Cassegrain telescope with piezoelectric actuators for a high-performance gyrostabilised viewfinder

Abstract

The invention relates to a telescope (1) comprising: - a primary mirror (10) mounted on a support (11); - a secondary mirror (20) surrounded by a secondary ring (21) securing the secondary mirror to an armature (30); - an articulated structure (40) arranged between the support (11) and the armature (30) in order to allow six degrees of freedom for the secondary mirror (20) with respect to the primary mirror (10); - a plurality of piezoelectric actuators (50) mounted so as to move the secondary mirror (20) by acting on the articulated structure (40); - at least one sensor (70) for sensing the alignment of the primary mirror (10) and of the secondary mirror (20); - an electronic processing unit electrically connected to the actuators and arranged to control the actuators (50) so as to maintain alignment between the primary mirror (10) and the secondary mirror (20).

Description

[0001] CASSEGRAIN TELESCOPE WITH PIEZO-ELECTRIC ACTUATORS FOR HIGH-PERFORMANCE GYRO-STABILIZED VIEWFINDER

[0002] The present invention relates to the field of optical devices, for example intended for aiming. More specifically, the present invention relates to the field of high-precision gyro-stabilized sights applied to on-board systems.

[0003] BACKGROUND OF THE INVENTION

[0004] Gyro-stabilized sights are known from the state of the art that can be used to form optronic devices commonly referred to as gyro-stabilized balls. In a manner known per se, a gyro-stabilized ball comprises a photosensitive sensor, optical elements carried by a mechanical structure to define an optical path to the photosensitive sensor, and an electronic processing unit connected to the photosensitive sensor to process the signals from said sensor.

[0005] In a constrained environment (severe vibrations, thermal gradients and / or load factor), the mechanical structure supporting the optical elements can deform and alter the optical path. For example, in a telescope, the mechanical structure supports two mirrors facing each other, thus defining the optical path between them. When a deformation of the mechanical structure occurs, the optical path itself is then deformed. Thus, to meet the objectives of improving the image quality of the optronic chain, a current solution consists of further stiffening the mechanical structure.

[0006] Unfortunately, such a mechanical structure is particularly expensive and is accompanied by difficulties in manufacturing or configuring the position and orientation of one mirror relative to the other. In addition, the use of the same material is necessary throughout the telescope in order to limit deformations linked to differential expansions and the thermal gradient, the temperature not being uniform over the entire mechanical structure.

[0007] SUBJECT OF THE INVENTION

[0008] The invention aims in particular to remedy at least in part the aforementioned drawbacks.

[0009] SUMMARY OF THE INVENTION

[0010] To this end, according to the invention, a telescope is provided comprising: a primary mirror mounted on a support, a secondary mirror surrounded by a secondary crown fixing the secondary mirror to a frame, an articulated structure which is arranged between the support and the frame to allow six degrees of freedom of the secondary mirror relative to the primary mirror, a plurality of piezoelectric actuators mounted to move the secondary mirror by acting on the articulated structure, at least one sensor for an alignment of the primary mirror and the secondary mirror, an electronic processing unit electrically connected to the actuators and arranged to control the actuators so as to maintain the alignment between the primary mirror and the secondary mirror.

[0011] Thus, a new telescope architecture is advantageously proposed. The chosen architecture allows the secondary mirror to be permanently aligned with respect to said primary mirror. Indeed, the sensor detects a misalignment or a deformation of the secondary mirror with respect to the primary mirror. The information is then received by the electronic processing unit which controls the piezoelectric actuators so as to realign the secondary mirror and the primary mirror. According to optional characteristics, used individually or all or part in combination:

[0012] - the frame comprises three arms which are arranged substantially in a triangle tangentially to the secondary crown and which each have two ends, the arms are fixed two by two by their ends forming a vertex of the triangle, the vertices being fixed to the articulated structure;

[0013] - the articulated structure comprises three pairs of legs, the legs of each pair having first ends spaced apart from each other and each connected to the support via one of the piezoelectric actuators, and second ends close to each other and connected to one of the vertices of the frame;

[0014] - the first end of each of the legs is connected to the piezoelectric actuator by a first spherical connection and the second end is connected to the armature by a second double pivot connection, the two connections being arranged to allow rotation of the armature relative to the support;

[0015] - each of the piezoelectric actuators is arranged to produce a translational movement in a longitudinal direction of the leg to which it is connected;

[0016] - the piezoelectric actuators are attached to the support;

[0017] - the sensor is a contactless sensor carried by a frame surmounting the support and arranged to detect a position of a target secured to the secondary mirror;

[0018] - the non-contact sensor is an inductive, or capacitive or eddy current position sensor;

[0019] - the electronic unit controls the actuators according to a control law designed to maintain alignment between the primary mirror and the secondary mirror.

