A 3-folded mirror system with a physical exit pupil.

Abstract

An optical imaging system (11) comprises a primary mirror (M1), a secondary mirror (M2), a tertiary mirror (M3), and an exit pupil (PS). Light rays (21, 22, 23) from a scene are reflected successively by these three mirrors towards the exit pupil. A light ray (21, 22, 23) from the scene is thus divided into an initial segment (210, 220, 230), a first intermediate segment (211, 221, 231), a second intermediate segment (212, 222, 232), and a terminal segment (213, 223, 233). M1 and M2 are oriented such that the second intermediate segment (232) of a parabasic ray (23) of the system intersects the initial segment (230) of said parabasic ray. M2 and M3 are oriented such that the first intermediate segment (231) of the parabasal ray of the system intersects the terminal segment (233) of said parabasal ray. An intermediate image plane (PI) is located between M1 and M2 or between M2 and M3. Figure for the abbreviation: Fig. 1b

Description

Title of the invention: Folded 3-mirror system having a physical exit pupil. technical field

[0001] The present invention relates to an optical imaging system comprising three mirrors. It also relates to an apparatus comprising such a system.

[0002] The field of the invention is more particularly, but not exclusively, that of compact reflective imaging optical systems, as well as optronic imaging devices embedded for example on nanosatellites which would include such a system, for imaging applications, for example in the thermal infrared. Prior art

[0003] In recent years, the development of nanosatellites has created a need for optical systems that are both high-performance and very compact. For satisfactory angular resolution, the focal length of the system must be large, which is contradictory to the size constraints of nanosatellites.

[0004] Moreover, optical systems with long focal lengths having an accessible exit pupil either have bulky optics or are cumbersome, which is again contradictory to the size constraints of nanosatellites.

[0005] One way to meet the needs of achromatic and athermic systems is through the use of reflective surfaces. Using combinations of focused mirrors reduces the system's size to a value smaller than its focal length. This type of optical architecture is therefore ideal for long focal length systems. However, these focused combinations suffer from central shutter problems that limit the optical system's performance, such as its aperture. By offsetting the mirror combinations, the central shutter can be eliminated and a larger aperture can be achieved, but problems related to the non-centrosymmetric distribution of optical aberrations in the field can arise, especially in the case of very fast and highly offset systems.

[0006] A classic geometry of such a system is called the TMA (for "Three Mirror Anastigmat"). The decentering of the mirrors will increase the size of the optical system. In order to limit the size of the optics, the "re-imaging" architecture is preferred for systems with long focal lengths and requiring an accessible exit pupil. Such an architecture consists of an upstream imaging optic, an intermediate image plane, and a re-imaging optic whose function is to project The intermediate image is projected onto the detector, and the entrance pupil of the imaging optics is conjugated with the physical exit pupil of the optical system, which is positioned after the re-imaging optics. However, this type of architecture further increases the overall size of the optical system due to the presence of an intermediate image plane. Moreover, these architectures must provide both good correction of aberrations in the image plane and control over the position of the exit pupil and pupillary aberrations. This generally results in systems that are more complex to manufacture, for an equivalent number of optical surfaces, than non-re-imaging systems.

[0007] Thus, combined with the need for re-imaging to cover the cooled infrared, requiring an infrared detector integrated into a cryostat and a cold diaphragm, the re-imaging TMA is particularly bulky.

[0008] To reduce the size of these off-center systems, one solution is to fold the optical path by arranging the mirrors along a circle. These "ball"-shaped architectures have the characteristic of an intersecting optical path and are therefore particularly relevant for catoptric systems with long focal lengths. Such a configuration, using four mirrors, was proposed by Rodgers US5309276A. A folded three-mirror architecture called "Grapefruit" US8616712B2 also exists. These geometries are compatible with high aperture constraints. The WO2022129770A2 system is also a folded system whose mirror configuration is called aZ

[0009] In the case of folded solutions such as the Pamplemousse and the aZ, the use of surfaces without rotational symmetry and which are not off-axis portions of centrosymmetric surfaces, called "freeform surfaces," is necessary. This type of surface offers a higher number of degrees of freedom than classical surfaces, such as conics or aspheric surfaces, and thus allows for better correction of off-axis catoptric systems exhibiting a distribution of aberrations in the field without rotational symmetry. Therefore, freeform surfaces make it possible to obtain well-corrected large aperture systems within a constrained volume.

[0010] The architecture proposed by Rodgers US5309276A, although reimageable, comprises four optical surfaces and is therefore complex to align. Making the system more compact while maintaining the aperture and / or field of view is also complicated due to the absence of freeform surfaces. Pamplemousse-type systems are composed of three freeform mirrors but do not have accessible exit pupils. Furthermore, the Pamplemousse system suffers from problems related to stray light management, requiring the use of large baffles at the system's input, which contradicts the objective of compactness. It may be possible to move the pupil of exit of a Pamplemousse at an accessible level, but in the absence of re-imaging, the compactness criterion of the system is not met.

[0011] The aZ system allows for better control of stray light than the Pamplemousse systems, but it also lacks an accessible exit pupil. It may be possible to move the exit pupil of a Pamplemousse system to an accessible level, but without re-imaging, the system's compactness criterion is not met.

[0012] The aim of the present invention is to provide a re-imaging optical system, preferably addressing the problem of satellite imaging in low Earth orbit, and meeting all or part of the following needs:

[0013] - Large opening, and / or

[0014] - Compatible with cooled infrared detection blocks (exit pupil accessible), and / or

[0015] - Compact (allowing integration into a nanosatellite of approximately 12U), and / or

[0016] - Achromatic (compatible from visible to thermal infrared), and / or

[0017] - Athermal, and / or

[0018] - easy to align, and / or

[0019] - with good management of parasitic flow. Description of the invention

[0020] This objective is achieved with an optical imaging system, comprising: - A primary mirror, a secondary mirror, and a tertiary mirror, - An exit pupil,

[0021] the primary mirror, the secondary mirror, and the tertiary mirror being adapted and arranged so that light rays from a scene are reflected first by the primary mirror, then by the secondary mirror and then by the tertiary mirror towards the exit pupil.

[0022] The system is preferably arranged so that a light ray coming from the scene is thus cut into an initial ray segment upstream of the primary mirror, a first intermediate ray segment between the primary mirror and the secondary mirror, a second intermediate ray segment between the secondary mirror and the tertiary mirror, and a terminal ray segment between the tertiary mirror and the exit pupil.

[0023] The primary and secondary mirrors can be oriented so that the second intermediate segment of a parabasal ray of the system intersects the initial segment of said parabasal ray.

[0024] The secondary and tertiary mirrors can be oriented so that the first intermediate segment of the parabasal ray of the system intersects the terminal segment of said parabasal ray.

