has a size of 3.5345 mm × 0.0065 mm. The first planar folding mirror (6) has a circular structure with a diameter of 16 mm. Both the front and rear surfaces are planar, with a thickness of 5 mm, and an inclination of 39.5° in the X direction. The positive lens (7) has a circular structure with a diameter of 52 mm. Both the front and rear surfaces are spherical, with a radius of curvature of -100.531 mm on the front surface and -41.871 mm on the rear surface. The thickness is 7.009 mm. The negative lens (8) has a circular structure with a diameter of 57 mm. Both the front and rear surfaces are spherical, with a radius of curvature of -108.951 mm on the front surface and -321.484 mm on the rear surface. The thickness is 5.300 mm. The curved prism (9) has a circular structure with a diameter of 73 mm. Both the front and rear surfaces are spherical. The radius of curvature of the front surface is -215.654 mm, the radius of curvature of the rear surface is -185.636 mm, the tilt of the front surface in the X direction is -6.373°, the tilt of the rear surface in the X direction is 5.630°, and the thickness along the optical axis is 14.516 mm. The quadric surface reflector (10) has a circular structure with a diameter of 73 mm, a center thickness of 10 mm, a radius of curvature of -222.781 mm, a conic coefficient of 0.0903, an eccentricity of 1.286 mm in the Y direction, and an inclination of -3.427° in the X direction.

8. The compact hyperspectral imager optical system according to claim 7, characterized in that: The distance between the aperture stop (1) and the freeform primary mirror (2) is 70 mm; The distance between the freeform primary mirror (2) and the freeform secondary mirror (3) is 76.649 mm; The distance between the freeform secondary mirror (3) and the first planar folding mirror (4) is 98.463 mm; The distance between the first planar folding mirror (4) and the slit (5) is 35.5 mm; The distance between the slit (5) and the second planar folding mirror (6) is 27.375 mm; The distance between the first planar folding mirror (6) and the positive lens (7) is 15mm; The interval between the positive lens (7) and the negative lens (8) is 83.570 mm; The distance between the negative lens (8) and the curved prism (9) is 68.689 mm; The distance between the curved prism (9) and the quadrature reflector (10) is 9.047 mm; The distance between the positive lens (7) and the area array detector (11) is 35.369 mm.

9. The compact hyperspectral imager optical system according to claim 8, characterized in that: The center thickness of the freeform primary mirror (2) is 10mm; The center thickness of the freeform secondary mirror (3) is 10 mm; The center thickness of the first planar folding mirror (4) is 5 mm; The center thickness of the first planar folding mirror (6) is 5 mm; The center thickness of the positive lens (7) is 7.009 mm; The center thickness of the negative lens (8) is 5.300 mm; The thickness of the curved prism (9) along the optical axis is 14.516 mm; The center thickness of the quadric surface reflector (10) is 10 mm.

10. An imaging method based on the compact hyperspectral imager optical system according to any one of claims 1-9, characterized in that, Includes the following steps: Step 1: Align the compact hyperspectral imager optical system of any one of claims 1-7 with any position of the ground target, and acquire the target image at that position through the area array detector (11); Step 2: Move the compact hyperspectral imager optical system to make it sweep along a fixed path to the ground target, so that the array detector (11) can obtain target images of the ground target at multiple different locations in the space. Step 3: The images of ground objects obtained by the area array detector (11) at multiple different locations in the space are stitched together to obtain a data cube that combines ground object maps and atlases.

Images