[0020] Other characteristics and advantages of the invention will emerge from reading the following description of particular and non-limiting embodiments of the invention.

[0021] BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Reference will be made to the attached drawings, including:

[0023] [Fig. 1] Figure 1 is a perspective view of a telescope, according to a first embodiment of the invention;

[0024] [Fig. 2] Figure 2 is a side view of the telescope shown in Figure 1;

[0025] [Fig. 3] Figure 3 is a top view of the telescope shown in Figure 1;

[0026] [Fig. 4] Figure 4 is a perspective view of an element of the telescope according to the first embodiment of the invention;

[0027] [Fig. 5] Figure 5 is a representation of the telescope provided with sensors according to the first embodiment of the invention;

[0028] [Fig. 6] Figure 6 is a top view of the telescope illustrated in Figure 5 to better show the sensors; [Fig. 7] Figure 7 is a side view of the telescope illustrated in Figure 5 to better show the sensors; [Fig. 8] Figure 8 is a perspective view of the telescope, according to a second embodiment of the invention;

[0029] [Fig. 9] Figure 9 is a top view of the telescope shown in Figure 8; [Fig. 10] Figure 10 is a perspective view of an actuator included in the telescope shown in Figure 8;

[0030] [Fig. 11] Figure 11 is a perspective view of the telescope, according to a third embodiment of the invention;

[0031] [Fig. 12] Figure 12 is a perspective view of an actuator and leg included in the telescope illustrated in Figure 11;

[0032] [Fig. 13] Figure 13 is a perspective view of the telescope illustrated in Figure 11;

[0033] [Fig. 14] Figure 14 is a perspective view of the actuator shown in Figure 12;

[0034] [Fig. 15] Figure 15 is a flowchart illustrating the operation of the invention.

[0035] DETAILED DESCRIPTION OF THE INVENTION

[0036] With reference to Figures 1 to 7, a telescope 1 is described according to a first embodiment of the invention.

[0037] The telescope 1 is here a Cassegrain type telescope and comprises a primary mirror 10 mounted on a support 11 and a secondary mirror 20 arranged to be opposite the primary mirror 10.

[0038] The primary mirror 10 comprises a reflective concave upper surface and a lower surface, opposite the upper surface and carried by the support 11. The primary mirror 10 comprises a central hole having a central axis X extending orthogonally to said lower surface. In a manner known per se, a photosensitive sensor is intended to be positioned opposite the central hole, perpendicular to the central axis X.

[0039] The support 11 has an upper surface 11.1 facing said lower surface of the primary mirror 10, and a lower surface opposite the upper surface 11.1. The upper surface 11.1 is connected to said lower surface of the primary mirror 10 by means of studs known per se and arranged to fix the primary mirror 10 to the support 11 while limiting as much as possible the stresses on the primary mirror 11.

[0040] The secondary mirror 20 comprises a convex reflecting lower surface and an upper surface, opposite said lower surface. In addition, the secondary mirror 20 also comprises a cylindrical peripheral surface. The secondary mirror 20 has a central axis aligned with the central axis X of the primary mirror 10 to define an optical axis of the telescope 1. The central axis of the secondary mirror 20 is therefore coincident with the central axis X previously defined.

[0041] A secondary crown 21 matches the shape of the cylindrical peripheral surface of the secondary mirror 20. In other words, the secondary crown 21 encircles the secondary mirror 20. The secondary crown 21 carries said secondary mirror 20 by means of at least one flexible element and preferably, by means of three flexible elements arranged at 120° from each other.

[0042] The telescope 1 also comprises a frame 30 secured, in part, to the secondary crown 21 surrounding the cylindrical peripheral surface of the secondary mirror 20 and ensuring the attachment of the secondary mirror 20 to the frame 30. With reference to FIG. 3, the frame 30 comprises three arms 30.1, 30.2 and 30.3 which are arranged substantially in a triangle tangentially to the secondary crown 21 and which have their central part curved towards the secondary crown 21 and attached thereto. The arms 30.1, 30.2 and 30.3 each have two ends and are attached two by two by said ends forming a vertex 31 of the triangle. Therefore, the frame 30 comprises three vertices 31.1, 31.2, 31.3 each formed by the junction of two of the arms 30.1, 30.2 and 30.3 of the frame 30.