[0025] The system according to the invention may further include an intermediate image plane located between the primary mirror and the secondary mirror or between the secondary mirror and the tertiary mirror.

[0026] The intermediate image plane can be located along the second intermediate segment.

[0027] The image plane can be located closer to the secondary mirror than to the primary mirror and / or only the tertiary mirror.

[0028] The primary, secondary and tertiary mirrors can be arranged so that the initial segment of the parabasal ray of the system intersects the terminal segment of said parabasal ray.

[0029] An angle formed between: - a tangent to a reflective surface of the primary mirror arranged to reflect light rays from the scene, at the point of contact between this reflective surface and the parabasal ray of the system, and - a tangent to a reflective surface of the tertiary mirror arranged to reflect light rays from the secondary mirror, at the level of the contact between this reflective surface and the parabasal ray of the system

[0030] is preferably greater than 110°.

[0031] An angle formed between: - a normal to an axis connecting a center of the secondary mirror (preferably located at the contact between the reflective surface of the secondary mirror and the parabasal radius of the system) and a crossing or intersection point between: • a tangent to a reflective surface of the primary mirror arranged to reflect light rays from the scene, at the point of contact between this reflective surface and the parabasal ray of the system, and • a tangent to a reflective surface of the tertiary mirror arranged to reflect light rays from the secondary mirror, at the point of contact between this reflective surface and the parabasal ray of the system - a tangent to a reflective surface of the secondary mirror arranged to reflect light rays from the primary mirror, at the level of the contact between this reflective surface and the parabasal ray of the system

[0032] is preferably less than 10°.

[0033] The secondary mirror may have the largest dimension of its reflective surface arranged to reflect light rays from the primary mirror at least 5 times smaller: - that the largest dimension of the primary mirror's reflective surface arranged to reflect light rays from the scene, and - that the largest dimension of the reflective surface of the tertiary mirror arranged to reflect light rays coming from the secondary mirror.

[0034] The primary mirror and tertiary mirror can be made on the same substrate so that these two mirrors are joined together.

[0035] The primary mirror and the tertiary mirror may be adjacent and / or separated by a distance of less than 1% of the focal length of the primary mirror.

[0036] The system according to the invention may include at least one alignment marker, preferably between the primary mirror and the tertiary mirror and / or on the periphery of the primary mirror and / or the tertiary mirror.

[0037] For all primary, secondary and tertiary mirrors, the angle between the parabasal ray incident on the reflective surface of the mirror considered and the parabasal ray reflected by the reflective surface of the same mirror considered is preferably always of the same sign.

[0038] The exit pupil may be located outside of any folded optical path between the primary mirror and the secondary mirror.

[0039] The system according to the invention may include a first light emitter or detector located downstream of the exit pupil relative to the tertiary mirror.

[0040] The system according to the invention may include at least one baffle disposed laterally to the exit pupil and / or the first detector, and arranged to block the passage of light rays which would arrive at the first detector without being reflected successively by each of the primary, then secondary and then tertiary mirrors and preferably without passing through the exit pupil.

[0041] The system according to the invention may include a dichroic blade placed between the tertiary mirror and the first detector or light emitter, preferably between the tertiary mirror and the exit pupil, said dichroic blade being arranged to direct light rays from the tertiary mirror to at least one other detector and / or to direct light rays to the tertiary mirror from another light emitter.

[0042] The tertiary mirror can be arranged to focus light rays from the scene towards, preferably onto, the light detector.

[0043] The detector can be an infrared detector placed in a cryostat, the system according to the invention then preferably comprising a transparent infrared window placed upstream of the exit pupil relative to the tertiary mirror.

[0044] The backspace of the system, defined as the distance between an image plane of the system and a point of intersection between:

[0045] - a straight line perpendicular to a photosensitive surface of the first detector and passing roughly through its center and

[0046] - the ray, reflected by the primary mirror, closest to the detector,

[0047] may be greater than 20% of the focal length of the system.

[0048] Preferably: - the largest dimension of the primary mirror's reflective surface arranged to reflect light rays from the scene, and - the largest dimension of the reflective surface of the tertiary mirror arranged to reflect light rays from the secondary mirror

[0049] are substantially identical to within 50%.

[0050] The primary mirror may be concave, and / or the secondary mirror may be convex, and / or the tertiary mirror may be concave.

[0051] The primary mirror can be inclined from 10 to 20° relative to an entrance pupil of the system.

[0052] The tertiary mirror can be inclined from 5° to 25° with respect to the orientation of the parabasal ray incident on the tertiary mirror.

[0053] The secondary mirror can be positioned at a distance from the intermediate image plane of between 1 and 3 times the focal length of the secondary mirror.

[0054] The primary mirror may be a freeform or "free surface" mirror, and / or the secondary mirror may be a freeform mirror, and / or the tertiary mirror may be a freeform mirror.

[0055] The system according to the invention may include: - the tertiary mirror on one side of the initial segment of the parabasal ray, and the secondary mirror and exit pupil on the other side of the initial segment of the parabasal ray, and / or - the tertiary mirror on one side of the first intermediate segment of the parabasal ray, and the exit pupil on the other side of the first intermediate segment of the parabasal ray, and / or - the entrance pupil on one side of the second intermediate segment of the parabasal ray, and the primary mirror and exit pupil on the other side of the second intermediate segment of the parabasal ray, and / or - the secondary mirror and the entrance pupil on one side of the terminal segment of the parabasal ray, and the primary mirror on the other side of the terminal segment of the parabasal ray.

[0056] According to yet another aspect of the invention, an imaging device is proposed, comprising an optical imaging system according to the invention, said device being a satellite. Description of the figures and methods of realization

[0057] Other advantages and features of the invention will become apparent upon reading the detailed description of implementations and embodiments, which are by no means limiting, and the following accompanying drawings:

[0058] [Fig.1a] [Fig.1a] is a schematic diagram of the architecture of a first mode of implementation of system 11 according to the invention, which is the preferred embodiment of the invention; the parabasal ray 23 is represented by dashed lines and passes through the center of each mirror M1, M2, M3, with a limited number of references for better readability,

[0059] [Fig.lb] [Fig.lb] corresponds to [Fig.la] with indications of the relevant references,

[0060] [Fig.2] [Fig.2] is a principle illustration of a first mounting variant mirrors M1 and M3 in the first embodiment of the system in Figure 1 (Figure 1 referring to [Fig.1a] or 1b),

[0061] [Fig. 3] [Fig. 3] is a principle illustration of a second mounting variant of mirrors M1 and M3 in the first system embodiment of Figure 1,

[0062] [Fig. 4] [Fig. 4] is a principle illustration of a third mounting variant mirrors M1 and M3 in the first embodiment of the system of Figure 1,

[0063] [Fig.5] [Fig.5] is a front view of the cases of Figures 2 and 3 and illustrates the positioning of an alignment marker,

[0064] [Fig.6] [Fig.6] is a first example of an embodiment of the first mode of Figure 1, in the particular case of a volume of 12U, with a total field of view of 1.8x1.44°, F / 2.44, and a focal length of 300mm,

[0065] [Fig.7] [Fig.7] illustrates the optical path of the parasitic rays 61, 62, 63 originating from sources outside the useful field and passing through the entrance pupil PE reaching the detector 41 and / or the image plane IM directly or by specular reflection on one or more mirrors for the system described [Fig.6] for a bare detector i.e. without the baffles 51, 52. Baffling (by baffles 51, 52) allows to eliminate the stray light.

