An imaging optical system comprising three mirrors

Abstract

An imaging optical system (1) comprising three mirror surfaces (M1, M2, M3) having a configuration adapted to block stray light rays that can otherwise reach an image sensor (2) of the system, while permitting a large field, a high aperture and a good system compactness. The system can also incorporate two entrance baffles (11, 12) respectively arranged on both sides of the optical entrance of the system. The function of the two entrance baffles can be limited to intercepting light rays originating from a field that is angularly far from the entrance field available for each captured image. The two entrance baffles can thus have a reduced length upstream, so that the system has a small size.

Description

Technical Field

[0001] This article relates to an imaging optical system comprising three mirrors, and to an optoelectronic imaging device comprising the same system. Background Technology

[0002] Imaging optics systems comprising three mirrors are used in a wide range of applications. Specifically, these systems can be telescope-type systems, and the article "Concurrent engineering of anext-generation freeform telescope: optical design" by A. Bauer et al., published in SPIE Proc, May 14, 2019, Vol. 10998, pp. 109980W-1 to 109980W-8, proposes several new configurations of imaging optics, each consisting of three freeform mirrors. A general challenge with imaging optics is, in particular, their size and the possibility of reducing the amount of stray light superimposed on the image formed by the mirrors. Conventionally, one or more baffles are used, arranged in a manner suitable for reducing the amount of stray light reaching the image sensor of this imaging optics; however, some of these baffles, especially the most effective ones, significantly increase the size of the system. In addition to increasing its size, such baffles also increase the rotational inertia of the imaging optics during the applied rotation, in order to scan large scenes captured in several consecutive images. One application requiring rapidly rotating imaging optics with high angular acceleration is the supply of electro-optical pods intended for monitoring and detecting aircraft, such as helicopters or drones. Therefore, it is crucial to obtain low levels of stray light in the captured images while minimizing the baffles incorporated into the imaging optics.

[0003] [ Figure 1[Figure 1] is a diagram of one of the configurations mentioned in the article by A. Bauer et al. listed above. This imaging optical system, generally indicated by reference numeral 1, is a type comprising three mirrors: a primary mirror designated M1, a secondary mirror designated M2, and a tertiary mirror designated M3. These mirrors are adapted and arranged such that light originating from a scene located in the system's incident field is first reflected by mirror M1, then by mirror M2, and then by mirror M3 to form an image of the scene in the system's focal plane designated PF. Therefore, any light originating from the scene that contributes to image formation is divided into an initial segment upstream of mirror M1, a first intermediate ray segment between mirror M1 and mirror M2, a second intermediate ray segment between mirror M2 and mirror M3, and a terminal ray segment between mirror M3 and the focal plane PF. The secondary mirror M2 may be convex, and the tertiary mirror M3 may be concave. The direction of curvature of the primary mirror M1 may vary depending on its position on the mirror. Three mirrors, M1, M2, and M3, have free reflective surfaces. In a known manner, free surfaces do not exist within any surface having rotational symmetry.

[0004] Throughout this text, the terms upstream and downstream are defined relative to the propagation direction of light rays originating from the scene and forming the image in the focal plane PF. Furthermore, the terms paraxial ray or principal ray are used to refer to a ray originating from the scene and passing through the center of the entrance pupil of system 1 to form the image in the focal plane PF, having zero angular deviation relative to the optical axis of the system. [In [ Figure 1 In the figure, paraxial rays are designated by reference numeral RP, their initial segment by reference numeral RP0, their first and second intermediate segments by reference numerals RP1 and RP2 respectively, and their terminal segment by reference numeral RP3. Rays originating from elements of the scene located at the boundary of the incident field of system 1 and passing through the edge of the system's incident pupil are called field edge rays.

[0005] exist[ Figure 1 In the system, mirrors M1 and M2 are oriented such that the second intermediate segment RP2 of the paraxial ray RP intersects its initial segment RP0. This configuration of mirrors M1 and M2 is called the α configuration. Furthermore, mirrors M2 and M3 are oriented such that the terminal segment RP3 of the paraxial ray RP passes through the lateral side of mirror M2, which is opposite to the lateral offset of mirror M1 relative to mirror M2. In this way, the terminal segment RP3 of the paraxial ray RP does not intersect the first intermediate segment RP1 of the ray. This configuration of mirrors M2 and M3 is called the z configuration. Therefore, system 1 has an overall optical configuration called the α-z configuration.