[0043] Each arm 30.1, 30.2, 30.3 has an inner surface opposite the secondary crown 21 and an outer surface, opposite the inner surface.

[0044] The telescope 1 also comprises an articulated structure 40 and a plurality of piezoelectric actuators 50.

[0045] The piezoelectric actuators 50 are secured to the support 11 by any suitable means and for example by screwing, bolting, etc.

[0046] Each of the piezoelectric actuators 50 comprises at least one piezoelectric crystal (not shown) and at least one strain gauge (not shown) arranged to measure the elongation of said crystal.

[0047] The articulated structure 40 comprises three pairs of legs 41, 42 and 43.

[0048] Referring to Figure 2, the legs of pair 41 are referenced 41.1 and 41.2, the legs of pair 42 are referenced 42.1 and 42.2, and the legs of pair 43 are referenced 43.1 and 43.2.

[0049] Each of the legs extends in a longitudinal direction specific to it. Thus, leg 41.1 extends along an axis X41.1. Equivalently, leg 41.2 extends along an axis X41.2. Equivalently, leg 42.1 extends along an axis X42.1. Equivalently, leg 42.2 extends along an axis X42.2. Equivalently, leg 43.1 extends along an axis X43.1. Equivalently, leg 43.2 extends along an axis X43.2.

[0050] Furthermore, the pairs of legs 41, 42 and 43 have first ends spaced apart from each other and each connected to the support 11 via one of the piezoelectric actuators 50, and second ends close to each other and connected to one of the vertices 31.1, 31.2 and 31.3 of the frame 30.

[0051] Referring to Figure 4, each of the legs 41.1, 41.2, 42.1, 42.2, 43.1 and 43.2 is connected by its first end to one of the piezoelectric actuators 50 by means of a spherical connection 61 (or ball joint). The spherical connection 61 allows rotation around three orthogonal axes allowing three degrees of freedom. On the other hand, each of the legs 41.1, 41.2, 42.1, 42.2, 43.1 and 43.2 is connected by its second end to one of the vertices 31.1, 31.2 and 31.3 of the frame 30 by means of a double pivot connection 60. The double pivot connection 60 allows rotation around two orthogonal axes allowing two degrees of freedom.

[0052] In the extension of the legs 41.1, 41.2, 42.1, 42.2, 43.1 and 43.2, the piezoelectric actuators 50 are responsible for the actuation of a prismatic link 62. The prismatic link 62 allows a translation and makes it possible to achieve a degree of freedom.

[0053] As a result, each of the legs 41.1, 41.2, 42.1, 42.2, 43.1 and 43.2 of the articulated structure 40 comprises a double pivot link 60, a spherical link 61 and a prismatic link 62. All of these links allow six degrees of freedom of the secondary mirror 20 with respect to the primary mirror 10. The six degrees of freedom correspond to three translations in an orthonormal frame of reference (not shown) and three rotations (not shown). The three rotations correspond to heading, rolling or pitching.

[0054] With reference to figures 5, 6 and 7, the telescope 1 comprises at least one sensor 70 of a relative alignment of the primary mirror 10 and the secondary mirror 20. Preferably, the telescope 1 comprises at least two sensors 70 of the relative alignment of the primary mirror 10 and the secondary mirror 20. Preferably, the telescope 1 comprises three sensors 70.

[0055] Each sensor 70 is a contactless position sensor. Said sensor 70 is arranged to detect a position of a target secured to the secondary mirror 20. More precisely, said sensor 70 comprises a movable part and a fixed part. The movable part of the sensor 70 is adjacent to one of the vertices (31.1, 31.2 and 31.3) of the frame 30. The fixed part of the sensor 70 is, for its part, fixed to a frame 2 surmounting the support 11. This fixed part corresponds to the target secured to the secondary mirror 20.

[0056] The frame 2 surmounts the support 11. Preferably, the frame 2 is shaped into a cylinder.

[0057] Finally, the telescope 1 comprises an electronic processing unit (not shown), electrically connected to the piezoelectric actuators 50 and to the sensors 70 and arranged to control the piezoelectric actuators 50 as a function of the signals supplied by the sensors 70 so as to maintain the alignment between the primary mirror 10 and the secondary mirror 20. More precisely, the electronic processing unit is arranged to control the elongation of the piezoelectric actuators 50 and to measure said elongation by means of the strain gauges.

