Catadioptric optical system, imaging device and artificial satellite

Abstract

To provide a compact, high-performance catadioptric optical system. [Solution] The optical system comprises, in order of light passage from the object side, a primary mirror section consisting of an aspherical concave mirror with its concave surface facing the object side, a secondary mirror section arranged on the object side of the primary mirror section and consisting of an aspherical convex mirror with its convex surface facing the image side, and a refractive system arranged on the image side of the secondary mirror section, the refractive system comprising a front group with positive refractive power and a rear group with negative refractive power separated by a maximum air gap, the refractive system comprising at least five lenses, where PG is the Petzval sum of the refractive system obtained by normalizing the focal length of each surface by the focal length of the entire system, and PM is the Petzval sum of the reflective system obtained by normalizing the focal length of each surface by the focal length of the entire system, 0.6<|PG / PM|<1.2…(1) If the refractive power of the refractive system is φG and the refractive power of the entire system is φ0, then 0.05<|φG / φ0|<1.5…(2) 7 <PM<20…(3) It is characterized by satisfying the conditions (1), (2), and (3).

Description

[Technical Field]

[0001] The present invention relates to a catadioptric optical system consisting of a primary mirror section made of a concave reflecting mirror, a secondary mirror section made of a convex reflecting mirror, and a refractive section that corrects the imaging performance of the primary mirror section and the secondary mirror section, or to an imaging device using the same, or to an artificial satellite equipped with the same. [Background technology]

[0002] Among telephoto imaging optical systems with long focal lengths, there are catadioptric imaging optical systems that have both a reflecting system and a refractive system. In particular, catadioptric systems used in astronomical observation and photography often use Cassegrain optical systems, which can reduce the overall length by folding back light rays with two reflecting surfaces, a primary mirror and a secondary mirror. Here, the overall length is the distance on the optical axis from the frontmost surface of the optical system (the secondary mirror surface, which folds back light rays) to the final surface, plus the back focus.

[0003] The Ritchey-Chrétien optical system, an improvement on the classic Cassegrain optical system, uses aspherical surfaces close to hyperbolic surfaces on both the primary and secondary mirrors to correct spherical aberration and coma. However, astigmatism and curvature of field remain with the Ritchey-Chrétien optical system alone, limiting the field of view. Therefore, in order to obtain a wide field of view, the Ritchey-Chrétien optical system is sometimes combined with a corrective optical system that flattens the field of view.

[0004] Patent Document 1 discloses a telescope that combines a Ritchey-Chréchant optical system or a Cassegrain optical system with a corrective optical system consisting of three lenses made of the same glass material. Patent Document 2 discloses a catadioptric optical system that combines a spherical reflective system and a refractive system to correct aberrations, thereby achieving diffraction-limited performance within a field of view with a half angle of view of approximately 0.246°. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6395036 [Patent Document 2] Patent No. 6879723 Summary of the Invention [Problem to be solved by the invention]

[0006] In classical Cassegrain optical systems, the field of view is limited mainly by coma aberration, while in Ritchey-Chretien optical systems, it is limited mainly by astigmatism and curvature of field. Generally, these aberrations are corrected and a wide field of view is obtained by combining a refractive system. Patent documents 1 and 2 show optical systems in which a refractive system is placed between the secondary mirror and the image plane.

[0007] In the optical system disclosed in Patent Document 1, the correction optical system is constructed using the same glass material, making it difficult to correct chromatic aberration. Furthermore, only the number of lens elements and the surface irregularities are shown, and no specific data is provided, so it is unclear to what extent each aberration is corrected.

[0008] In the optical system disclosed in Patent Document 2, the total optical length is shortened by folding the optical path using a reflecting system, and aberrations are effectively corrected using a refractive system. However, when considering the weight of the entire optical system, the configuration is prone to increase the weight of the entire optical system because a lens with a larger effective diameter than the secondary mirror is placed near the secondary mirror. This is undesirable, particularly when considering applications of the optical system for artificial satellites, as it is expected to increase the cost of materials and processing for the lenses, as well as the launch cost due to the increased weight of the optical system and the structure that holds it.