[0006] System 1 further includes image sensor 2 arranged such that the photosensitive surface S of this image sensor is superimposed on the focal plane PF. The photosensitive surface S extends from the upstream boundary L. AMExtending to the downstream boundary LAV, the upstream boundary L of the photosensitive surface S of image sensor 2 AM and downstream boundary L AV These boundaries are defined by their corresponding projections onto the initial segment of the paraxial ray RP0 and by the propagation direction of the paraxial ray RP in this initial segment.

[0007] Technical issues

[0008] Therefore, the objective of this invention is to propose a new imaging optical system that reduces the amount of stray light reaching the image sensor.

[0009] An additional objective of this invention is to have a small size for the imaging optical system.

[0010] Another additional objective of the present invention is that the imaging optical system can have a large incident field and / or a large incident pupil.

[0011] Another objective of this invention is to enable the low-cost manufacture of imaging optical systems. Summary of the Invention

[0012] To achieve at least one of these or other objectives, a first aspect of the invention proposes an imaging optical system comprising three mirrors of the type described above, wherein the secondary and tertiary mirrors are oriented such that the upstream boundary of the photosensitive surface of the image sensor is offset downstream relative to a straight line connecting the upstream edge of the primary mirror to the upstream edge of the secondary mirror or to the upstream edge of a screen surrounding the secondary mirror. In this way, the secondary mirror or the screen surrounding it intercepts light rays that otherwise propagate directly from the primary mirror to the photosensitive surface of the image sensor in a straight line.

[0013] As indicated above, the upstream and downstream edges of the primary mirror and corresponding secondary mirrors are defined with respect to their respective projections onto the initial segment of the paraxial ray and with respect to the propagation direction of the paraxial ray in this initial segment. Similarly, the downstream edge of the upstream boundary of the photosensitive surface of the image sensor is parallel to the initial segment of the paraxial ray and oriented according to the propagation direction of the paraxial ray in this initial segment.

[0014] The system's configuration, in which the secondary mirror is thus located between the image sensor and the primary mirror, makes it possible to block any stray light that would otherwise reach the image sensor via the primary mirror, especially light that would enter through the system's optical entrance and be reflected toward the image sensor by the primary mirror.

[0015] According to an improvement of the invention, the system may further include a first incident baffle superimposed on the initial segment of the first field-edge edge rays, located on a first side of the incident field identical to that of the image sensor, and opposite the tertiary mirror. In this case, the first incident baffle may have a downstream edge that engages the terminal segments of the second field-edge edge rays. These second field-edge edge rays may be opposite the first field-edge edge rays in a beam of light entering the system and forming an image, particularly when the system has a plane of symmetry common to the three mirrors. This first incident baffle blocks some light that would otherwise enter the system via its optical entrance, light that is directed directly toward the image sensor. Additionally, this first incident baffle may have a length starting from its downstream edge such that the upstream edge of this first incident baffle intercepts light that would otherwise enter the system via its optical entrance toward the tertiary mirror and be reflected toward the image sensor by the tertiary mirror.

[0016] Thanks to the features of this invention, according to which the upstream boundary of the photosensitive surface of the image sensor is offset downstream relative to the straight line connecting the respective upstream edges of the primary and secondary mirrors, the first incident baffle can have a reduced length of an initial segment parallel to the edge rays of the first field edge. The size of the system incorporating the first incident baffle is thus reduced.

[0017] According to a complementary improvement of the invention, the system may further include a second incident baffle superimposed on the initial segment of the edge rays of the second field, located on a second side of the incident field identical to that of the tertiary mirror, and opposite the image sensor. This second incident baffle may then have an upstream edge connected to the tertiary mirror, or a downstream edge connected to a screen surrounding the tertiary mirror, or a downstream edge connected to an opaque support for the tertiary mirror. Alternatively, the downstream edge of the second incident baffle may be located downstream of the straight line connecting the upstream boundary of the photosensitive surface of the image sensor to the downstream edge of the first incident baffle. This second incident baffle further reduces light that would otherwise enter the system via its optical entrance, which is directed directly toward the image sensor.