[0066] [Fig.8] [Fig.8] illustrates the optical path of the parasitic rays 64 originating from sources outside the useful field and passing through the entrance pupil PE reaching the detector 41 directly or by specular reflection on one or more mirrors for the system illustrated in [Fig.6] in the case where the detector 41 is enclosed in a box (such as a cryostat 43)

[0067] [Fig.9] [Fig.9] illustrates the Sag (also called sagitta surface or mapping of the arrows in French) of the mirror Ml (in mm), a Sag of a mirror corresponding to the distance of the reflective surface of this mirror from the tangent plane to this reflective surface at the level of the intersection between this reflective surface and the parabasal ray,

[0068] [Fig. 10] [Fig. 10] illustrates the Sag of mirror M2 (in mm)

[0069] [Fig. 11] [Fig. 11] illustrates the Sag of mirror M3 (in mm), and

[0070] [Fig. 12] The [Fig. 12] illustration is a second example of an embodiment of the first mode of figure 1, featuring a dichroic plate 70 for placing a visible detector.

[0071] These embodiments being in no way limiting, variants of the invention may be considered, in particular, comprising only a selection of features described or illustrated hereafter, isolated from the other described or illustrated features (even if this selection is isolated within a sentence including these other features), if this selection of features is sufficient to confer a technical advantage or to differentiate the invention from the prior art. This selection includes at least one preferably functional feature without structural details, and / or with only a portion of the structural details if this portion alone is sufficient to confer a technical advantage or to differentiate the invention from the prior art.

[0072] We will first describe, with reference to figures 1 to 12, a first embodiment of the optical imaging system 11 according to the invention.

[0073] This system links a folded reimaging catoptric architecture.

[0074] The system 11 comprises a primary mirror M1, a secondary mirror M2, and a tertiary mirror M3.

[0075] The mirrors M1, M2, M3 can be in a coplanar arrangement (with a plane of symmetry, which corresponds to the plane of figure 1) or non-coplanar (M1, M2, M3 contained in a sphere, without this plane of symmetry).

[0076] The system 11 includes an exit pupil PS, physically materialized by a component (for example, an optical element, or preferably a diaphragm, etc.). The exit pupil PS is located outside the folded optical path between M1 and M3, making it accessible. This exit pupil PS is accessible from outside the system 11 without having to enter the space between the reflective surfaces of the mirrors M1, M2, and M3.

[0077] Preferably, the position of the exit pupil PS with respect to this optical path (more precisely, of the ray reflected by M2 closest to the exit pupil PS) is between 1 and 3 times the diameter (or largest dimension) of the exit pupil PS.

[0078] In the embodiment of system 11 illustrated in Figure 1, it is noted that system 11 also includes an entrance pupil PE which is the optical conjugate of the exit pupil PS by the tertiary, secondary, and primary mirrors. In Figure 1, the position of this entrance pupil PE is located at the entrance of system 11, but depending on the variants considered, the position of PE can vary; this entrance pupil PE could, for example, be located on or almost on the reflective surface of mirror ML. To limit the size of the optics, it is advantageous for the pupil The entrance pupil PE is either close to the mirror Ml, before, after or on top of it. Thus, depending on the variant considered, the entrance pupil PE can be positioned upstream of Ml, or can be coincident or substantially coincident with Ml, or can be located downstream of Ml (it can be real or virtual).

[0079] The entrance pupil PE is the optical conjugate of the exit pupil PS by the tertiary mirror M3, secondary mirror M2 and primary mirror M1.

[0080] The system 11 includes a first light emitter or detector 41 located downstream of the exit pupil PS relative to the tertiary mirror M3.

[0081] The primary mirror M1, the secondary mirror M2, and the tertiary mirror M3 are adapted and arranged so that light rays 21, 22, 23 from a scene are reflected first by the primary mirror M1, then by the secondary mirror M2 and then by the tertiary mirror M3 towards the exit pupil PS.

[0082] Among these light rays, the following are illustrated in Figure 1: - The upper marginal radius 21 of the light beam coming from the imaged scene, - The lower marginal radius 22 of the light beam coming from the imaged scene, - The parabasal ray 23 from the imaged scene,

[0083] The system 11 is arranged so that a light ray 21, 22, 23 from the scene is thus cut into an initial segment 210, 220, 230 of radius upstream of the primary mirror M1, a first intermediate segment 211, 221, 231 of radius between the primary mirror and the secondary mirror M2, a second intermediate segment 212, 222, 232 of radius between the secondary mirror and the tertiary mirror M3, and a terminal segment 213, 223, 233 of radius between the tertiary mirror and the exit pupil.

[0084] This has therefore been illustrated in Figure 1: - the initial segment 210 of radius 21, - the initial segment 220 of radius 22, - the initial segment 230 of radius 23, - the first intermediate segment 211 of radius 21, - the first intermediate segment 221 of radius 22, - the first intermediate segment 231 of radius 23, - the second intermediate segment 212 of radius 21, - the second intermediate segment 222 of radius 22, - the second intermediate segment 232 of radius 23, - the terminal segment 213 of radius 21, - the terminal segment 223 of radius 22, and - the terminal segment 233 of radius 23.

[0085] The primary mirror Ml (more precisely its reflective surface arranged to reflect rays 21, 22, 23) is concave.

[0086] The primary mirror Ml is inclined:

[0087] - of more than 10° and / or less than 20° relative to the PE entrance pupil of the system 11 (so as to eliminate the occlusion of system 11) (these values ​​correspond to the angle between the plane of PE and the plane tangent to the reflective surface of Ml at the intersection between this reflective surface and the parabasal ray 23), preferably when the entrance pupil PE is not coincident with Ml, and / or

[0088] - of more than 10° and / or less than 20° with respect to the parabasal radius (these values correspond to the angle between the parabasal ray 23 and the normal to the tangent plane to the reflective surface of M1 at the level of the intersection between this reflective surface and the parabasal ray 23), preferably when the entrance pupil PE coincides with ML

[0089] The primary mirror M1 reflects the light 21, 22, 23 in the direction of the secondary mirror M2.

