Astronomical optical device and associated telescope.

Abstract

The present invention relates to an optical telescope device (1), comprising: - a mirror (2) having a first concave reflective outer face (2a) and a first flat inner face (2b) opposite the first outer face (2a), the first outer face (2a) being polished; - a reinforcement structure (3) fixed to the mirror (2), the reinforcement structure (3) having a second flat inner face (3b) and a second free outer face (3a), the second inner face (3b) being in contact with the first inner face (2b) along a fixing plane; the second free outer face (3a) being concave, the mirror (2) and the reinforcement structure (3) being symmetrical with respect to the fixing plane. Figure for the abstract: Fig. 1

Description

Title of the invention: Astronomical optical device and associated telescope. technical field

[0001] The present invention relates to the field of high-resolution imaging. It relates more particularly to telescope mirrors for space applications, which must meet the problem of mass reduction while maintaining good optical performance. STATE OF THE ART

[0002] Space telescope mirrors can be very large, with diameters of one or more meters. The size of such mirrors is a key factor in improving the performance of optical systems. However, for space applications, this parameter is constrained by the major requirement of minimizing the mirror's mass.

[0003] To meet this constraint, the mirrors are lightened on their back face. To achieve this, recesses are machined into the back face. These weight reductions decrease the mass of the mirror but also affect the rigidity of the part.

[0004] However, the rigidity of the mirror helps to limit measurement bias related to its deformation when measured on the ground. Indeed, a discrepancy between ground measurements and in-orbit performance can be observed due to deformations affecting the mirror subjected to demanding environmental conditions. The greater the rigidity, the better the opto-mechanical performance obtained. This will also help to maintain the same performance between the mirror integrated into its telescope in orbit and on the ground.

[0005] Mirror designers are therefore constantly searching for an appropriate compromise between the mass and rigidity of the mirror, in order to optimize the opto-mechanical performance of said mirrors.

[0006] To remedy this lack of rigidity, various mirror reinforcement structures have been developed, including the so-called "close-back" structure, consisting of a reinforcement plate covering the back of the mirror in order to stiffen it.

[0007] However, these different solutions have the drawback of complicating the positioning of a mirror support device which, in order to minimize the mirror's deformation, must be placed on the mirror's neutral fiber. The neutral fiber is understood to be a zone of equilibrium of the mirror where the effect of gravity is minimized, that is to say, the zone of least deformation of the mirror.

[0008] In other words, these reinforcement solutions modify the location of the neutral fiber of the mirror and make its characterization complex or even impossible, so that the positioning of the mirror support device is made difficult. EXPOSED

[0009] One object of the present invention is to remedy the aforementioned drawbacks.

[0010] In particular, an object of the present invention is to propose an optical device exhibiting better performance in terms of rigidity relative to mass.

[0011] To this end, according to a first aspect, an optical telescope device is proposed, comprising: - a mirror comprising a first concave reflective outer face and a first flat inner face opposite the first outer face, the first outer face being polished; - a reinforcement structure fixed to the mirror, the reinforcement structure comprising a second flat inner face and a second free outer face, the second inner face being in contact with the first inner face according to a fixing plane;

[0012] the second free external face being concave, the mirror and the reinforcing structure being symmetrical with respect to the fixing plane.

[0013] The optical device according to the invention exhibits, due to its symmetry, improved opto-mechanical performance while simplifying the characterization of its neutral fiber.

[0014] Advantageously, the mirror comprises a first honeycomb structure formed over the entire thickness of the mirror and the reinforcement structure comprises a second honeycomb structure formed over the entire thickness of the reinforcement structure.

[0015] In a preferred embodiment, the alveolated structures comprise triangular alveolated portions.

[0016] Preferably, the mirror and the reinforcement structure are made of the same material, chosen from the following materials: glass-ceramic, Zerodur, aluminum alloy, silicon carbide or other ceramics.

[0017] Advantageously, the diameter of the mirror and the reinforcement structure is between 1 and 5 meters, preferably between 1.5 meters and 2.5 meters.

[0018] Advantageously, the optical device has a surface mass between 5 and 50 kg / m2, preferably between 25 and 50 kg / m2, even more preferably between 40 and 50 kg / m2.

[0019] Advantageously, the optical device includes a central orifice formed over the entire thickness of the optical device and passing through the optical device.