[0018] Preferably, the second incident baffle may have an upstream edge located upstream of the straight line connecting the downstream edge of the first incident baffle to the downstream boundary of the photosensitive surface of the image sensor. Thus, the first and second incident baffles cooperate to intercept all light that would otherwise enter the system via its optical entrance, light that is directed directly toward the image sensor or toward the tertiary mirror.

[0019] Again, thanks to the features of the present invention, according to which the upstream boundary of the photosensitive surface of the image sensor is offset downstream relative to the straight line connecting the respective upstream edges of the primary and secondary mirrors, the second incident baffle can have a reduced length of the initial segment parallel to the edge rays of the second field edge. The size of the system incorporating the second incident baffle is thus also reduced.

[0020] In a preferred embodiment of the invention, at least one of the following additional features may be reproduced, either alone or in combination thereof:

[0021] - At least one of the primary mirror, secondary mirror, and tertiary mirror may have a free-reflecting surface;

[0022] - The longitudinal dimension of the image sensor determines the first viewpoint of the system, and the system can be adapted such that this first viewpoint is greater than or equal to 9°, preferably greater than or equal to 18°. The system's incident field can therefore be relatively large, and preferably less than 45°;

[0023] - The image sensor may have a matrix arrangement, in which the lateral dimension of the image sensor perpendicular to its longitudinal dimension determines the second viewpoint of the system. In this case, the system may also be adapted such that the second viewpoint is greater than or equal to 12°, preferably greater than or equal to 24°, and preferably less than 60°;

[0024] The system can have an aperture number N of less than 5, preferably less than 2, where N equals f / D, and f is the focal length of the system and D is the size of the entrance pupil. A larger aperture number N will result in a larger entrance pupil.

[0025] - The value of focal length f can be less than or equal to 100mm, preferably less than or equal to 20mm and greater than 2mm;

[0026] - The image sensor may be of a type that is at least partially sensitive to spectral intervals ranging from 360 nanometers (nm) to 14 micrometers (μm) in wavelength values ​​of light originating from a scene. More specifically, the image sensor may be a thermal sensor of the type of calorimeter or microbolometer.

[0027] The system may further include a pupillary septum located at the primary or tertiary mirror, preferably at the tertiary mirror. For both locations, the pupillary septum can have a simple, particularly circular, square, or rectangular, opening shape. Furthermore, its size is smaller when located at the tertiary mirror. Advantageously, the pupillary septum can be formed from the outer edge of the primary or tertiary mirror.

[0028] - The system may further include a spectral separation device arranged between the tertiary mirror and the image sensor, and an additional image sensor arranged in the image of the system's focal plane, the image having been formed by the spectral separation device;

[0029] - Primary, secondary, and tertiary mirrors can be housed within a sphere with a diameter between 2 and 6 times the value of the system's focal length f; and

[0030] - At least one of the primary, secondary, and tertiary mirrors may include a rigid portion made of injected polymeric material and optionally a reflective metallic layer.

[0031] Finally, a second aspect of the invention provides an optoelectronic imaging device comprising the system indicated in the first aspect above. This device may be, but is not limited to, an airborne vehicle guidance system, a thermal camera, a visual aid, or an optoelectronic pod for monitoring and detection. Attached Figure Description

[0032] Referring to the accompanying drawings, the features and advantages of the invention will become more apparent from the following detailed description of some non-limiting embodiments, the drawings of which include:

[0033] [ Figure 1 [This is an optical diagram of an imaging optical system known prior to this invention;]

[0034] [ Figure 2 [This is an optical diagram of the imaging optical system according to the present invention;]

[0035] [ Figure 3a ] corresponds to [ Figure 2 The features of the present invention are also described.