[0058] With reference to Figures 8 to 10, a telescope 200 is described according to a second embodiment of the invention. The telescope 200 differs from the first embodiment in that the connection of the piezoelectric actuators 50 to the support 11 and to the legs 41, and the connection of the articulated structure 40 to the frame 30 have been modified.

[0059] With reference to figure 8, the support 11 comprises, on its upper face 11.1, at least one base 201 and preferably at least three bases 201. The bases 201 are here fixed on the support 11 by any suitable means and for example by screwing, bolting, etc.

[0060] At least one fixing plate 202 is arranged on each of the bases 201. Here, at least two fixing plates 202 are arranged on each of the bases 201. Preferably, the fixing plates 202 extend projecting from the support 11. Furthermore, the fixing plates 202 are inclined with respect to the support 11 and more precisely oriented towards the secondary mirror 20.

[0061] With particular reference to FIG. 10, each fixing plate 202 comprises a first straight edge 202.1 and a second curved edge 202.2. The fixing plates 202 arranged on the same base 201 are adjacent at their first edge 202.1, along an axis of symmetry. The second edge 202.2 comprises, in particular, a recess 203.

[0062] At least one piezoelectric actuator 50 is arranged on each of the bases 201 and preferably at least two piezoelectric actuators 50 are arranged on each of the bases 201. More precisely, one piezoelectric actuator 50 is provided per fixing plate 202. Each piezoelectric actuator 50 is housed, at least in part, in the recess 203 of the corresponding fixing plate 202.

[0063] Each piezoelectric actuator 50 is connected, at a lower end, to the fixing plate 202 via a first flexible element 204. Furthermore, each piezoelectric actuator 50 is connected, at an upper end, to an actuation force guide member 205 via a second flexible element 204.

[0064] The actuation force guide member 205 is here substantially shaped as a beam. Said guide member 205 is also connected to the fixing plate 202 via a third flexible element 204. The actuation force guide member 205 is also connected to the articulated structure 40. More precisely, the guide member 205 is connected to the first end of one of the legs 41.1, 41.2, 42.1, 42.2, 43.1, 43.2 via a first flexible element 204.1, a rigid support plate 206 and a second flexible element 204.2, the second flexible element 204.2 being perpendicular to the first 204.1. The first flexible element 204.1 connects the actuating force guide member 205 to the rigid support plate 206. The second flexible element 204.2 connects the rigid support plate 206 to the first end of one of the legs.Thus, the guide member 205 is arranged to amplify the movements in the direction of the leg 41.1, 41.2, 42.1, 42.2, 43.1, 43.2. The guide member 205 provides two degrees of freedom for the actuation of the leg.

[0065] The flexible elements 204 described are shaped to exhibit flexion. For example, the flexible elements 204 are elastic hinges each in the form of an elastically deformable tongue about a longitudinal axis of the tongue. It will be noted that the arrangement described comprises at least five flexible elements 204. The flexible elements 204.1 and 204.2 form a double pivot with two axes perpendicular to each other.

[0066] This arrangement of piezoelectric actuator 50, fixing plate 202, guide member 205, rigid support plate 206 and flexible elements 204 is applicable to each of the legs 41.1, 41.2, 42.1, 42.2, 43.1, 43.2.

[0067] Referring to Figures 8 and 12, the second end of the leg is connected to one of the vertices 31.1, 31.2 and 31.3 of the frame 30. More specifically, a torsion blade 207 is arranged between the second end of the leg and one of the vertices 31.1, 31.2 and 31.3. The torsion blade 207 extends substantially along the axis of the leg and has a cross-shaped cross-section allowing elastic torsion of the torsion blade 207 around a longitudinal central axis of the torsion blade 207. The torsion blade 207 is in reality connected to one of the vertices 31.1, 31.2 and 31.3 by means of a first flexible tongue 208, a circular rigid plate 209 and a second flexible tongue 210. The first flexible tongue 208 is substantially perpendicular to the axis of one of the legs 41.1, 41.2, 42.1, 42.2, 43.1, 43.2. The second flexible tab 210 is substantially perpendicular to the axis of one of the legs 41.1, 41.2, 42.1, 42.2, 43.1, 43.2 and substantially perpendicular to the first flexible tongue 208. The circular rigid plate 209 is arranged between said first tongue 208 and said second tongue 210.

[0068] The torsion blade 207, the first flexible tongue 208, the circular rigid plate 209 and the second flexible tongue 209 constitute a ball joint which gives the structure three degrees of freedom in rotation.