[0020] Advantageously, the mirror comprises a first peripheral face connecting the first outer face to the first inner face, and the reinforcing structure comprises a second peripheral face connecting the second external face to the second internal face, the first honeycomb structure comprising first walls, the second honeycomb structure comprising second walls, the first walls and the second walls being aligned to define internal cavities of the optical device, the central orifice being delimited by a central face, at least one among the central face and / or one of the peripheral faces, and at least one among the first walls and the second walls, being configured to put said internal cavities into gaseous communication with the outside of the optical device.

[0021] Advantageously, the optical device comprises tenons each consisting of a first tenon portion and a second tenon portion, each first tenon portion protruding from the first peripheral face, each second tenon portion protruding from the second peripheral face.

[0022] The invention also relates to a telescope comprising a frame and an optical device as defined above. DESCRIPTION OF THE FIGURES

[0023] Other features, objectives and advantages of the invention will become apparent from the following description, which is purely illustrative and not limiting, and which should be read in conjunction with the accompanying drawings on which:

[0024] Fig. 1 illustrates in perspective an optical device according to the present application;

[0025] Fig.2 illustrates in exploded view the optical device of Fig.1;

[0026] Figure 3 illustrates a front view of the rear of the mirror in Figure 1; and

[0027] [Fig.4] schematically illustrates a radial cross-sectional view of the optical device of [Fig.1].

[0028] Throughout the figures, similar elements bear identical references. DETAILED DESCRIPTION

[0029] Fig. 1 illustrates an optical device 1 according to a first embodiment of the invention, and Fig. 2 illustrates the optical device 1 of Fig. 1 in exploded view.

[0030] The optical device 1 extends and is centered along a central axis Z and includes along this axis Z a mirror 2 and a reinforcing structure 3.

[0031] The mirror 2 comprises a first external face 2a, a first internal face 2b, and a first peripheral face 2c. The first peripheral face 2c defines a diameter of the mirror 2 and connects the first external face 2a to the first internal face 2b. The first peripheral face 2c is in the form of a ring, thus connecting the first external face 2a to the first internal face 2b.

[0032] The first external face 2a is reflective. The first internal face 2b is flat and has the same diameter as the first external face 2a, said diameter corresponding to the diameter of mirror 2. The first inner face 2b is perpendicular to the central axis Z of the optical device 1.

[0033] The reinforcement structure 3 comprises a second free outer face 3a, a second inner face 3b, and a second peripheral face 3c. The second peripheral face 3c defines a diameter of the reinforcement structure 3 and connects the second outer face 3a to the second inner face 3b. The second peripheral face 3c takes the form of a ring, thus connecting the second outer face 3a to the second inner face 3b.

[0034] The diameter of the reinforcement structure 3 is identical to the diameter of the mirror 2, so that said diameters therefore define a diameter of the optical device 1. The diameter of the optical device 1 is for example between 1 and 5 meters, preferably between 1.5 meters and 2.5 meters.

[0035] The second inner face 3b is flat and extends perpendicularly to the central axis Z of the optical device 1, opposite the first inner face 2b. More precisely, the second inner face 3b is fixed to the first inner face 2b according to a fixing plane P illustrated in [Fig. 4]. For example, the second inner face 3b and the first inner face 2b are fixed by gluing.

[0036] Fig. 4 illustrates a schematic radial cross-sectional view of the optical device 1, showing the profiles of the different faces of the mirror 2 and the reinforcement structure 3.

[0037] The fixing plane P is a plane perpendicular to the Z axis of the optical device 1. The first external face 2a and the second external face 3a are both concave, i.e., rounded towards the fixing plane P. As illustrated in [Fig. 4], the fixing plane P thus constitutes a plane of symmetry of the optical device 1, the reinforcing structure 3 being symmetrical to the mirror 2 with respect to the fixing plane P. Also, the central axis Z includes the center of each of the first and second external faces 2a and 3a, this center forming the deepest point of their respective said face 2a and 3a. The thickness of the optical device 1 is therefore minimal at the Z axis (between the respective centers of the first external face 2a and the second external face 3a), and therefore increases as we move away from the Z axis until it is maximal at the radial end of the optical device 1 formed by the first peripheral face 2c and the second peripheral face 3c.For example, for an optical device 1 with a diameter of approximately 2 meters, the maximum thickness of the optical device 1 on its peripheral face (formed by the combination of the first and second peripheral faces 2c and 3c) is between 200 and 400 millimeters, more specifically around 270 millimeters. In such an example, the first external face 2a, and by symmetry the second external face 3a, each have a thickness between 1 and 10 millimeters, for example, around 4 millimeters. Obviously, the maximum thickness of the optical device 1 on its peripheral face depends on the degree of concavity or curvature of the optical device 1, and this example is not limiting.