[0090] The secondary mirror M2 (more precisely its reflective surface arranged to reflect rays 21, 22, 23) is convex.

[0091] The secondary mirror M2 is oriented so as to be opposite the mirrors M1 and M3.

[0092] The secondary mirror M2 is arranged to reflect the light 21, 22, 23 arriving from the primary mirror M1 towards the tertiary mirror M3.

[0093] The system 11 further includes an intermediate image plane PI located between the primary mirror and the secondary mirror or between the secondary mirror and the tertiary mirror, i.e. on the first intermediate segment or on the second intermediate segment of the radius 21, 22 and / or 23.

[0094] This intermediate image plane is defined as the intersection of the marginal rays 21, 22 on the first intermediate segment or on the second intermediate segment of the rays 21, 22. This intermediate image plane can also be defined as the intersection of the marginal rays 21, 22 of a point in the field seen by the detector 41 or coming from the emitter 42 described later.

[0095] In the particular case of Figure 1, the intermediate image plane PI is located along the second intermediate segment of rays 21, 22 and 23.

[0096] The system 11 includes an image plane IM of the system 11 (distinct from the intermediate image plane PI and located outside the system 11 on the side of the exit pupil PS). The image plane IM is optically conjugate to the intermediate image plane PI by the mirrors located downstream of PI, either M2 / M3 or M3.

[0097] The secondary mirror M2 is positioned at a distance from the intermediate image plane PI of between 1 and 3 times the focal length of the secondary mirror M2, in order to maintain the compactness of the system.

[0098] The secondary mirror M2 is substantially smaller in size than the mirrors M1 and M3.

[0099] The secondary mirror M2 has the largest dimension of its reflective surface arranged to reflect light rays from the primary mirror at least 5 times smaller (preferably 5 to 20 times smaller): - that the largest dimension of the reflective surface of the primary mirror M1 arranged to reflect the light rays coming from the scene, and - that the largest dimension of the reflective surface of the tertiary mirror M3 arranged to reflect the light rays coming from the secondary mirror.

[0100] The largest dimension of a reflective surface is understood to mean, for example: - its diameter, in the case of a circular or hemisphere reflective surface, - its length, in the case of a rectangular reflective surface,

[0101] or generally the length of the largest segment connecting two points of this surface.

[0102] Typically, the secondary mirror M2 has a diameter 5 to 20 times smaller than that of the mirrors M1 and M3.

[0103] The tertiary mirror M3 (more precisely its reflective surface arranged to reflect rays 21, 22, 23) is concave.

[0104] The tertiary mirror M3 is inclined by more than 5° and / or less than 25° with respect to the orientation of the parabasal ray incident on the tertiary mirror M3, these values ​​corresponding to the angle between the parabasal ray 23 and the normal to the plane tangent to the reflective surface of M3 at the level of the intersection between this reflective surface and the parabasal ray 23 (so as to fold the optical path, thus guaranteeing the compactness of the architecture and its folded structure).

[0105] The tertiary mirror M3 reflects the rays from the reflection on the secondary mirror M2 towards the exit pupil PS.

[0106] The tertiary mirror M3 is arranged to focus towards, preferably on, the light detector 41 the light rays 21, 22, 23 coming from the scene.

[0107] The arrangement and orientation of the mirrors M1, M2, M3 ensure a system 11 without a central shutter and without vignetting. The mirrors M1, M2, M3 have the functions of defining the optical path as well as correcting the main aberrations in the image plane IM and / or PI and ensuring the conjugation of the entrance pupil PE located near the mirror M1 with the exit pupil PS.

[0108] It is noted that system 11 comprises:

[0109] - the tertiary mirror M3 on one side of the initial segment 230 of the parabasal ray 23, and the secondary mirror M2 and exit pupil PS on the other side of the initial segment 230 of the parabasal ray 23, and / or

[0110] - the tertiary mirror M3 on one side of the first intermediate segment 231 of the parabasal ray 23, and the exit pupil PS on the other side of the first intermediate segment 231 of the parabasal ray 23, and / or

[0111] - the entrance pupil PE on one side of the second intermediate segment 232 of the radius parabasal 23, and the primary mirror Ml and the exit pupil PS on the other side of the second intermediate segment 232 of the parabasal ray 23, and / or

[0112] - the secondary mirror M2 and the entrance pupil PE on one side of the terminal segment 233 of the parabasal ray 23, and the primary mirror Ml on the other side of the terminal segment 233 of the parabasal ray 23.

[0113] The primary mirror M1 and secondary mirror M2 are oriented so that the second intermediate segment 232 of the parabasal ray 23 of the system 11 intersects the initial segment 230 of said parabasal ray 23.

[0114] The parabasal ray 23 is defined as the field radius defined as zero passing through the center of the entrance pupil PE and / or the exit pupil PS.

[0115] Preferably, the zero field is considered to be the average field with respect to the total field of view of the system defined by the dimensions of the receiver 41.

[0116] This parabasal ray 23, or main ray, is the light ray which comes from the scene and participates in the image in the image focal plane IM by passing through a center of the entrance pupil PE and / or the exit pupil PS of the system 11 ((and preferably through the center of the receiver(s) and / or emitter(s) 41, 42).

[0117] The system 11 is designed so that the parabasal radius trajectory 23 is folded back on itself (forming two successive alphas).

[0118] The secondary mirrors M2 and tertiary mirror M3 are oriented so that the first intermediate segment 231 of the parabasal ray 23 of the system intersects the terminal segment 233 of said parabasal ray 23.

[0119] The system 11 is a compact folded reimager catoptric system which has an intermediate image plane PI and a conjugation of pupils PE and PS allowing to have a physical exit pupil PS, an entrance pupil PE close to M1 thus limiting the size of the mirrors M1, M2, M3.

[0120] The configuration of the mirrors M1, M2, M3 is such that they have an arrangement called "aa". The mirrors, however, have different functions than the grapefruit system: M1 constitutes the head imaging lens, M2 is diverging and close to the intermediate image plane, so that it behaves like a "field lens" and M3 constitutes the re-imaging system.

[0121] The intermediate image plane PI is located closer to the secondary mirror M2 than to the primary mirror M1 and / or to the tertiary mirror M3.

[0122] The primary mirror M1, secondary mirror M2 and tertiary mirror M3 are arranged so that the initial segment 230 of the parabasal ray of the system intersects the terminal segment 233 of said parabasal ray.