[0069] In other words, legs 41.1, 41.2, 42.1, 42.2, 43.1,

[0070] 43.2 are connected to the frame 30 by a ball joint comprising articulation axes perpendicular to each other.

[0071] The arrangement described is applicable to all legs 41.1, 41.2, 42.1, 42.2, 43.1, 43.2.

[0072] According to Figures 8 and 9, at least one pillar 211 is arranged on the support 11 and preferably at least three pillars 211 are arranged on the support 11. The three pillars 211 are arranged on the support 11 equidistant from each other. The three pillars 211 are fixed to the support 11 by any suitable means and for example by screwing, bolting, etc.

[0073] Furthermore, each pillar 211 is arranged between two bases 201 and, consequently, between two fixing plates 202. Each pillar 211 extends vertically and parallel to the central axis X, opposite the vertices 31.1,

[0074] 31.2 and 31.3 of the frame 30. Each pillar comprises a flat upper face 212 and a flat rear face 213. The upper face 212 is perpendicular and adjoining the rear face 213. The upper face 212 of each pillar 211 is substantially parallel to said support 11. The rear face 213 of each pillar 211 is perpendicular to said support 11. Each pillar 211 also comprises a front face 214 shaped as a triangular prism. Said front face 214 therefore comprises a first lateral slope and a second lateral slope.

[0075] A first sensor support 215 is fixedly mounted on each pillar 211. The first sensor support 215 has the shape of a bracket fixed to the rear face 213 of the pillar 211 to extend projecting and facing the upper face 212. The first sensor support 215 comprises an orifice facing the upper face 212 of the pillar. A first sensor 216 is housed, at least in part, in the orifice of the first sensor support 215. A second sensor 217 is arranged on the upper face 212 of the pillar, facing said first sensor 216. The first sensor 216 and the second sensor 217 define an air gap in which an electrically conductive blade 218 extends flat. 1 and secured to the armature 30. Said blade 218.1 matches the shape of the upper face 212 of the pillar 211. The blade 218.1 is theoretically horizontal.

[0076] A second sensor support 219 is fixedly mounted on each pillar 211. The second sensor support 219 has a lower portion fixed on the first lateral slope of the front face 214 of the pillar 211 and an upper portion extending parallel and spaced apart from the first lateral slope of the front face 214. More specifically, the second sensor support 219 extends vertically on the front face 214 of the pillar 211. The upper portion of the second sensor support 219 comprises an orifice facing said first slope. A third sensor 220 is housed, at least in part, in the orifice of the second sensor support 219. A fourth sensor 221 is arranged on the first slope of the front face 214 of the pillar, opposite said third sensor 220. The third sensor 220 and the fourth sensor 221 define an air gap in which a blade 218 extends flat.2 electrically conductive and secured to the frame 30. Said blade 218.2 matches the shape of the first slope of the front face 214 of the pillar 211. The blade 218.2 is theoretically vertical.

[0077] Optionally, a third sensor support 222 is fixedly mounted on each pillar 211. The third sensor support 222 extends over the second lateral slope of the front face 214 of the pillar 211. More precisely, the third sensor support 222 extends vertically over the front face 214 of the pillar 211. Just like the first sensor support 215 and the second sensor support 219, the third sensor support 222 comprises an orifice opposite said second slope. A fifth sensor 223 is housed, at least in part, in the orifice of the third sensor support 222. A sixth sensor 224 is arranged on the second slope of the front face 214 of the pillar, opposite said fifth sensor 223. The fifth sensor 223 and the sixth sensor 224 define an air gap in which an electrically conductive blade 218.3 extends, secured to the frame 30. Said blade 218.3 follows the shape of the second slope of the front face 214 of the pillar 211. The blade 218.3 is also a vertical blade.

[0078] Therefore, the set of blades 218.1, 218.2 and 218.3 form, in the present case, a single piece shaped into a triangular prism. This piece matches the shape of the front face 214 of the pillar 211.

[0079] Furthermore, the first 216, second 217, third 220, fourth 221, fifth 223 and sixth 224 sensors are position sensors, here inductive.

[0080] The structure formed by the first sensor support 215, the second sensor support 219 and the third sensor support 222, the blades 218.1, 218.2 and 218.3 as well as all of the sensors is identical for each pillar 211.

[0081] With reference to figures 11 to 14, a telescope is described

[0082] 300 according to a third embodiment of the invention. The telescope 300 differs from the second embodiment in that the actuation force guide member and the fixing plate have been modified.