[0009] The present invention provides a catadioptric optical system that has diffraction-limited imaging performance over a wide field of view and is compact relative to its focal length. [Means for solving the problem]

[0010] In order to solve the above problems, the catadioptric optical system of the present invention comprises, in order of light passage from the object side, a primary mirror unit consisting of an aspherical concave mirror with its concave surface facing the object side, a secondary mirror unit arranged on the object side of the primary mirror unit and consisting of an aspherical convex mirror with its convex surface facing the image side, and a refractive system arranged on the image side of the secondary mirror unit, The refractive system consists of a front group with positive refractive power and a rear group with negative refractive power, separated by a maximum air gap. The refractive system consists of at least five lenses, Let PG be the Petzval sum of the refractive system obtained by normalizing the focal length of each surface by the focal length of the entire system, and PM be the Petzval sum of the reflective system obtained by normalizing the focal length of each surface by the focal length of the entire system. 0.6<|PG / PM|<1.2 …(1) If the refractive power of the refractive system is φG and the refractive power of the entire system is φ0, then 0.05<|φG / φ0|<1.5 …(2) 7 <PM<20 …(3) It is characterized by satisfying the conditions (1), (2), and (3). [Effects of the Invention]

[0011] According to the present invention, it is possible to realize a catadioptric optical system that has diffraction-limited imaging performance over a wide field of view and is compact relative to its focal length. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a cross-sectional view of an optical system according to a first embodiment. [Figure 2] Longitudinal aberration diagram of the optical system of Example 1 [Figure 3] FIG. 10 is a cross-sectional view of the optical system of the second embodiment. [Figure 4] Longitudinal aberration diagram of the optical system of Example 2 [Figure 5] FIG. 10 is a cross-sectional view of the optical system of the third embodiment. [Figure 6] Longitudinal aberration diagram of the optical system of Example 3 [Figure 7] FIG. 10 is a cross-sectional view of an optical system according to a fourth embodiment. [Figure 8] Longitudinal aberration diagram of the optical system of Example 4 [Figure 9] FIG. 10 is a cross-sectional view of an optical system according to a fifth embodiment. [Figure 10] Longitudinal aberration diagram of the optical system of Example 5 [Figure 11] 1 is a schematic diagram of an imaging device of the present invention. [Figure 12]1 is a schematic diagram of an artificial satellite equipped with an optical system according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Figures 1, 3, 5, 7, and 9 are cross-sectional views of catadioptric optical systems (hereinafter simply referred to as optical systems) of Examples 1 to 5, respectively. Figures 2, 4, 6, 8, and 10 are longitudinal aberration diagrams of the optical systems of Examples 1 to 5, respectively. The optical systems of each embodiment are suitable for imaging devices such as digital video cameras, digital still cameras, broadcast cameras, silver halide film cameras, and surveillance cameras, as well as optical instruments mounted on artificial satellites to image the Earth and other celestial bodies.

[0014] In the cross-sectional views of the optical system of each embodiment, the left side is the object side and the right side is the image side. The optical system of each embodiment is composed of a plurality of lens groups, and each lens group may include an aperture stop. The optical system of each embodiment has a primary mirror unit consisting of a reflecting surface M1 with a concave surface facing the object side, a secondary mirror unit consisting of a reflecting surface M2 with a convex surface facing the image side, and a refractive system located on the image side of the secondary mirror unit, and light from the object is reflected by the concave mirror M1 and the convex mirror M2 and then forms an image through the refractive system. IP is an image plane. When the optical system of each embodiment is used as an imaging optical system, the imaging surface of an imaging element (photoelectric conversion element) such as a CCD sensor or CMOS sensor or the surface of a silver halide film is located on the image plane IP. In the spherical aberration diagram of the longitudinal aberration diagram, Fno indicates the F-number, the solid line indicates spherical aberration for the d-line (wavelength 587.6 nm), the two-dot chain line indicates spherical aberration for the g-line (wavelength 435.8 nm), the one-dot chain line indicates spherical aberration for the C-line (wavelength 656.3 nm), the long-dashed line indicates spherical aberration for light with a wavelength of 950.0 nm, and the dashed line indicates the sine condition. In the astigmatism diagram, the solid line ΔS indicates the amount of field curvature at the sagittal image plane, and the dashed line ΔM indicates the amount of field curvature at the meridional image plane. Distortion aberration is shown for the d-line. The chromatic aberration diagram shows lateral chromatic aberration for the g-line, C-line, and light with a wavelength of 950.0 nm. ω is the half angle of view (°).

[0015] Next, a description will be given of the configuration common to the optical systems of each embodiment. As mentioned above, the optical systems of each embodiment reflect light from an object first off a reflecting surface M1 with a concave surface facing the object side, and then off a reflecting surface M2 with a convex surface facing the image side, and then form an image through the refractive system. The refractive system is composed of a front group Gp with positive refractive power and a rear group Gn with negative refractive power, separated by a maximum air gap.