[0123] An angle 0i formed between: a tangent Tl to a reflective surface of the primary mirror Ml arranged to reflect the light rays 21, 22, 23 coming from the scene, at the level of the contact between this reflective surface and the parabasal ray 23 of system 11, and a tangent T3 to a reflective surface of the tertiary mirror M3 arranged to reflect the light rays 21, 22, 23 coming from the secondary mirror M2, at the level of the contact between this reflective surface and the parabasal ray 23 of the system 11,

[0124]

[0125] is greater than 100°, preferably greater than 110°. An angle 02 formed between: the normal A to the axis connecting the center of mirror M2 (located at the contact between the reflective surface of mirror M2 and the parabasal ray 23 of system 11) and P on the one hand, P being the crossing point or intersection between T1 and T3, and a tangent T2 to a reflective surface of the secondary mirror M2 arranged to reflect light rays coming from the primary mirror M1, at the level of the contact between this reflective surface and the parabasal ray 23 of system 11 (i.e. the tangent T2 to M2 at its center) on the other hand

[0126] is less than 10°.

[0127] Another advantage of the system lies in the positioning and orientation of the mirrors M1, M2, M3.

[0128] The primary and tertiary mirrors (M1 and M3 respectively) are adjacent.

[0129] Depending on the variant considered, the primary and tertiary mirrors (M1 and M3 respectively) are: manufactured separately and mounted on a common support, as illustrated in [Fig.2], via a mounting mechanism 32, fabricated on a single substrate. Thus, preferably, and as illustrated in Figures 3 and 4, the primary and tertiary mirrors are fabricated on the same substrate 30 so that these two mirrors M1, M3 are fixed to each other. This eliminates the need to align mirrors M1 and M3 with each other, and in particular simplifies alignment. of such a system 11, using alignment techniques classically employed for TMAs. It is thus possible to place artifacts 31 (also called alignment markers or "fiducials") on the substrate containing M1 and M3 and outside the useful areas of the mirrors, for example between mirrors M1 and M3 on the substrate 30 (as well as on the back of M2), allowing for simpler alignment of the system. In an extreme configuration of this variant, M1 and M3 can even be a single mirror described by a single equation, as illustrated in [Fig. 4]. In [Fig. 4], the solid black lines define the shape of mirrors M1 and M3, and the dashed line between the two specifies that the two mirrors are defined by the same equation, only the solid part being optically useful. Thus, only two mirrors (M1+M3 on the one hand, M2 on the other) need to be aligned.

[0130] For example, a mirror M1, M2, and / or M3 made of Zerodur, aluminum, silicon carbide, aluminum alloy (e.g., Al-Si), beryllium, aluminum composite matrix, or titanium composite matrix can be used on a substrate made of Zerodur, aluminum, silicon carbide, aluminum alloy (e.g., Al-Si), beryllium, aluminum composite matrix, or titanium composite matrix, but not polished. Preferably, the mirrors and substrates are made of the same material.

[0131] The primary mirror M1 and the tertiary mirror M3 are adjacent and separated by a distance of less than 1% of the focal length of the primary mirror M1.

[0132] In the present description, the radius of the best sphere Rm of a mirror M1, M2, and M3 is defined respectively as the radius of curvature that minimizes the least-squares deviation from the reflective surface of that mirror. The focal length fM of a mirror Mi (where i is a positive integer) can then be defined by the following formula:

[0133] [Math.l] r Rm< JM,~ 2

[0134] For all primary mirrors M1, secondary mirrors M2, and tertiary mirrors M3, the angle 0a, 0b, or 0C, respectively, between the parabasal ray 23 incident on the reflecting surface of the considered mirror M1, M2, or M3 and the parabasal ray 23 reflected by the reflecting surface of the same considered mirror M1, M2, or M3, respectively, always has the same sign. Thus, in the proposed architecture, all angles (defined with respect to the parabasal ray 23 incident on a surface) have the same sign, unlike in the classical TMA.

[0135] For improved sensitivity, the system 11 preferably uses a cooled detector 41. This detector 41 is cooled to cryogenic temperatures and is therefore placed in a vacuum chamber called a cryostat 43. A mechanical system also A cooled enclosure surrounds detector 41 to limit the instrumental background and is opened by a cold diaphragm. To ensure optimal radiometric performance, it is preferable that the optical system 11 have an accessible exit pupil PS, which should be positioned on the cold diaphragm of the detector block.

[0136] Thus, for infrared imaging applications using a cooled detector 41, the system 11 includes an infrared-transparent window (for example, made of germanium or silicon), i.e., having a transmission coefficient of at least 40% over a wavelength range of at least 2 pm to 8 pm (for example, between 1.2 and 10 pm for silicon, or between 1.8 and 18 pm (especially between 8 and 12 pm for long-wave infrared [LWIR] for germanium), positioned upstream of the exit pupil PS and allowing the cryostat 43 to be optically opened while maintaining its seal. In such a configuration, the distance between the exit pupil PS and the image focal plane IM is preferably between 10 and 40 mm in order to reduce the energy consumption required for cooling the detector 41 and its mechanical element. surrounding him.

[0137] The detector 41 is therefore preferably an infrared detector placed in a cryostat 43, the system comprising a transparent window (not illustrated) in the infrared (IR) placed upstream of the exit pupil relative to the tertiary mirror.

[0138] The system 11, thanks to the presence of the physical exit pupil PS, allows efficient and compact management of the parasitic flux by the insertion of a mechanism called baffle 51, 52 (in this description the term baffle can be replaced by the term panel or screen) between the exit pupil PS and the detector 41 (which is an optical sensor, for example visible CMOS detector or quantum or thermal infrared sensor).

[0139] For example, a baffle in the form of a panel (preferably made of Kovar, Invar or titanium when it is necessary to cool to cryogenic temperature) is used that absorbs as much light as possible, typically by a surface treatment such as matte black paint, anti-reflective treatment, surface texturing, etc.

[0140] Thus, the system 11 includes at least one baffle 51 disposed laterally to the exit pupil PS and / or to the first detector 41 (i.e. placed on the sides of the pupil PS or of the detector 41, without obstructing the front or rear face of the pupil PS or of the detector 41), and arranged to block the passage of light rays which would arrive at the first detector 41 without being reflected successively by each of the primary mirrors M1, then secondary mirror M2 then tertiary mirror M3 and without passing through the exit pupil PS.

[0141] Figure 7 shows, for the case of system 11 presented in Figure 6, the paths of stray light 61, 62, 63 likely to illuminate detector 4L. The essential part of the Stray light 61, 62, 63 is well managed by baffles 51, 52 between the pupil PS and the detector 41 or by a cold screen integrated into a cryostat 43 in which the detector 41 is located.

[0142] A current trend in optical payloads deployed on nanosatellites is to increase the number of detection channels in order to perform multispectral imaging, i.e., imaging in several distinct spectral ranges. This generally requires multiplying the imaging channels; however, the exclusive use of mirrors makes it possible to create multispectral imagers in which the detection channels all share the same optical path and only separate before reaching detector 41, for example, by using a dichroic plate 70 placed upstream of detector 41.