[0036] [ Figure 3b ] corresponds to [ Figure 3a [This is to illustrate other features of the invention;]

[0037] [ Figure 4 ] corresponds to [ Figure 2 Improvements to the present invention; and

[0038] [ Figure 5 [Display combined] Figure 2 The system's photoelectric imaging device. Detailed Implementation

[0039] For clarity, the dimensions of the elements shown in these figures do not correspond to actual dimensions or ratios of actual dimensions. Furthermore, the same numbers indicated in different figures refer to the same or functionally identical elements. It can be assumed that the planes of the figures constitute the planes of symmetry of the represented imaging optical system, but such symmetry is not essential to the invention. In practice, the three mirrors of each system can be angled, such that the paraxial ray segments are not coplanar.

[0040] exist[ Figure 1 ]arrive[ Figure 4 In the diagram, an orthogonal coordinate system x, y, z is used such that the x-axis is perpendicular to the plane of the diagram, the z-axis is parallel to the initial segment RP0 of the paraxial ray RP and oriented along the direction of propagation of the ray in this segment, and the y-axis is oriented such that the terminal segments of the rays that contribute to the formation of the image in the focal plane PF have a positively oriented projection on the y-axis, the rays being oriented according to their propagation directions. The yz plane, the plane in the diagram, can be the plane of symmetry of system 1, encompassing the plane of symmetry of the reflecting surfaces of each of mirrors M1, M2, and M3. The terms upstream and downstream are defined by comparing the corresponding positions of the projections of the boundaries or edges of the optical components on this z-axis with respect to the z-axis. Specifically, for mirror M1, and correspondingly M2, the downstream edge B... AV1 Accordingly B AV2 With upstream edge B AM1 Accordingly B AM2 relatively.

[0041] exist[ Figure 1 The straight line D0 indicated in the diagram connects mirrors M1 and M2, and is labeled B respectively. AM1 and B AM2 The upstream edge. It shows that image sensor 2 is still at least partially offset upstream of this line D0 about the z-axis. Due to these relative positions of line D0 and image sensor 2, stray light can propagate directly from mirror M1 to image sensor 2. This stray light can originate from the scene facing the optical entrance of system 1, be reflected by mirror M1 towards image sensor 2, and then reach image sensor 2 directly through the upstream side of mirror M2. In [ Figure 1 The reference numeral R1 in the figure indicates the ray of this stray light. Since the ray of stray light R1 is slightly tilted relative to the optical axis of system 1 at its optical entrance, meaning that it is slightly tilted relative to the z-axis before being reflected by mirror M1, its elimination by the field edge mask placed at the optical entrance of system 1 will require this mask to have a large length in the direction upstream of the optical entrance.

[0042] [ Figure 2 Display and [ Figure 1This is a system of the same type as 1, but modified by the present invention. In the present invention, mirrors M2 and M3 are positioned and tilted such that the photosensitive surface S of the image sensor 2 is completely offset upstream relative to line D0. In other words, the upstream boundary L of the photosensitive surface S is... AM Located on the downstream side of line D0. In this way, stray light no longer propagates directly from mirror M1 to image sensor 2: light rays similar to ray R1 are all blocked by the present invention. Obviously, the upstream edge of mirror M2 can be replaced by the upstream edge of the peripheral screen of mirror M2 extending upstream of the mirror to define line D0.

[0043] exist[ Figure 2 In system 1, mirror M3 constitutes the entrance pupil.

[0044] for[ Figure 2 In an embodiment of the present invention, the dimension of the photosensitive surface S of the image sensor 2, presented in the yz plane of the figure, is such that the associated viewing angle is equal to 18°. In the general part of this document, this dimension of the photosensitive surface S is referred to as the longitudinal dimension, and the associated viewing angle is referred to as the first viewing angle. This first viewing angle is hereinafter referred to as α1.