[0038] The mirror 2 and the reinforcing structure 3 are made of the same material, for example Zerodur, glass-ceramic, glass, silicon carbide or any other suitable material such as other ceramics, or aluminum alloys. Preferably, they are made of Zerodur, which is a material exhibiting very high thermal stability and a good mass-to-stiffness ratio suitable for optical polishing (its coefficient of thermal expansion being approximately 0.05 x 10⁶ K⁻¹ ±0.10).

[0039] Thus, the mirror 2 and the reinforcing structure 3 have substantially the same mass. In this way, and thanks to the symmetry of the optical device 1, the optical device 1 includes a neutral fiber carried by the plane of symmetry P, and is therefore particularly easy to characterize.

[0040] Also, the optical device 1 may include a central orifice 11 passing through the optical device 1 in its entire thickness, the function of which may depend on the design of the optical device, and may, for example, be to allow light rays to pass through. In particular, the central orifice 11 may be defined by an annular central face resulting from the alignment of a first central ring 1la extending in the mirror 2 between the first inner face 2b and the first outer face 2a and a second central ring 11b extending in the reinforcing body 3 between the second outer face 3b and the second outer face 3a. In any plane perpendicular to the Z-axis, the central orifice has as its center a point along the Z-axis.

[0041] To lighten the optical device 1, the mirror 2 and the reinforcing structure 3 each comprise a honeycomb structure symmetrical to each other with respect to the fixing plane P, illustrated in figures 2 and 3.

[0042] Fig. 3 illustrates a front view of the first internal face 2b, which is identical to the second internal face 3b visible in Fig. 2.

[0043] More specifically, the mirror 2 comprises a first honeycomb structure 4 extending through the thickness of the mirror 2 along the axial direction Z between the first external face 2a and the first internal face 2b (not shown in [Fig. 4] for clarity). To save mass, the first honeycomb structure 4 opens onto the first internal face 2b, as illustrated in [Fig. 3]. Conversely, the first honeycomb structure 4 does not open onto the first internal face 2a, which is the reflecting face of the optical device 1. The first external face 2a is therefore solid, as illustrated in [Fig. 1]. The honeycomb structure 4 comprises a plurality of honeycomb portions 4a defined by walls 4b machined in the thickness of the mirror 2 along the Z axis. The honeycomb portions 4a are in other words blind cavities limited by the walls 4b which extend in the thickness of the mirror 2 along the Z axis and open onto the first inner face 2b.The first inner face 2b is therefore not. solid, and includes axial ends 4c of the walls 4b of the honeycomb structure 4. The first inner face 2b is flat, so that the walls 4b extend axially over a greater distance as said walls 4b are moved radially away from the central axis Z, because the central axis Z carries the center of the concavity of the first outer face 2a. For example, in the case of the optical device 1 with a diameter of 2 meters and a maximum thickness of 270 millimeters on its peripheral face le, said walls 4b extend axially over a distance of between approximately 21 millimeters at the Z axis and approximately 130 millimeters at the peripheral face le.

[0044] Similarly, the reinforcement structure 3 includes a second honeycomb structure 5 extending through the thickness of the reinforcement structure 3 along the axial direction Z between the second external face 3a and the second internal face 3b. To further reduce mass, the second honeycomb structure 5 opens onto the second internal face 3b. In order to preserve symmetry and, in particular, to maintain a similar mass between the mirror 2 and the reinforcement structure 3, the second external face 3a is solid, like the first external face 2a. Furthermore, symmetrically to the first honeycomb structure 4, the second honeycomb structure 5 comprises a plurality of honeycomb portions 5a defined by walls 5b machined into the thickness of the reinforcing structure 3 along the Z axis. The honeycomb portions 5a are, in other words, blind cavities limited by the walls 5b which extend into the thickness of the reinforcing structure 3 and open onto the second internal face 3b.The second internal face 3b is therefore also not solid and includes axial ends 5c of the walls 5b of the second honeycomb structure 5. This second internal face 3b being planar and perpendicular to the central axis Z of the optical device 1, the walls 5b extend axially in the same way over a greater distance as said walls 5b are moved radially away from the central axis Z. The dimensions of the walls 5b of the second honeycomb structure 5 are identical to those of the walls 4b of the first honeycomb structure 4.