[0143] Thus, particularly for multispectral imaging applications, the system 11 includes a dichroic plate positioned upstream of the PS exit pupil (or the IR transparent window or port, if present), thus multiplying the PS exit pupil and image planes. This allows for the integration of a second detector 42, operating, for example, in the visible or near-infrared range, while maintaining the ability to adapt the aperture of system 11 to these detectors by placing a diaphragm at the PS exit pupil. Parallax is eliminated by using a single imaging channel before the beam splitter, which facilitates image co-registration.

[0144] Thus, as illustrated in [Fig. 12], the system 11 preferably comprises a dichroic blade 70 placed between the tertiary mirror M3 and the first detector or emitter 41 of light, preferably between the tertiary mirror M3 and the exit pupil PS, said dichroic blade 70 being arranged to direct the light rays from the tertiary mirror M3 to at least one other detector 42 and / or to direct light rays to the tertiary mirror from another emitter 42 of light.

[0145] The exit pupil PS is located outside of any optical path of rays 21, 22, 23 (from the scene and / or from an emitter 41 and / or 42) folded between the primary mirror M1 and the secondary mirror M2.

[0146] The exit pupil PS is located outside any optical path of rays 21, 22, 23 (from the scene and / or from an emitter 41 and / or 42) folded between the entrance pupil PE and the primary mirror ML

[0147] The exit pupil PS is located outside any optical path of rays 21, 22, 23 (from the scene and / or from an emitter 41 and / or 42) folded between the secondary mirror M2 and the tertiary mirror M3.

[0148] The back distance of the system d^f, defined as the distance between an image plane IM of the system (distinct from the intermediate image plane PI and located outside the system 11 on the exit pupil side PS) and a point of intersection between:

[0149] - a straight line perpendicular to a photosensitive surface of the first detector 41 and passing substantially through its center (or barycenter of this photosensitive surface) and

[0150] - the ray, reflected by the primary mirror, closest to detector 41,

[0151] is greater than 20% (allowing for example the insertion of a dichroic plate 70) and / or preferably less than 40% of the focal length of system 11.

[0152] This focal length of the system 11 is defined as being equal to f=d / tan(0) d being the distance between two points of the field in the image plane IM, one of the points of the field being the zero field or the field defined by the parabasal ray 23 (ray passing through the center of a pupil PE and / or PS and through the center of the receiver 41 or emitter 42), the other point of the field being a point of the field displaced by an angle 0 with respect to the first point of the field in the object space.

[0153] This ensures accessibility of the PS exit pupil and the possibility of inserting elements such as a dichroic blade.

[0154] It can be seen in the figures that: - the largest dimension L1 of the reflective surface of the primary mirror Ml arranged to reflect light rays from the scene, and - the largest dimension L3 of the reflective surface of the tertiary mirror M3 arranged to reflect the light rays coming from the secondary mirror

[0155] are substantially identical to within 50%.

[0156] In other words:

[0157] 0.5 L3 < L1 < 1.5 L3

[0158] or

[0159] 0.5 L1 < L3 < 1.5 L1

[0160] Typically, the diameter of the tertiary mirror M3 is substantially identical to that of the primary mirror M1 (within 50%).

[0161] The primary mirror Ml is a freeform or "free surface" mirror as illustrated in [Fig.9].

[0162] A freeform surface is defined as a surface without translational and / or rotational symmetry with respect to the axes normal to the mean plane.

[0163] The secondary mirror M2 is a freeform mirror as illustrated in [Fig. 10].

[0164] The tertiary mirror M3 is a freeform mirror as illustrated in [Fig.1 1].

[0165] The use of freeform mirrors ensures good image quality for this type of geometry. Mirrors also allow for a single optical path to visualize different spectral bands (visible, NIR, SWIR, MWIR, LWIR). The system's achromaticity is thus ensured by the use of mirrors, and aberration correction is achieved through the use of freeform surfaces. A multispectral configuration can be obtained by inserting of a dichroic plate placed between M3 and the detector. If the dichroic plate is placed between M3 and the exit pupil, then it is possible to adapt the aperture of the system according to the spectral band. The parallax problem is eliminated by using a single optical path, which facilitates image co-registration.

[0166] Thus, the system 11 comprises the entrance pupil PE, 3 reflective freeform optical surfaces M1, M2 and M3, the exit pupil PS and at least one detector 41, located in the image focal plane IM of the system. Each mirror M1, M2, or M3 is described using the curvature c of its reflective surface and coefficients associated with a polynomial expansion describing the freeform surfaces, expressed for example in the form:

[0167] [Math.2]

[0168] where Pij is a two-dimensional polynomial whose coefficients are weighted by Cy, and where x, y, z and p are respectively:

[0169] x: coordinate of a point on the reflective surface of the considered mirror along the X-axis

[0170] y: coordinate of a point on the reflective surface of the considered mirror along the Y axis

[0171] z: coordinate of a point on the reflective surface of the considered mirror along the Z-axis, also called the "arrow", at the point with coordinates (x, y) on the plane tangent to this reflective surface at the intersection between this reflective surface and the parabasal radius 23

[0172] p : p^xj) - yjx2 + y2

[0173] the X, Y and Z axes forming an orthonormal or orthogonal coordinate system

[0174] Their positioning is defined by the distance between each surface along the direction of propagation of the parabasal ray, as well as the angle between the normal to the mirror and the parabasal ray.

[0175] The distortion can also be constrained below 3%, without causing a significant increase in the volume of the system.

[0176] An example of the first embodiment of system 11 is given in Figure 6 for a system 11 with an aperture of f / 2.44 and a total field of view of 1.8 x 1.44. The equivalent focal length of system 11 is 300 mm. This system 11 is designed for use with a cooled detector 41 operating in the LWIR between 8 and 12.5 pm with a pixel pitch of 15 pm. In this [Fig. 6], each of the rays represents a point in the field: a solid line that corresponds to a radius of field called zero, - a dashed line that corresponds to an intermediate field radius, - a line made of dashes and dots that corresponds to a radius of the maximum field,

[0177] The application area envisaged by this example is far-infrared satellite imaging. The system 11 is contained within a volume of 12U (200 x 200 x 300 mm) and is therefore very compact considering its focal length of 300 mm. Detailed specifications for the example in Figure 6 are given in Tables 1 and 2 below, and the freeform surfaces are described using a XY polynomial basis. The polynomial coefficients are described by the corresponding polynomial (X²Y²), the coefficient associated with the polynomial P(X,Y) = X²Y²Y. Figures 9, 10, and 11 below provide an overview of the shape of the mirrors once the best sphere has been subtracted. Figures 9, 10, and 11 illustrate, respectively, the distribution of the sagitta (also called Sag) of the mirror M1, M2, or M3 once the best sphere has been subtracted (in mm). A dichroic plate 70 is further placed upstream of the exit pupil PS to illustrate the multispectral configuration of the invention, as illustrated in [Fig. 12].