[0016] In such an optical system, in order to realize good imaging performance that satisfies the diffraction limit over a wide field of view while keeping the entire optical system small, it is necessary to appropriately set the configuration of the entire optical system. By configuring the reflecting surfaces M1 and M2 of the optical system described above to fold back light rays, it is possible to obtain an optical system that is compact relative to its focal length. Furthermore, it is desirable to configure both reflecting surfaces M1 and M2 as aspherical surfaces that are close to hyperbolic surfaces to form a Ritchey-Chretien optical system that eliminates spherical aberration and coma. This makes it easy to correct spherical aberration and coma, including in the refractive system, which will be explained later.

[0017] In the case of a configuration consisting only of reflective surfaces as described above, the field of view in which good imaging performance can be obtained is limited primarily by astigmatism and curvature of field. For this reason, in order to obtain good imaging performance over a wider field of view, it is necessary to include a refractive system and effectively correct the astigmatism and curvature of field that remain in the reflective system.

[0018] The refractive system described above comprises, in order from the object side, a front group Gp with positive refractive power and a rear group Gn with negative refractive power, and is configured so that light rays traveling toward the image side via the reflecting system are converged by the front group Gp with positive refractive power.

[0019] With this configuration, the effective beam diameter of the entire refractive system can be kept small, and the shading of the center of the reflecting surface M1 due to the arrangement of the refractive system can be kept small.

[0020] For the same reason, it is desirable that the refractive surface closest to the object side has a convex shape facing the object side, and it is further desirable that the lens closest to the object side has positive refractive power.

[0021] Since chromatic aberration does not occur in a reflective system, it is desirable to effectively correct the chromatic aberration that occurs in the refractive system itself.In addition, it is desirable to effectively correct the spherical aberration and coma that occur in the refractive system.

[0022] In a refractive system, in order to maintain an appropriate Petzval sum, which contributes to field curvature, and to effectively correct chromatic aberration over a wide wavelength range from visible to near-infrared, it is necessary to appropriately select the Abbe number and refractive index of the glass material used in the refractive system. Since glass materials have a certain distribution of Abbe number and refractive index, and it is not possible to select a glass material with any desired characteristics, it is necessary to select appropriate glass materials for the multiple positive and negative lens elements in the refractive system to balance Petzval sum and chromatic aberration.

[0023] In order to obtain a refractive system that satisfactorily corrects chromatic aberration, spherical aberration, and coma, as well as astigmatism and field curvature that occur in a reflective system, the refractive system is composed of at least five lenses.

[0024] The front group Gp of the refractive system is preferably composed of, from the object side, a lens group with negative refractive power made of a radiation-resistant glass material (radiation-resistant glass material), and a lens group with positive refractive power. By composing the front group Gp of multiple lenses including positive and negative lenses, it is possible to correct spherical aberration and axial chromatic aberration to a certain extent within the front group Gp, making it easier to correct aberrations throughout the refractive system.

[0025] Furthermore, it is more preferable that the refractive front group Gp be composed of, in order from the object side, a positive lens, a negative lens and a positive lens.

[0026] In outer space, the intensity of radiation such as electron beams and proton beams is stronger than on Earth, and it is known that ordinary glass materials become discolored when exposed to radiation for a long time.

[0027] Considering use in outer space, it is preferable that the glass material of the lens on the object side, which is most exposed to radiation, has a radiation-resistant function that minimizes coloration due to radiation, and that the lens on the image side is protected from radiation.

[0028] As described in Patent No. 6829548, for example, the radiation resistance function is such that, when the absorbed dose from irradiation with cobalt-60 gamma rays is 0.5 MGy, the change Δλ5 in λ5 after exposure to gamma rays relative to λ5 before exposure to gamma rays is 20 nm or less, and the change Δλ80 in λ80 after exposure to gamma rays relative to λ80 before exposure to gamma rays is 230 nm or less, where λ5 is the wavelength at which the light transmittance, including reflection losses, of a 10 mm thick parallel plate sample is 5%, and λ80 is the wavelength at which the light transmittance is 80%.

[0029] Radiation resistance can be achieved by adding ions or compounds of Ce, Sn, Sb, etc. to the glass material. Synthetic quartz, which is less likely to be discolored by radiation exposure, can also be used as the radiation-resistant glass material.