[0045] Image sensor 2 can be matrix-type, in which case its photosensitive surface S has another dimension parallel to the x-axis. This other dimension has been referred to as the lateral dimension of the photosensitive surface S in the general part of this paper. For [ Figure 2 In an embodiment of [ ], this lateral dimension of the photosensitive surface S of the image sensor 2 makes the associated viewing angle, referred to as the second viewing angle, equal to 24°. Therefore, [ Figure 2 System 1 has a large total field: 18° x 24°. However, it is possible to obtain a larger or smaller field with this configuration of imaging optics. According to the embodiments described herein, when the photosensitive surface S of image sensor 2 is rectangular, this image sensor is preferably oriented such that the maximum lateral dimension of its photosensitive surface is perpendicular to the plane of symmetry of system 1, i.e., perpendicular to […]. Figure 2 The plane of ].

[0046] for[ Figure 2 The example shown is an embodiment where the image sensor 2 has 240 pixels, each 12 micrometers (μm), in its vertical dimension and 320 pixels in its horizontal dimension. The system 1 has a focal length f of 9 millimeters (mm) and an aperture number N of 1.5, corresponding to an entrance pupil size of 6 mm.

[0047] [ Figure 3a ]and[ Figure 3b Repeating and [ Figure 2The same embodiment of the invention also shows that the three mirrors M1, M2, and M3 of system 1, as well as the image sensor 2, are housed within a sphere with a diameter of 40 mm, designated by SPH. System 1 is therefore particularly compact and suitable for integration into photoelectric imaging devices, such as airborne vehicle guidance systems, thermal cameras, visual aids, and photoelectric pods for monitoring and detection. Figure 5 The attached figure shows the electro-optical pod, designated by reference numeral 20, used for monitoring and detection. The electro-optical pod is carried on the UAV 30 and integrated with system 1.

[0048] In some possible embodiments of the invention, some or all of the optical components of System 1 may be made by three-dimensional printing, commonly referred to as 3D printing.

[0049] In other possible embodiments, some or all of the optical components of system 1 may be made of injected polymeric materials. Such other embodiments may have a particularly low cost. Furthermore, at least one of the mirrors M1, M2, and M3 thus formed by injection can directly produce a self-positioning system for the mirror.

[0050] Each of mirrors M1, M2, and M3 may consist of a rigid base portion that provides the shape of its reflective surface, and a reflective metal layer deposited on its surface. The rigid base portion may be made of a solid 3D printed material or may be based on an injection-molded polymer. For mirror M2, the base portion and the reflective layer of this mirror are respectively located in […]. Figure 2 In the figure, M2b and M2r are used to refer to the figures.

[0051] [ Figure 3a ]and[ Figure 3b Further illustrating the addition of two incident baffles to System 1 to further reduce the amount of stray light that could otherwise reach Image Sensor 2. The incident baffle, designated by reference numeral 11, is referred to herein as the first incident baffle, and the incident baffle, designated by reference numeral 12, is referred to herein as the second incident baffle. The optical entrance of System 1 is designated by reference numeral O. Incident baffle 11 is located on the edge close to the optical entrance O of Image Sensor 2, and incident baffle 12 is located on the opposite edge of the optical entrance O. Thus, incident baffle 12 is close to mirror M3. In effect, due to the α-z configuration of System 1, Image Sensor 2 is located close to or very close to the optical entrance O, while being laterally offset relative to the optical entrance O in a direction opposite to mirror M3.

[0052] exist[ Figure 3a ]and[ Figure 3bIn the yz plane, the incident field of system 1 is defined between the two field edge ray rays, designated RM1 and RM2 in the attached figures. The initial segments of these field edge ray rays RM1 and RM2 thus form the angle α1 introduced above between them. An incident baffle 11 is superimposed on the initial segment of the field edge ray RM1, and an incident baffle 12 is superimposed on the initial segment of the field edge ray RM2. Furthermore, incident baffle 11 can extend downstream to the terminal segment of the field edge ray RM2, and incident baffle 12 can extend downstream to the upstream edge B of mirror M3. AM3 In other words, the downstream edge B of the incident baffle 11 AV11 It can be located on the end segment of the edge ray RM2 at the edge of the field, and on the downstream edge B of the incident baffle 12. AV12 Upstream edge B of the mating mirror M3 AM3 .exist[ Figure 3a ]and[ Figure 3b Outside the yz plane, incident baffles 11 and 12 are preferably superimposed on the edge rays of the field edge rays RM1 and RM2 near the field edge.