[0083] With reference to Figure 11, at least one fixing plate 301 is arranged on each of the bases 201. Here, at least two fixing plates 301 are arranged on each of the bases 201. Preferably, the fixing plates

[0084] 301 extend projecting from the support 11. Furthermore, the fixing plates 301 are inclined with respect to the support 11 and more precisely oriented towards the secondary mirror 20.

[0085] Each fixing plate 301 comprises a first straight edge 301.1 and a second edge 301.2 shaped like a staircase. The fixing plates 301 arranged on the same base 201 are adjacent at their first edge 301.1, along an axis of symmetry.

[0086] At least one piezoelectric actuator 50 is arranged on each of the bases 201 and preferably at least two piezoelectric actuators 50 are arranged on each of the bases 201. Each piezoelectric actuator 50 extends parallel to the corresponding leg 41.1, 41.2, 42.1, 42.2, 43.1, 43.2. Therefore, the orientation of said actuator 50 is, in the present case, no longer radial.

[0087] A piezoelectric actuator 50 is provided per fixing plate 301. Each piezoelectric actuator 50 is adjacent to said second edge 301.2 of the corresponding fixing plate 301.

[0088] With reference to figure 14, each piezoelectric actuator 50 is connected by at least two connections, of the embedding connection type, to said corresponding fixing plate 301.

[0089] More precisely, each piezoelectric actuator 50 is connected at a lower end, to the fixing plate 301, by means of a lower ring

[0090] 302 secured to said fixing plate 301.

[0091] Furthermore, each piezoelectric actuator 50 is connected at an upper end to the fixing plate 301 by means of an upper ring

[0092] 303 secured to said fixing plate 301.

[0093] On the other hand, the upper ring 303 is linked to an actuating force guide member 304.

[0094] With reference to Figures 12 and 14, the actuation force guide member 304 is here substantially shaped as a deformable parallelogram whose opposite sides are systematically parallel and connected to each other by flexible elements 305, the upper ring 303 and the plate 301 forming two opposite sides of the deformable parallelogram, the other two opposite sides being formed by beams. The actuation force guide member 304 is also connected to the articulated structure 40. More precisely, the guide member 304 is connected to the first end of one of the legs 41.1, 41.2, 42.1, 42.2, 43.1,

[0095] 43.2 via flexible elements 204.1 and

[0096] 204.2 forming a double pivot as in the second embodiment described.

[0097] Thus, the guide member 304 is arranged to amplify the movements in the direction of the leg 41.1, 41.2, 42.1,

[0098] 42.2, 43.1, 43.2. The guide member 304 provides two degrees of freedom perpendicular to the axis of the leg. The connection of the guide member 304 to said leg 41.1,

[0099] 41.2, 42.1, 42.2, 43.1, 43.2 is a double cardan-style joint. It will be noted that torsional stresses are stored by the guide member 304 when an actuating force is provided by the piezoelectric actuator 50 because said actuator is connected without play and by embedding-type connections to the guide member 304 and to the fixing plate 301.

[0100] The described flexible elements 305 are shaped to exhibit flexion. For example, the flexible elements 305 are, like the flexible elements 204.1 and 204.2, elastic hinges in the form of a tongue elastically deformable about a longitudinal axis of the tongue. It will be noted that the described arrangement comprises at least six flexible elements.

[0101] This arrangement of piezoelectric actuator 50, fixing plate 301, guide member 304, rigid support plate 206 and flexible elements 305 is applicable to each of the legs 41.1, 41.2, 42.1, 42.2, 43.1, 43.2. With reference to figure 13, the second end of the leg is connected to one of the vertices 31.1, 31.2 and 31.3 of the frame 30 in a manner identical to the second embodiment described, according to a ball joint which gives the structure three degrees of freedom.

[0102] In the present case, the arrangement of the position sensors on the support 11 is identical to the second embodiment. The operation of the present invention will now be described.

[0103] The position sensors 70, 216, 217, 220, 221, 223, 224 make it possible to measure the movements (translations and orientations) of the frame 30 and therefore of the secondary mirror 20 with respect to the primary mirror 10. When a movement is detected by one of the sensors 70, 216, 217, 220, 221,

[0104] 223, 224 the signal provided by the sensors contains corresponding information transmitted to the processing unit which controls the piezoelectric actuators 50 to maintain the alignment along the X axis of the secondary mirror 20 with respect to the primary mirror 10.