[0045] The first honeycomb structure 4 and the second honeycomb structure 5 are symmetrical with respect to the fixation plane P. In other words, the walls 4b of the first honeycomb structure 4 and the walls 5b of the second honeycomb structure 5 are aligned with each other along directions parallel to the Z-axis. Thus, the axial ends 4c of the first honeycomb structure 4 are in contact with the axial ends 5c of the second honeycomb structure 5. The bonding of the second inner face 3b to the first inner face 2b involves bonding the ends 5c of the walls 5b of the second honeycomb structure 5 to the ends 4c of the walls 4b of the first honeycomb structure 4. The adhesive used could, for example, be DP490 adhesive. Internal cavities are thus formed by bonding the first honeycomb structure 4 and of the second honeycomb structure 5 between them. These internal cavities are each made up of a honeycomb portion 4a of the mirror 2 on one side and of a honeycomb portion 5a of the reinforcing body 3 aligned axially along the Z axis with said honeycomb portion 4a on the other side.

[0046] It should be noted that the internal cavities formed by bonding the first honeycomb structure 4 to the second honeycomb structure 5 are not airtight, in order to prevent air from becoming trapped, which would be incompatible with use under vacuum in space. The respective walls 4b and 5b of the first honeycomb structure 4 and the second honeycomb structure 5 are thus configured to allow gaseous communication between the exterior and interior of these internal cavities and of the optical device 1, so as to ensure air continuity and equal pressure between the exterior and interior of the optical device 1. For example, the walls 4b of the first honeycomb structure 4 and / or the walls 5b of the second honeycomb structure 5 may be perforated or notched to allow air circulation.The same applies to at least one of the peripheral faces 2c, 3c and / or one of the central rings 1a, 11b defining the central orifice 11, in order to establish gaseous communication between the exterior and interior of the optical device 1. If only one of the first honeycomb structure 4 and the second honeycomb structure 5 comprises walls 4b, 5b which allow air circulation (for example by being perforated), then the central ring 1a, 11b defining the portion of the central orifice 11 and / or the peripheral face 2c, 3c of the part (the part being the reinforcing body 3 or the mirror 2) comprising said honeycomb structure 4, 5 also allow air circulation, preferably by also being perforated.

[0047] Preferably, the honeycomb portions 4a and 5a are triangular, in order to maximize the ratio between the mass gain and the bonding surface. Alternatively, the honeycomb portions 4a and 5a may be circular or rectangular. When the honeycomb portions 4a, respectively 5a, are polygonal, each of said honeycomb portions 4a, respectively 5a, includes vertices 10 formed by junctions between walls 4a, respectively 5a. Said vertices 10 may be substantially circular and thicker than said walls 4a, respectively 5a, in order to limit the loss of rigidity of the optical device 1 due to the machining of the first and second honeycomb structures 4 and 5.

[0048] In the example of the optical device 1 described above (with a diameter of 2 meters and a maximum thickness of 270 millimeters on its peripheral face), when the honeycomb portions 4a and 5a are triangular or rectangular, the vertices 10 of said honeycomb portions may have a radius between 10 and 12 millimeters, in order to increase the bonding surface and limit the loss of rigidity due to the machining of the first and second honeycomb structures 4 and 5. Furthermore, in In this example, the walls 4b and 5b of the first and second honeycomb structures 4 and 5 can have a thickness between 3 and 4 millimeters and define triangular honeycomb portions composed of equilateral triangles with sides of length between 300 and 350 millimeters, for example, 320 millimeters. Thus, the optical device 1 can have a mass of approximately 175 kg.

[0049] Furthermore, the optical device 1 may include a plurality of pins 6 for attaching external mirror mounting devices to the optical device 1, which are connected to a telescope structure. Such mirror mounting devices may be arms extending from the telescope structure to the pins 6 to which they are attached, so as to support and secure the optical device 1.