[0053] [ Figure 3a The complete paths of field edge rays RM1 and RM2 within system 1 are shown, along with their contribution to the image formed on the photosensitive surface S of image sensor 2. Field edge ray RM1 contributes to the formation of an image at the downstream boundary L of the photosensitive surface S of image sensor 2. AV The image is formed at the field edge, and the edge ray RM2 helps to form at the upstream boundary L. AM An image is formed at that location.

[0054] In the yz plane and [ Figure 3b In the attached figures, reference numeral F0 denotes the incident field of system 1, reference numerals F1 and F2 denote angular fields outside the incident field F0 but angularly close to it, and reference numerals F3 and F4 denote angular fields located angularly about the incident field F0 on opposite sides of fields F1 and F2, respectively. Fields F1 and F2 are therefore called adjacent fields of the incident field F0, and fields F3 and F4 are called non-adjacent fields of the incident field F0.

[0055] Light rays originating from adjacent fields F1 and F2, and reflected by mirror M1, then by mirror M2, and finally by mirror M3, reach the image sensor 2 outside its photosensitive surface S. Generally, light rays originating from non-adjacent fields F3 and F4 do not follow the nominal path within system 1, passing sequentially through the three mirrors. However, if the light emanates from a non-adjacent field F4, it is directed directly toward the image sensor 2; or if the light emanates from a non-adjacent field F3 and the mirror, it will be reflected on mirror M3 before reaching the image sensor 2.

[0056] Placing the image sensor 2 close to the optical entrance O of the system 1, as provided by the α-z configuration, makes it possible to prevent stray light from the adjacent field F1 from being reflected towards the image sensor 2 by the mirror M3. The function of the incident baffle 11 therefore includes intercepting stray light from non-adjacent fields F3 that can be reflected towards the image sensor 2 by the mirror M3, but does not include intercepting stray light from adjacent fields F1 that also head towards the mirror M3. Therefore, the length of the incident baffle 11 upstream of the system 1 can be relatively short.

[0057] The incident baffle 11 also intercepts portions of light rays from non-adjacent fields F4 that are simultaneously oriented toward the image sensor 2, meaning that those rays from non-adjacent fields F4 are less tilted relative to the z-axis. These are actually intercepted by the downstream portion of the incident baffle 11.

[0058] Furthermore, those rays from non-adjacent fields F4 that are most tilted relative to the z-axis and oriented towards image sensor 2 are intercepted by the incident baffle 12. To intercept these rays, the incident baffle 12 may have an upstream edge B upstream of line D1. AM12 The straight line D1 will be at the downstream edge B of the incident baffle 11. AV11 The downstream boundary L of the photosensitive surface S connected to the image sensor 2 AV However, due to the characteristic that mirror M2 intercepts light rays that would otherwise propagate in a straight line between mirror M1 and image sensor 2, the incident baffle 12 does not need to intercept stray light rays from adjacent field F2 that would otherwise be reflected towards image sensor 2 on mirror M1, nor does it need to intercept the less oblique light rays from non-adjacent field F4. Therefore, the α-z configuration of system 1 makes it possible to place image sensor 2 close to its optical entrance O so that only the most oblique parasitic light rays from non-adjacent field F4 are intercepted by incident baffle 12, without requiring incident baffle 12 to intercept light rays from adjacent field F2 or less oblique light rays from non-adjacent field F4. The upstream edge B of incident baffle 12 AM12 Therefore, it is located on line D1, and does not necessarily need to extend upstream beyond line D1. Thus, the incident baffle 12 can also have an upstream length, meaning a shorter length extending in front of the optical entrance O. Furthermore, the downstream edge B of the incident baffle 12... AV12 It can be positioned on line D2, which will point to the downstream edge B of the incident baffle 11. AV11 The upstream boundary L of the photosensitive surface S connected to the image sensor 2 AM Instead of the upstream edge B of the mating mirror M3 AM3 .

[0059] Thanks to the reduced upstream length of the two incident baffles 11 and 12, the entire system 1 containing these incident baffles 11 and 12 thus has a smaller size.