[0178] [Tables] Ml M2 M3 67.053 6.311 74.673 8.628 Cur&ure * ) -2.42282F- S -1.0204065 - 3 ~4>673W " 3 ü Radius of 122.10 m.ia@ 0.404 '~2 & 3 4S 1.052 vin -kW,g -138.383 0.144 - 1.629 -393, S 5.f 0.063 c.sos 203.180 0.174 .7272 0.568 870.743 :0.525: 0.269 2987.258 -0.12: ■S%S:23.£ -b- 4 JŒ'S 7.144F r 4 0.ü?2 jrcrs -aoss 2.:3835 4 5 0.020 &G33- 2.872F 4 5 0.035 ,"472 0.119 iLOæBsiiiiii 0.184 A'2T4 ai&o 1.0735 - S 0.319 W8 0.065 A'gyi 0.237 -1.485F + 7 0.342 .w?3 &E53< o.:iss:' Aïrs 0.43 S -8.8238 4 6 0.154 OT7 O.GES -7.717.?

[0179] Table 1 gives the description of the reflective surfaces of each of the mirrors M1, M2 and M3. The terms in the left-hand column in XiYj format correspond to the Py = x'yj of equation Math. 2 and the terms of the 3 rightmost columns correspond to Cÿ of equation Math. 2 respectively for the mirror M1, M2 or M3.

[0180] [Tables2] ngne Surface Type Spreader Material fmmj xpj 7 - Q8J Standard: — Standard — — — 2 Failure Coordinate ] ^2.372 •- 3 Failure Coordinate — — .10374 4 Polynomial Extent — MIRROR — Failure Coordinate — 1G474 s Failure Coordinate -2G2.203 7 Failure Coordinate — - 1&Œ13 ■8: Polynomial Extent — MIRROR 9 Failure Coordinate — — lgO-13 IG Failure Coordinate 2G&BQ7 - Failure Coordinate — 5.1GS t J Polynomial Extent MIRROR 73 Failure Coordinate — — 92 CS 14 Failure Coordinate - J.9Ï.567 : — ^5 Break Coordinate — — -5.392 2 6 Standard — 17' Break Coordinate — — 1S Break Coordinate -3.GG0 -> . -• : 19 Break Coordinate •- — -5.392 25 Standard GERMANIUM 21 Break Coordinate — — 5.392 22 Break Coordinate - LOGO — — 23 Break Coordinate — — -3.392 24 Standard -> : 25 Break Coordinate — — 26 Break Coordinate —4LOGO Break Coordinate — -5202 8 - IMG Standard — ; ~ :

[0181] Table 2 gives the positioning of the reflective surfaces according to the standard or usual representation of the Zemax software.

[0182] The table has been translated into French, but the following expressions in Table 2 have the following translation in Zemax: - Break Coordinate: Coordinate Break - Extended Polynomial

[0183] Line 0 corresponds to the Object line and line 28 corresponds to the image line.

[0184] The columns "Surface Type", "Thickness", "Material" and "Tilting" X" correspond respectively to the columns "Surface Type", "Thickness", "Material" and "Tilt About X" in Zemax.

[0185] The proposed system 11 is compatible with aperture numbers from 2 to 5.5, and for diagonal fields of view of less than 20°. The spectral range extends from the visible (400 nm) to the far infrared (50 pm).

[0186] The volume of this system 11 is defined by the diameter of the smallest envelope that can encompass the entire system 11, preferably at least all the mirrors M1, M2, M3. The diameter of the envelope of this system 11 is less than 3 times the diameter of the entrance pupil PE, making it a compact system. By way of example, the diameter of the envelope of state-of-the-art systems (for a re-imaging TMA or Pamplemousse with an accessible exit pupil and having the same focal length and aperture) is at least 4 or 3.5 times the diameter of the entrance pupil PE.

[0187] Unlike a conventional TMA system, the intermediate image plane PI is not accessible in system 11, and therefore cannot be used to eliminate stray light 61, 62, 63, 64. However, this is not a problem because most of the stray light 61, 62, 63 is blocked by baffles 51 near detector 41 as illustrated [Fig. 7]. Adding baffles 51 near detector 41 does not increase the size of system 11 and allows for a compact architecture.

[0188] When used with a cooled detector 41, the cryostat 43 and especially the cold screen (which will be blackened from the inside, unlike the cryostat) also act as a baffle, blocking most of the stray light. A small out-of-field area 64 remains that can illuminate the detector 41 after specular refurbishment on M3. A small baffle 52 can be added behind M2 as shown in [Fig. 8] to eliminate this as well.

[0189] A comparison of the compactness between a state-of-the-art reimaging TMA, a state-of-the-art Grapefruit with an accessible exit pupil and a system 11 according to Figure 1 is given in Table 3 below, for a focal length of 240 mm, a total field covering the detector diagonal of 2.3°, and open at F / 3.

[0190] [Tables3] System Volume (mm3) Difference with TMA TMA reimage 111x200x279 - Grapefruit with pupil exit 150 x 200 x 245 + 15% Proposed Architecture U1X163X 199 -42%

[0191] Table 3 therefore illustrates a comparison of the overall dimensions between a state-of-the-art reimaging TMA, a state-of-the-art Pamplemousse with an accessible exit pupil, and the proposed folded reimaging system 11. The overall dimensions of the state-of-the-art TMA are used as a reference. The difference in overall dimensions between the state-of-the-art TMA and the state-of-the-art Pamplemousse is explained by the fact that the mirrors of the state-of-the-art Pamplemousse are larger than those of the reimaging architectures.

[0192] Thus, the system 11 described above has a number of advantages compared to systems established in the state of the art.

[0193] Initially, the folded system 11 offers greater compactness than a conventional TMA-type architecture according to the state of the art. However, folded architectures like the Pamplemousse, according to the state of the art, typically suffer from stray light problems and are bulky when the exit pupil is accessible. System 11 proposes a solution for good stray light management 61, 62, 63, 64 by inserting at least one baffle 51 between the exit pupil and the detector 4L. This baffle 51 does not increase the size of the optical system 11.

[0194] The presence of an accessible PS exit pupil also allows the aperture to be adapted to the desired spectral range. If the system 11 includes an aperture diaphragm located at the PS pupil, this makes it possible to reduce the aperture of the system 11 without introducing vignetting.