[0105] With reference to Figure 6, the quantities measured by the different sensors 70, 216, 217, 220, 221, 223, 224 are as follows:

[0106] These quantities correspond to distances measured between the fixed part of the sensor 70 and the mobile part of the sensor 70, between the first sensor 216 and the second sensor 217, between the third sensor 220 and the fourth sensor 221 or between the fifth sensor 223 and the sixth sensor.

[0107] 224. This measurement is preferably carried out at the three vertices (31.1, 31.2 and 31.3) of the frame 30. Thus, each of the vertices is respectively named A, B and C to facilitate notation.

[0108] The relative position of the secondary mirror with respect to the primary mirror is thus determined by the following matrix:

[0109] For j = 1, 2 or 3, each element lll of the matrix 1 corresponds to a distance measured by the sensor 70, 216, 217, 220, 221, 223, 224.

[0110] Subsequently, we will denote the Laplace variable by s.

[0111] A first system describes the link between the forces generated by the six actuators, which we will denote Fi, and a position and orientation vector of the secondary mirror 20, which we will denote X.

[0112] The vector X is defined by six quantities including three positions (x, y and z) and three angles (0, <5, and W).

[0113] A matrix F brings together all the forces generated

[0114] Fi.

[0115] The said first system can be described by a transfer matrix which will be noted G (s) and which is obtained by finite element modeling.

[0116] The corresponding real system is denoted G* .

[0117] The vector X is then defined as follows:

[0118] A second system describes the link between said generated forces Fi and the nine lengths l measured by the sensors 70, 216, 217, 220, 221, 223, 224. This system can be described by a transfer matrix, which will be noted H (s), also obtained by finite element modeling. It will be noted that at least six measurements recorded by the sensors 70, 216, 217, 220, 221, 223, 224 are required to invert the matrix H (s). In the present case, between six and nine measurements are obtained by said sensors. The corresponding real system is noted H*.

[0119] The vector l is then defined as follows:

[0120] A gain matrix G(0) corresponds to the continuous gain of the system G(s).

[0121] Furthermore, N corresponds to the force factor of an actuator, mentioned in the manufacturer's specification sheet. In the case of Figure 8, N=M(s=0) i.e. the transfer function M(s) becomes a gain matrix N when s is zero.

[0122] With reference to Figure 15, a control law makes it possible to ensure that the alignment of the secondary mirror 20 is maintained with respect to the primary mirror 10. The control law illustrated in Figure 15 makes it possible to control the position and orientation of the secondary mirror 20 with respect to the primary mirror 10, using the piezoelectric actuators 50.

[0123] The control law includes:

[0124] - a diagonal correction matrix K (s) 100 in which each diagonal component is a corrector Ki(s) allowing the loop to be stabilized according to the degree of freedom n°i; each corrector is chosen as being a proportional corrector and the gain is chosen so as to stabilize the total loop;

[0125] - a gain matrix G (O) -1 101 resulting from the finite element modeling of the system G(s), arranged to linearize and decouple the system up to the first mechanical modes;

[0126] 1

[0127] - a gain matrix — Z6102 arranged to convert the calculated forces into controllable voltage for each of the actuators 50;

[0128] - a real actuator M* (s) 103;

[0129] - a real model of the telescope T* (s) 104 defined as follows '

[0130] - a plurality of sensors C* (s) 105 and

[0131] - an estimator f 106 arranged to find the state X from the measured lengths l.

[0132] The estimator 106 can, for example, be obtained by the following relation:

[0133] Where the measurement matrix H(0) is inverted in the least squares sense with a pseudo-inverse of H(0) denoted H(0) # :

[0134] This matrix allows to obtain more measurements from the sensors than degrees of freedom and thus reduce errors and parasitic signals.

[0135] To improve the estimator 106, this static inversion can be replaced by a frequency inversion.

[0136] The pseudo-inverse of the matrix H (s) then becomes:

[0137] Fl (s) is a diagonal matrix of dimension 6x6 and of type low-pass filter of unity gain at low frequency F(0) = 1. This filtering makes it possible to preserve the quasi-static gain at low frequency while filtering the high frequencies. F2 (s) is a diagonal matrix of dimension nxn of filters with n the number of sensors used. This filtering makes it possible to improve the signal to noise ratio (SNR) of each sensor 70, 216, 217, 220, 221, 223, 224 by selecting its useful frequency band.

[0138] Of course, the invention is not limited to the embodiments described but encompasses any variant falling within the scope of the invention as defined by the claims.

[0139] In particular, the telescope may have a structure different from that described above and for example a so-called “off-axis” telescope structure (see reflecting telescopes or reflection telescopes) in particular for applications linked to laser-based digital telecommunications.