[0060] [ Figure 4 ] Again corresponds to [ Figure 2 The embodiments of the present invention are shown to illustrate the possible integration of an additional image sensor into system 1. Reference numeral 13 denotes a spectral separation device, such as a dichroism separator. Device 13 generates an image PF' of the focal plane PF. An additional image sensor 2' may then be arranged such that its photosensitive surface is superimposed on the image PF' of the focal plane. As an example, the additional image sensor 2' may be silicon-based and sensitive to the visible light range.

[0061] It should be understood that the invention can be reproduced by modifying minor aspects of the embodiments described in detail above, while still retaining at least some of the enumerated advantages. Specifically, the imaging optical system according to the invention can be used in applications other than those mentioned. Furthermore, any values ​​mentioned are for illustrative purposes only and can be varied depending on the specific application. Those skilled in the art will know how to effortlessly adjust the values ​​of focal length, angle of view, entrance pupil size, etc., for each application.

Claims

1. An imaging optical system (1) comprising three mirrors, said three mirrors comprising: - Main mirror (M1) - Secondary mirror (M2), and - A third-order mirror (M3), which is adapted and arranged such that light from a scene located in the incident field of the system is first reflected by the primary mirror, then by the secondary mirror, and then by the tertiary mirror to form an image of the scene in the focal plane (PF) of the system. The light rays originating from the scene and contributing to the formation of the image are therefore divided into: - The initial section upstream of the main mirror (M1), - The first intermediate ray segment between the primary mirror and the secondary mirror (M2), - The second intermediate ray segment between the secondary mirror and the tertiary mirror (M3), and - The terminal ray segment between the third-order mirror and the focal plane (PF), Furthermore, in the imaging optical system: - The primary mirror (M1) and the secondary mirror (M2) are oriented such that the second intermediate ray segment of the paraxial ray of the system (1) intersects with the initial segment of the paraxial ray, and - The secondary mirror and the tertiary mirror (M3) are oriented such that the terminal ray segment of the paraxial ray passes through the lateral side of the secondary mirror, the lateral side being opposite to the lateral offset of the primary mirror relative to the secondary mirror, such that the terminal ray segment of the paraxial ray does not intersect with the first intermediate ray segment of the paraxial ray. The system (1) further includes an image sensor (2) arranged such that the photosensitive surface (S) of the image sensor is superimposed on the focal plane (PF), the photosensitive surface extending from the upstream boundary (L). AM ) extends to the downstream boundary (L AV The upstream and downstream boundaries of the photosensitive surface of the image sensor are defined with respect to the corresponding projections of the upstream and downstream boundaries onto the initial segment of the paraxial ray and with respect to the propagation direction of the paraxial ray in the initial segment of the paraxial ray. The system (1) is characterized in that the secondary mirror (M2) and the tertiary mirror (M3) are oriented such that the upstream boundary (L) of the photosensitive surface (S) of the image sensor (2) is such that... AM The line D0 is offset downstream relative to the main mirror (M1), and the upstream edge B of the main mirror (M1) is offset downstream. AM1 Connected to: - The upstream edge B of the secondary mirror (M2) AM2 ,or - The upstream edge of the screen surrounding the secondary mirror. This allows light from the primary mirror, or the screen block around the secondary mirror, to propagate directly from the primary mirror to the photosensitive surface of the image sensor in the straight line D0 in other ways. The upstream edge B of the main mirror (M1) AM1 and downstream edge B AV1 Correspondingly, the upstream edge B of the secondary mirror (M2) AM2 and downstream edge B AV2 Regarding the upstream edge B of the main mirror surface AM1 and the downstream edge B AV1 Correspondingly, the upstream edge B of the secondary mirror AM2 and the downstream edge B AV2 The corresponding projection onto the initial segment of the paraxial ray and defined with respect to the propagation direction of the paraxial ray in the initial segment of the paraxial ray. And the upstream boundary (L) of the photosensitive surface (S) of the image sensor (2) AM The direction of the downstream offset relative to the straight line D0 is parallel to the initial segment (RP0) of the paraxial ray and is oriented according to the propagation direction of the paraxial ray (RP) in the initial segment (RP0) of the paraxial ray.