[0050] Each of the tenons 6 consists of a first portion of tenon 6a and a second portion of tenon 6b.

[0051] The first tenon portions 6a project outwards from the optical device 1 from the first peripheral face 2c, and the second tenon portions 6b project outwards from the optical device 1 from the second peripheral face 3c, symmetrically to the first tenon portions 6a with respect to the fixing plane P.

[0052] In order to gain rigidity of the tenons 6, each of the first portions of tenon 6a, respectively of the second portions of tenon 6b, may include a reinforcing part 8 extending from the first peripheral face 2c, respectively from the second peripheral face 3c, towards the interior of the optical device 1.

[0053] The first tenon portions 6a and the second tenon portions 6b each include an axial end 9 included in the fixing plane P, the axial ends 9 of each of the first tenon portions 6a being thus in contact with the axial end 9 of one of the second tenon portions 6b, by symmetry, so as to form a tenon 6.

[0054] Thus, the pins 6 are arranged at the level of the neutral fiber of the optical device 1 characterized by the fixing (and symmetry) plane P. The symmetry of the optical device 1 thus makes it easy to characterize the neutral fiber of the optical device 1 and therefore to optimally position the pins 6, which are subject to forces from the mirror fixing devices, and consequently make it possible to filter the mechanical forces transmitted to the optical device 1 and to guarantee the stability of the optical performance.

[0055] For example, the thickness of the tenon portions 6a and 6b in the radial direction is 8 millimeters. The tenon portions 6a and 6b can be machined directly from the block of material in which the mirror 2 and the reinforcing structure 3 are machined. Glue can be applied to the contacting surfaces of the tenon portions 6a and 6b.

[0056] An optical device 1 according to the present application and the example described above (diameter of 2 meters and maximum thickness of 270 millimeters on its peripheral face 1), with triangular honeycomb portions conforming to the examples described above, was evaluated and compared to an optical device comprising a mirror according to the prior art known as "close-back", comprising a plate on the rear face of the mirror.

[0057] The evaluated data have been reported in the following table: Optical device 1 as requested. Close-back type mirror. Surface mass ps = 48 kg / m² ps = 34 kg / m². Frequency of the first mode (free-free modal analysis): 184 Hz 130 Hz. Maximum stress in the mirror for a quasi-static load under 1 g: X-axis: 1.027 MPa; X-axis: 0.235 MPa; Y-axis: 1.169 MPa; Y-axis: 0.275 MPa; Z-axis: 0.405 MPa; Z-axis: 0.483 MPa. Maximum stress in the mirror for a quasi-static load under 15 g: X-axis: 15.401 MPa; X-axis: 3.521 MPa; Y-axis: 17.535 MPa; Y-axis: 4.119 MPa; Z-axis: 6.075 MPa; Z-axis: 7.249 MPa. Gravity effect for 1 g s⁻¹ According to the radial direction, off tilt and in focus: PTV = 874.739 nm RMS = 64.507 nm Off tilt and in focus: PTV = 613.325 nm RMS = 104.931 nm Tilt term: PTV = 51.668 nm RMS = 13.241 nm Tilt term: PTV = 227.269 nm RMS = 58.353 nm In focus term: PTV = 0.674 nm RMS = 0.194 nm In focus term: PTV = 1.790 nm RMS = 0.515 nm

[0058] The optical device 1 according to the invention, by virtue of its improved rigidity relative to its symmetrical structure, allows for the optimization of the optical device 1's mass by increasing the size of the honeycomb portions 4a and 5a and reducing the thickness of the optical device 1's components. Although its surface mass is slightly greater than that of the "close-back" mirror due to the difference in the rear structure formed by the reinforcement structure 3, the gain in rigidity of the optical device 1 allows for better opto-mechanical performance.

[0059] The superior rigidity of the optical device 1 according to the present application is demonstrated by modal analysis. Indeed, for two parts of equal rigidity, the frequency of the first mode and the mass are inversely proportional. Now, although the reinforcement structure 3 is heavier than the plate of the "close-back" type mirror, the first mode frequency of the optical device 1 is higher than that of the "close-back" type mirror, which allows us to deduce that its rigidity is superior.