[0030] Correcting chromatic aberration is often a challenge in optical systems with long focal lengths, such as telescopes. Suppressing axial chromatic aberration is particularly important in the front group of a refractive system. Axial chromatic aberration can generally be improved by selecting glass materials with low chromatic dispersion or by combining lenses with positive and negative refractive powers and different chromatic dispersions. However, there are fewer types of radiation-resistant glass materials available than standard glass materials, making it difficult to select low-dispersion glass materials with an Abbe number of 70 or higher. For this reason, it is preferable to correct chromatic aberration by configuring the front group with positive refractive power with a group made of radiation-resistant glass material with weak negative refractive power and a group made of low-dispersion glass material with strong positive refractive power.

[0031] In such a configuration, if the group made of radiation-resistant glass were to consist of a single lens, it would likely have a meniscus shape with a large radius of curvature, making it difficult to center the lens using a bell clamp method, for example. Therefore, by configuring the group made of radiation-resistant glass closest to the object with, in order from the object side, a positive lens and a negative lens, it becomes easier to maintain the shape of the lenses making up the front group in a shape that facilitates centering.

[0032] Furthermore, such a configuration of positive and negative lenses made of radiation-resistant glass material is a configuration that makes it easy to ensure the total thickness of the center thickness and the total thickness of the edge thickness of the radiation-resistant glass material required to protect the image-side lenses from radiation.

[0033] Next, the conditions that the optical system of each embodiment must satisfy will be described.

[0034] The optical system of each embodiment satisfies the following expressions (1) to (3). 0.6<|PG / PM|<1.2 …(1) 0.05<|φG / φ0|<1.5 …(2) 7 <PM<20 …(3)

[0035] In equation (1), PG represents the Petzval sum of the refractive system obtained by normalizing the focal length of each surface by the focal length of the entire system, and PM represents the Petzval sum of the reflective system obtained by normalizing the focal length of each surface by the focal length of the entire system.

[0036] In the formula (2), φG represents the refractive power of the refractive system, and φ0 represents the refractive power of the entire optical system.

[0037] The condition in formula (1) is a condition regarding the ratio of the Petzval sums of the refractive system and the reflective system. If the lower limit of formula (1) is not met, it becomes difficult for the refractive system to sufficiently correct the field curvature that occurs in the reflective system. If the upper limit of formula (1) is exceeded, the field curvature is over-corrected by the refractive system.

[0038] The condition of formula (2) is a condition regarding the ratio of the refractive power of the refractive system to the refractive power of the entire optical system. If the lower limit of formula (2) is not met, the overall length of the optical system becomes long, which is disadvantageous for miniaturization. If the upper limit of formula (2) is met, it becomes difficult to ensure the back focus of the optical system.

[0039] The condition in equation (3) is related to the Petzval sum of the reflecting system. There is an imaging surface on the image side of the primary mirror, and since an imaging device will be placed there, a sufficient amount of extraction must be ensured. Here, the extraction amount is the distance from the primary mirror surface to the imaging position. To increase the amount of extraction while keeping the overall length small, the Petzval sum of the reflecting system must be maintained appropriately. Designs that shorten the overall length of the reflecting system and those that increase the amount of extraction both increase the Petzval sum of the reflecting system. Therefore, if the lower limit of equation (3) is exceeded, it becomes difficult to ensure a sufficient amount of extraction while keeping the overall length small. If the upper limit of equation (3) is exceeded, it becomes difficult to correct the image plane using a refractive system. Furthermore, the secondary mirror diameter relative to the primary mirror diameter increases due to the arrangement of the reflecting system, resulting in significant pupil obstruction by the secondary mirror.

[0040] It is more preferable that the optical system of each embodiment satisfies the following expressions (4) and (5). 2 <f0 / R1<80 …(4) νdGp2> νdGp1 …(5)

[0041] In equation (4), f0 represents the focal length of the entire optical system, and R1 represents the radius of curvature of the surface of the refractive system closest to the object.

[0042] In formula (5), νdGp1 represents the largest Abbe number among the glass materials that make up the group of radiation-resistant glass materials that are arranged on the object side of the front group, and νdGp2 represents the smallest Abbe number among the glass materials that make up the front group and that are not included in the group of radiation-resistant glass materials on the object side.