2. The imaging optical system (1) according to claim 1, characterized in that, At least one of the primary mirror (M1), the secondary mirror (M2), and the tertiary mirror (M3) has a free-reflecting surface.

3. The imaging optical system (1) according to claim 1 or 2, further comprising a first incident baffle (11), the first incident baffle (11) being superimposed on the initial segment of the first field edge edge ray, located on a first side of the same incident field as the image sensor (2), opposite to the third-order mirror (M3), and the first incident baffle having a downstream edge B of the terminal ray segment of the second field edge edge ray. AV11 .

4. The imaging optical system (1) according to claim 3, further comprising a second incident baffle (12), the second incident baffle (12) being superimposed on the initial segment of the edge ray of the second field, located on the second side of the incident field identical to that of the tertiary mirror (M3), opposite to the image sensor (2), and the second incident baffle having an upstream edge B connected to the tertiary mirror. AM3 Alternatively, it can be connected to a screen surrounding the tertiary mirror or to the downstream edge B of an opaque support for the tertiary mirror. AV12 Otherwise, the downstream edge B of the second incident baffle AV12 Located at the upstream boundary (L) of the photosensitive surface (S) of the image sensor (2) AM ) connected to the downstream edge B of the first incident baffle (11) AV11 Downstream of line D2.

5. The imaging optical system (1) according to claim 4, characterized in that, The second incident baffle (12) has a downstream edge B located at the first incident baffle (11). AV11 The downstream boundary (L) of the photosensitive surface (S) connected to the image sensor (2) AV The upstream edge B of the straight line D1 AM12 .

6. The imaging optical system (1) according to claim 1 or 2, characterized in that, The longitudinal dimension of the image sensor (2) determines the first viewpoint (α1) of the system. The system (1) is adapted such that the first angle of view (α1) is greater than or equal to 9°.

7. The imaging optical system (1) according to claim 6, characterized in that, The system (1) is adapted such that the first angle of view (α1) is greater than or equal to 18°.

8. The imaging optical system (1) according to claim 6, characterized in that, The image sensor (2) has a matrix arrangement, and the horizontal dimension of the image sensor, which is perpendicular to the vertical dimension, determines the second viewpoint of the system. The system (1) is further adjusted so that the second angle of view is greater than or equal to 12°.

9. The imaging optical system (1) according to claim 8, characterized in that, The system (1) is adapted such that the second angle of view is greater than or equal to 24°.

10. The imaging optical system (1) according to claim 1 or 2, having an aperture number N of less than 5, wherein the aperture number N is equal to f / D, where f is the focal length f of the system and D is the size of the entrance pupil of the system.

11. The imaging optical system (1) according to claim 10, having an aperture value N of less than 2.

12. The imaging optical system (1) according to claim 1 or 2, further comprising a pupillary septum located at the primary mirror (M1) or the tertiary mirror (M3).

13. The imaging optical system (1) according to claim 12, characterized in that, The pupillary septum is located at the third-order mirror (M3).

14. The imaging optical system (1) according to claim 1 or 2, further comprising a spectral separation device (13) disposed between the third-order mirror (M3) and the image sensor (2), and an additional image sensor (2') disposed in an image (PF') of the focal plane (PF) of the system, the image having been formed by the spectral separation device.

15. The imaging optical system (1) according to claim 1 or 2, characterized in that, The primary mirror (M1), the secondary mirror (M2), and the tertiary mirror (M3) are housed in a sphere whose diameter is between 2 and 6 times the value of the focal length f of the system.

16. The imaging optical system (1) according to claim 1 or 2, characterized in that, At least one of the primary mirror (M1), the secondary mirror (M2), and the tertiary mirror (M3) includes a rigid portion (M2b) made of injected polymeric material and optionally a reflective metal layer (M2r).

17. An optoelectronic imaging device (100) comprising a system (1) according to any one of the preceding claims, the device being selected from airborne vehicle guidance devices, thermal cameras, visual aids, and optoelectronic pods for monitoring and detection.

Images