[0195] Another advantage of this system 11, compared to the established state-of-the-art Pamplemousse systems, lies in the ease of alignment of the system 11. Since the mirrors M1 and M3 can be made on the same substrate 30, alignment techniques specific to classical TMA, and very well mastered, can be used here.

[0196] Thus, it is possible to combine the compactness of folded systems like Pamplemousse or AlphaZ with the alignment techniques of classic TMAs, while avoiding the problems of stray light.

[0197] The main application area envisaged for this invention is space infrared imaging, for systems embedded on nanosatellites or microsatellites.

[0198] Multispectral imaging applications can be envisaged.

[0199] Thus, the system 11 is typically integrated into an imaging device according to the invention such as a satellite or microsatellite according to the invention.

[0200] Of course, the invention is not limited to the examples just described and many modifications can be made to these examples without departing from the scope of the invention.

[0201] For example, in the preceding description, each element 41, 42 can be a detector or emitter of light of any wavelength, preferably between 400 nm and 40 pm.

[0202] Generally, the system 11 may include as detector type a CCD, CMOS, MCT, T2SL, QWIP, microbolometer, InSb or InGaAs.

[0203] Generally, the system 11 may include a laser or a diode as the type of emitter.

[0204] Of course, the various features, forms, variants, and embodiments of the invention can be combined with one another in various ways, provided they are not incompatible or mutually exclusive. In particular, all the variants and embodiments described above are combinable.

Claims

Demands

1. Optical imaging system (11), comprising: - A primary mirror (M1), a secondary mirror (M2), and a tertiary mirror (M3) - An exit pupil (PS), the primary mirror, the secondary mirror, and the tertiary mirror being adapted and arranged so that light rays (21, 22, 23) from a scene are reflected first by the primary mirror, then by the secondary mirror, and then by the tertiary mirror towards the exit pupil, the system being arranged so that a light ray (21, 22, 23) coming from the scene is thus cut into an initial segment (210, 220, 230) of radius upstream of the primary mirror (M1), a first intermediate segment (211, 221, 231) of radius between the primary mirror and the secondary mirror (M2), a second intermediate segment (212, 222, 232) of radius between the secondary mirror and the tertiary mirror (M3), and a terminal segment (213, 223,233) of radius between the tertiary mirror and the exit pupil, the primary (M1) and secondary (M2) mirrors being oriented such that the second intermediate segment (232) of a parabasal ray (23) of the system intersects the initial segment (230) of said parabasal ray, the secondary (M2) and tertiary (M3) mirrors being oriented such that the first intermediate segment (231) of the parabasal ray of the system intersects the terminal segment (233) of said parabasal ray, the system further comprising an intermediate image plane (PI) situated between the primary mirror and the secondary mirror or between the secondary mirror and the tertiary mirror.

2. System according to claim 1, characterized in that the intermediate image plane (PI) is located along the second intermediate segment.

3. System according to claim 1 or 2, characterized in that the intermediate image plane is located closer to the secondary mirror (M2) than to the primary mirror and / or the tertiary mirror.

4. A system according to any one of the preceding claims, characterized in that the primary (M1), secondary (M2) and tertiary (M3) mirrors are arranged so that the initial segment (230) of the radius the parabasal of the system intersects the terminal segment (233) of said parabasal ray.

5. System according to any one of the preceding claims, characterized in that an angle (0J) formed between: - a tangent (T1) to a reflective surface of the primary mirror arranged to reflect light rays coming from the scene, at the level of the contact between this reflective surface and the parabasal ray of the system, and - a tangent (T3) to a reflective surface of the tertiary mirror arranged to reflect light rays coming from the secondary mirror, at the level of the contact between this reflective surface and the parabasal ray of the system is greater than 110°.

6. A system according to any one of the preceding claims, characterized in that an angle (02) formed between: - a normal (A) to an axis connecting a center of the secondary mirror (M2) located at the contact between the reflecting surface of the secondary mirror and the parabasal ray of the system and a point (P) of intersection between: • a tangent (T1) to a reflecting surface of the primary mirror arranged to reflect light rays from the scene, at the contact between this reflecting surface and the parabasal ray of the system, and • a tangent (T3) to a reflecting surface of the tertiary mirror arranged to reflect light rays from the secondary mirror, at the contact between this reflecting surface and the parabasal ray of the system - a tangent (T2) to a reflecting surface of the secondary mirror arranged to reflect light rays from the primary mirror,at the point of contact between this reflective surface and the parabasal radius of the system is less than 10°.

7. A system according to any one of the preceding claims, characterized in that the secondary mirror has the largest dimension of its reflective surface arranged to reflect light rays from the primary mirror at least 5 times smaller: - than the largest dimension of the reflective surface of the primary mirror arranged to reflect light rays from the stage, and - than the largest dimension of the reflective surface of the tertiary mirror arranged to reflect light rays from the secondary mirror.

8. System according to any one of the preceding claims, characterized in that the primary mirror and tertiary mirror are made on the same substrate (30) so that these two mirrors are fixed to each other.

9. System according to any one of the preceding claims, characterized in that the primary mirror and the tertiary mirror are adjacent and separated by a distance of less than 1% of the focal length of the primary mirror.

10. System according to any one of the preceding claims, characterized in that for all primary, secondary and tertiary mirrors, the angle between the parabasal ray incident on the reflective surface of the mirror considered and the parabasal ray reflected by the reflective surface of the same mirror considered is always of the same sign.

11. System according to any one of the preceding claims, characterized in that the exit pupil is located outside of any folded optical path between the primary mirror and the secondary mirror.

12. System according to any one of the preceding claims, characterized in that it comprises a first light emitter or detector (41) located downstream of the exit pupil relative to the tertiary mirror.

13. System according to claim 12, characterized in that it comprises at least one baffle (51) disposed laterally to the exit pupil and / or the first detector, and arranged to block the passage of light rays which would arrive at the first detector without being reflected successively by each of the primary, then secondary and then tertiary mirrors and without passing through the exit pupil.

14. System according to claim 12 or 13, characterized in that it comprises a dichroic plate placed between the tertiary mirror and the first detector or light emitter, preferably between the tertiary mirror and the exit pupil, said dichroic plate being arranged to direct light rays from the tertiary mirror to at least one other detector (42) and / or to direct light rays to the tertiary mirror from another light emitter (42).

15. System according to any one of claims 12 to 14, characterized in that the back distance of the system defined as the distance between an image plane (IM) of the system and a point of intersection between: - a straight line perpendicular to a photosensitive surface of the first detector and passing substantially through its center and - the ray, reflected by the primary mirror, closest to the detector, is greater than 20% of the focal length of the system.

16. Imaging apparatus (100), comprising a system (11) which conforms to any one of the preceding claims, said apparatus being a satellite.

Images