[0043] The condition of formula (4) is a condition regarding the ratio between the focal length f0 of the entire optical system and the radius of curvature RGp1 of the surface of the front group Gp closest to the object. If the lower limit of formula (4) is not met, light rays entering the refractive system are not sufficiently converged by the surface closest to the object of the refractive system, and the maximum effective diameter of the refractive system thereafter becomes large. If the upper limit of formula (4) is met, light rays are strongly bent by the object-side surface in the refractive system with the highest axial ray height, causing large spherical aberration and making it difficult to correct the spherical aberration in the refractive system.

[0044] Equation (5) is a condition related to the Abbe number of the glass material used in the lenses that make up the front group. By satisfying equation (5), the radiation-resistant glass material that makes up the front group will have negative refractive power, and the regular glass material will have positive refractive power, creating a relationship in which chromatic aberrations are cancelled out. If equation (5) is not satisfied, a relationship of chromatic aberration correction between a radiation-resistant glass material with positive refractive power and a regular glass material with negative refractive power will be required, and chromatic aberration correction over a wide wavelength range will become difficult because the glass materials, which have relatively large dispersions, will cancel out chromatic aberrations.

[0045] It is more preferable that the glass materials constituting the front group and not included in the group of radiation-resistant glass materials on the object side, for which the condition for the Abbe number is expressed in equation (5), satisfy the following equation (6) in addition to the condition of equation (5): (-4×10^-3)×νdGp2+1.78 <ndGp2 …(6)

[0046] In formula (6), ndGp2 represents the refractive index of the glass material that constitutes the front group and is not included in the group of radiation-resistant glass materials on the object side.

[0047] The condition in equation (6) relates to the refractive index ndGp2 of the glass material that makes up the front group and that is not included in the group of radiation-resistant glass materials on the object side. In order to correct chromatic aberration over a wide wavelength range, from the visible to the near-infrared, it is necessary to select glass materials that take anomalous dispersion into consideration. Glass materials that do not satisfy the condition in equation (6) have low anomalous dispersion, making it difficult to correct the secondary spectrum of axial chromatic aberration.

[0048] It is more preferable that the numerical ranges of the formulas (1) to (5) are the numerical ranges of the following formulas (1a) to (5a). 0.8<|PG / PM|<1.0 …(1a) 0.1<|φG / φ0|<1.0 …(2a) 10 <PM<15 … (3a) twenty five <f0 / R1<45 …(4a) 73<νdGp2<100 …(5a)

[0049] Furthermore, it is more preferable that the refractive rear group Gn be configured in the following order from the object side: a negative lens, a negative lens, a positive lens, and a positive lens. With this configuration, light rays can be bent gently toward the image side, thereby suppressing the occurrence of aberrations.

[0050] Furthermore, in optical systems intended for use in space, contamination of optical elements due to outgassing from materials such as plastics and adhesives, and the resulting degradation of optical performance and image quality, are often problems. Once contamination occurs in space, it is difficult to remove the contamination, so it is necessary to use materials with low outgassing and to remove the source of outgassing. One possible source of outgassing in an optical system is the adhesive used to bond lenses together. For this reason, it is more preferable that the refractive system does not include cemented lenses.

[0051] (Example) Numerical Examples 1 to 5 corresponding to Examples 1 to 5 are shown below. In the surface data of each numerical example, surface number i indicates the ith surface counted from the light incident side. r is the radius of curvature of the ith surface (mm), d is the lens thickness or air gap (mm) between the ith and (i+1)th surfaces, and nd is the refractive index of the material of the ith optical component at the d-line. νd is the Abbe number based on the d-line of the material of the ith optical component. The Abbe number νd is expressed as νd = (Nd-1) / (NF-NC), where Nd, NF, and NC are the refractive indices at the Fraunhofer d-line (587.6 nm), F-line (486.1 nm), and C-line (656.3 nm).

[0052] The above d, focal length (mm), F-number, and half angle of view (°) are all values ​​when the optical system of each example is focused on an object at infinity. BF represents back focus (mm). Back focus is the distance on the optical axis from the final surface of the optical system (the lens surface closest to the image) to the paraxial image plane, expressed as an air-equivalent length. The total lens length is the distance on the optical axis from the forefront of the optical system (reflective surface M2, which reflects light rays) to the final surface plus the back focus.