[0140] All of the prismatic connections and all of the actuators are advantageously offset as close as possible to the support in order to reduce the mass on the pairs of legs. The prismatic connections and the actuators could nevertheless be arranged in the vicinity of the frame 30.

[0141] The contactless position sensor can be inductive, capacitive or eddy current. Their number can also be revised, six sensors for example allowing to find the six degrees of freedom describing the position and orientation of the secondary mirror 20 relative to the primary mirror 10.

[0142] The actuator mounting plates may have a different shape than that described and, for example, each be arranged to carry two actuators instead of one. Alternatively, the plates may be mounted on individual bases.

[0143] The actuation force guide member 205, 304 may have a different shape from those described or may be omitted as in the first embodiment when the actuator produces a force which is always substantially in the direction of the leg.

[0144] Optionally, the elongation measurements of the piezoelectric actuators, carried out by the strain gauges, can be added to the measurements of the position sensors.

[0145] Optionally, a Bragg grating can be arranged on the armature. In this case, at least six strain gauges are added between the vertices of the armature and the secondary crown.

[0146] The addition of strain gauges makes it possible to increase the number of measurements, for example from six to twelve measurements and thus increase the number of rows of the matrix H ( s ) .

Claims

CLAIMS 1. Telescope (1, 200, 300), comprising: - a primary mirror (10) mounted on a support (11), - a secondary mirror (20) surrounded by a secondary crown (21) fixing the secondary mirror to a frame (30), - an articulated structure (40) which is arranged between the support (11) and the frame (30) to allow six degrees of freedom of the secondary mirror (20) relative to the primary mirror (10), - a plurality of piezoelectric actuators (50) mounted to move the secondary mirror (20) by acting on the articulated structure (40), - at least one sensor (70, 216, 217, 220, 221, 223, 224) of an alignment of the primary mirror (10) and the secondary mirror (20), - an electronic processing unit electrically connected to the actuators and arranged to control the actuators (50) so as to maintain the alignment between the primary mirror (10) and the secondary mirror (20).

2. Telescope (1, 200, 300) according to claim 1, in which the frame (30) comprises three arms (30.1, 30.2 and 30.3) which are arranged substantially in a triangle tangentially to the secondary crown (21) and which each have two ends, the arms are fixed two by two by their ends forming a vertex of the triangle (31), the vertices (31.1, 31.2 and 31.3) being fixed to the articulated structure (40).

3. Telescope (1, 200, 300) according to claim 2, wherein the articulated structure (40) comprises three pairs of legs (41, 42 and 43), the legs of each pair having first ends spaced apart from each other and each connected to the support (11) via one of the piezoelectric actuators (50), and second ends close to each other and connected to one of the vertices (31.1, 31.2 and 31.3) of the frame (30).

4. Telescope (1) according to claim 3, in which the first end of each of the legs is connected to the piezoelectric actuator (50) by a first spherical connection (61) and the second end is connected to the armature (30) by a second double pivot connection (60), the two connections being arranged to allow rotation of the armature (30) relative to the support (11).

5. Telescope (1) according to any one of claims 3 and 4, in which each of the piezoelectric actuators (50) is arranged to produce a translational movement in a longitudinal direction of the leg (41.1, 41.2, 42.1, 42.2, 43.1 and 43.2) to which it is connected.

6. Telescope (200, 300) according to claim 3, in which the first end of each of the legs is connected to the piezoelectric actuator (50) by means of an actuating force guide member (205, 304).

7. Telescope (1, 200, 300) according to any one of the preceding claims, in which the piezoelectric actuators (50) are attached to the support (11).

8. Telescope (1) according to any one of claims 1 to 7, in which the sensor (70) is a contactless sensor carried by a frame (2) surmounting the support (11) and arranged to detect a position of a target secured to the secondary mirror (20).

9. Telescope (1, 200, 300) according to claim 8, in which the non-contact sensor (70) is an inductive, or capacitive or eddy current position sensor.

10. Telescope (200, 300) according to claim 8, wherein at least one pillar (211) is mounted on the support (11), said pillar (211) comprising at least two inductive sensors (216, 217, 220, 221, 223, 224) arranged to detect the position of an electrically conductive blade which forms the target and which is integral with the frame (30) so as to extend between the sensors.

11. Telescope (1) according to any one of the preceding claims, in which the electronic unit controls the actuators (50) according to a control law arranged to maintain the alignment between the primary mirror (10) and the secondary mirror (20).

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