[0060] Furthermore, the root mean square (RMS) deformations (global, excluding tilt and focus, in terms of tilt and focus) under gravity along the Y-axis of the optical device 1, as required, are less significant than for the "close-back" type mirror. This result is directly related to the improved rigidity of the structure and the positioning of the pins 6, which allow the mirror mounting devices to be mounted at the neutral fiber of the optical device 1. The opto-mechanical performance of the optical device 1 is therefore improved. Indeed, the RMS deformations result from a mechanical calculation where the optical device 1, held by external mirror mounting devices, is subjected to a stress corresponding to its own weight (1g). The result of this calculation is the deformation of the optical face relative to the theoretical surface to be achieved.The greater this distortion, the more bias there will be during the manufacturing and measurement of the mirror, and the lower the achievable performance will be for that mirror.

[0061] Furthermore, the optical device 1 according to the present has the advantage of being machinable with the same machining tool for both the reinforcement structure 3 and the mirror 2, as these are identical because they are symmetrical, except that the first external face 2a is polished to be reflective, unlike the second external face 3b. Traditional machining techniques, such as CNC machines, can therefore be used. Moreover, unlike other existing reinforcement structures, the present reinforcement structure 3 requires only the material from which it is made (e.g., Zerodur) and adhesive. It follows from these various aspects that machining the optical device 1 is economically viable, in addition to its improved opto-mechanical performance.

[0062] For example, the optical device 1 can be a main mirror of a telescope on board a satellite.

Claims

Demands

1. Optical telescope device (1), comprising: - a mirror (2) having a first concave reflective outer face (2a) and a first flat inner face (2b) opposite the first outer face (2a), the first outer face (2a) being polished; - a reinforcement structure (3) fixed to the mirror (2), the reinforcement structure (3) having a second flat inner face (3b) and a second free outer face (3a), the second inner face (3b) being in contact with the first inner face (2b) along a fixing plane (P); characterized in that the second free outer face (3a) is concave, the mirror (2) and the reinforcement structure (3) being symmetrical with respect to the fixing plane (P).

2. Optical device (1) according to claim 1, wherein the mirror (2) comprises a first honeycomb structure (4) formed over the entire thickness of the mirror (2) and wherein the reinforcement structure (3) comprises a second honeycomb structure (5) formed over the entire thickness of the reinforcement structure (3).

3. Optical device (1) according to claim 2, wherein the honeycomb structures (4, 5) comprise triangular honeycomb portions (4a, 5a).

4. Optical device (1) according to any one of claims 1 to 3, wherein the mirror (2) and the reinforcement structure (3) are made of the same material selected from the following materials: glass-ceramic, Zerodur, aluminum alloy, silicon carbide or other ceramics.

5. Optical device (1) according to any one of claims 1 to 4, wherein the mirror (2) and the reinforcing structure (3) have a diameter of between 1 and 5 meters, preferably between 1.5 meters and 2.5 meters.

6. Optical device (1) according to any one of claims 1 to 5, comprising a surface mass of between 5 and 50 kg / m2, preferably between 25 and 50 kg / m2, even more preferably between 40 and 50 kg / m2.

7. Optical device (1) according to any one of claims 1 to 6, comprising a central orifice (11) formed over the entire thickness of the optical device (1) and passing through the optical device (1).

8. Optical device (1) according to claim 7 in combination with claim 2, wherein the mirror (2) comprises a first peripheral face (2c) connecting the first external face (2a) to the first internal face (2b) and the reinforcing structure (3) comprises a second peripheral face (3c) connecting the second external face (3a) to the second internal face (3b), the first honeycomb structure (4) comprising first walls (4b), the second honeycomb structure (5) comprising second walls (5b), the first walls (4b) and the second walls (5b) being aligned to define internal cavities of the optical device (1), the central orifice (11) being delimited by a central face (1a, 11b), at least one of the central face (1a, 11b) and / or one of the peripheral faces (2c, 3c), and at least one of the first walls (4b) and the second walls (5b),being configured to establish gaseous communication between said internal cavities and the exterior of the optical device (1).

9. Optical device (1) according to claim 8, comprising pins (6) each consisting of a first pin portion (6a) and a second pin portion (6b), each first pin portion (6a) projecting from the first peripheral face (2c), each second pin portion (6b) projecting from the second peripheral face (3c).

10. Telescope comprising a frame and an optical device (1) according to any one of claims 1 to 9.

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