[0053] An asterisk (*) next to a surface number indicates that the surface has an aspherical shape. The aspherical shape is expressed by the following equation, where x is the displacement from the vertex of the surface in the optical axis direction, h is the height from the optical axis in a direction perpendicular to the optical axis, R is the paraxial radius of curvature, K is the conic constant, and A4, A6, A8, A10, and A12 are aspherical coefficients of each order. x=(h 2 / R) / [1+{1-(1+K)(h / R) 2} 1 / 2 +A4×h 4 +A6×h 6 +A8×h 8 +A10×h 10 +A12×h 12

[0054] [Table 1]

[0055] [Table 2]

[0056] [Table 3]

[0057] [Table 4]

[0058] [Table 5]

[0059] The values ​​of the formulas (1) to (6) in each numerical example are summarized in the following Table 1. As shown in Table 1, the optical systems according to all of the examples satisfy each of the conditional formulas.

[0060] [Table 6]

[0061] 11 shows the configuration of an imaging device 10 as an optical apparatus of Example 1, which uses the optical system of each of the above Examples as an imaging optical system. The imaging device 10 has an imaging optical system configured using the optical system of any of Examples 1 to 5. The imaging device 10 also has an imaging element (light receiving element) 11, such as a CCD sensor or CMOS sensor, that receives an optical image formed by the imaging optical system and performs photoelectric conversion. The imaging device 10 may be a single-lens reflex camera with a quick-turn mirror, or a mirrorless camera without a quick-turn mirror.

[0062] 12 shows the optical system of each of the above-described embodiments mounted on an artificial satellite as an imaging optical system. The optical system of the present invention may be applied to an imaging device and mounted on an artificial satellite for observing and imaging the Earth and other celestial bodies from outer space.

[0063] The embodiments described above are merely representative examples, and various modifications and changes can be made to each embodiment when implementing the present invention. [Explanation of symbols]

[0064] M1 concave mirror M2 convex mirror Gp Refractive front group Gp1 Radiation-resistant glass lens on the object side of the front group of the refractive system Gn Refractive system rear group

Claims

1. The optical system comprises, in order of light passage from the object side, a primary mirror unit consisting of an aspherical concave mirror with its concave surface facing the object side, a secondary mirror unit arranged on the object side of the primary mirror unit and consisting of an aspherical convex mirror with its convex surface facing the image side, and a refractive system arranged on the image side of the secondary mirror unit. The refractive system consists of a front group with positive refractive power and a rear group with negative refractive power, separated by a maximum air gap. The refractive system consists of at least five lenses, Let PG be the Petzval sum of the refractive system obtained by normalizing the focal length of each surface by the focal length of the entire system, and PM be the Petzval sum of the reflective system obtained by normalizing the focal length of each surface by the focal length of the entire system. 0.6<|PG / PM|<1.2...(1) If the refractive power of the refractive system is φG and the refractive power of the entire system is φ0, then 0.05<|φG / φ0|<1.5...(2) 7<PM<20...(3) A catadioptric optical system characterized by satisfying the following conditions (1), (2), and (3).

2. The surface of the front group closest to the object side has a convex shape toward the object side, and the radius of curvature of the surface is R1. The focal length of the entire system is f0. 2<f0 / R1<80...(4) 2. The catadioptric optical system according to claim 1, wherein the following condition is satisfied:

3. 3. The catadioptric optical system according to claim 2, wherein the glass material of the lens closest to the object side of the refractive system is made of a radiation-resistant glass material.

4. Let νdGp1 be the largest Abbe number among the glass materials constituting the group made of radiation-resistant glass materials and arranged on the object side of the front group, and νdGp2 be the smallest Abbe number among the glass materials constituting the front group that are not included in the group made of radiation-resistant glass materials on the object side. νdGp2>νdGp1 (5) 4. The catadioptric optical system according to claim 3, wherein the following conditional expression (5) is satisfied:

5. 4. The catadioptric optical system according to claim 3, wherein the front lens group includes, in order from the object side, a group consisting of a positive lens made of a radiation-resistant glass material and a negative lens made of a radiation-resistant glass material.

6. 5. The catadioptric optical system according to claim 4, wherein the front lens group is configured in the following order from the object side: a positive lens, a negative lens, and a positive lens.

7. 7. The catadioptric optical system according to claim 6, wherein the rear group is configured in the following order from the object side: a negative lens, a negative lens, a positive lens, and a positive lens.

8. An optical instrument comprising: the catadioptric optical system according to claim 1; and a light-receiving element that receives light from the catadioptric optical system.

9. An artificial satellite equipped with the catadioptric optical system according to any one of claims 1 to 7.

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