DEFOR]X[ABLE MIRROR

Field of the invention [0001] The present invention is related to the field of space telescopes, large terrestrial telescopes and adaptive optics.

[0002] The present invention is particularly related to deformable mirrors used in astronomy. [0003] The present invention is also related to a method to fabricate such deformable mirrors.

State of the art

[0004] In astronomy, the larger the size of the primary mirror of a telescope, the finer the details of a scientific target that can be resolved. However, continuous mirrors with a diameter greater than 8 m constitute a real challenge to produce, especially to polish, and are impossible to transport via the present road or rail network. The solution found was to build very large primary mirrors out of independent segments. The Keck and the SALT telescopes, with diameters in the order of 10 m constitute the most successful implementations of this concept. The relative position and orientation between these segments is controlled in three rigid body degrees of freedom: piston, tip and tilt. Furthermore, their radius of curvature can be manually corrected by means of a warping harness in order to compensate for creep and manufacture surface errors. These segments need to be

polished with a paraboloid shape in order to properly focalize the incoming light into a single point, with the added difficulty that segments at different positions need to be polished to different shapes. This uniqueness of reflecting shape makes the production and the polishing of the segments an important aspect in the development and construction of future very large telescopes. An alternative approach passes by the creation of spherical instead of parabolic reflecting surfaces which would uniformize the shape to be polished in each segment, but this has the consequence of degrading the beam focalization and complicates the optical system subsequent to the primary mirror. [0005] An additional challenge faced by large telescopes is that the disturbances introduced by the atmosphere become increasingly important as the size of the telescope primary mirror increases. This fact calls for an image correction step introduced by deformable mirrors prior to image formation and constitutes one of the domains of application of adaptive optics.

[0006] Adaptive optics consists of employing deformable mirrors for the correction of light wavefronts and has demonstrated success in astronomy, ophthalmology, laser beam collimation for industrial and scientific applications and telecommunications.

[0007] The first deformable mirrors used in astronomy consisted of a continuous reflecting layer, deformed by linear actuators. This type of actuation exhibits, even today, advantages in terms of a high stroke and temporal and spatial frequency of actuation. However, this concept presents an inherent high complexity and thus an insufficient reliability, and high manufacturing and maintenance costs.

[0008] On the other hand, the benefits resulting from the simplicity of the unimorph/bimorph concept of actuation have been praised since the late 1970' s and the attempts for its introduction in adaptive optics systems have followed since then. The first successful implementation took place in 1994.

[0009] The degree of complexity of the shapes generated depends on the pattern of electrodes defined on the back of the active layer. An example of a layout according to the State of the Art as described in particular in Figure Ia. Said layout is especially suited for compensating atmospheric turbulence inside a circular aperture while the honeycomb layout provides a homogeneous pattern in which localized circular geometries can be repeated over the aperture and fractally scaled (figure Ib) .

[0010] US7019888 Bl of March 28, 2006 is describing the utilization of an insulating layer for shielding the electrodes and traces from each other and from external electrical fields. It is considered that the layer of active material is shielded by the electrodes. [0011] WO 2004/057407 of July 8, 2004, Al is describing several configurations of hybrid deformable mirrors which simultaneously employ the bimorph and linear actuation principles. It proposes indeed the utilization of linear actuators for inducing the tilt of the mirrors and one of the configurations presented has all the elements that compose the device within the foot-print of the reflecting surface. [0012] EP0793120 Al of September 3, 1997 is describing a particular design of a bimorph deformable mirror with enhanced sensitivity which is achieved by employing multiple layers of active materials. One important aspect is the utilization of an elastic sealant

layer which has the main aim of rigidifying the structure during polishing in order to facilitate this operation. [0013] US 2005/0088734 Al of April 28, 2005 is describing a method for assembling a primary mirror of limitless size constituted by segment modules. It focuses mostly on the aspects of feasibility of space transportation and automatic assembling rather than those of reducing the production efforts by taking advantage of mass production methods. [0014] US4484798 of November 27, 1984 is related to a manufacture procedure for rigid segments focusing on the deposition of reflective layers on concave substrates. [0015] US 2006/0221473 Al, of October 5, 2006 is describing a complex design of a segment support structure for controlling the negative impact that the deformations of such a support structure of a segmented mirror have on the alignment and position of these segments. These deformations occur in response to changes in gravity direction, wind loads and thermal gradients. [0016] The development of bimorph mirrors has overcome numerous challenges, including the production of active layers of brittle materials with high aspect ratios, the gluing of these layers to the optical substrates, with the subsequent distortion of the optical surface, the wiring of the independent electrodes to the voltage amplification electronics which was very time consuming, degraded the optical quality and required a significant volume on the back of the mirror. At present, the challenges posed to bimorph technology deal with the increase of spatial and temporal frequency capabilities. An increase in the number of actuators calls for the possibility of segmentation of the aperture into several mirrors. The increase of temporal frequencies will probably pass by the introduction of compliant back

structures like those proposed by BAE (document "Planet- finder class zonal bimorph deformable mirror" - presentation to the 1 ^st coordination meeting of an adaptive optics proposal (s) as part of framework program 7, Institute d ^r Astrophysique de Paris, 29 ^th and 30 ^th March 2006) or damping foams.

[0017] The existing studies for the conceptual design of the future TMT (Thirty Meter Telescope) have considered almost hexagonal segments with a straight-to- straight distance of about 1 m and a constant inter-segment gap of 2 to 4 mm. This is illustrated in the document "Development of the primary mirror segment support assemblies for the Thirty Meter Telescope" (PONSLET, E. et al, Optomechanical Technologies for Astronomy, edited by Atad-Ettedgui, Eli; Antebi, Joseph; Lemke, Dietrich, Proceedings of the Society of Photo-Optical Instrumentation Engineers (SPIE), Volume 6273, pp. 627319 (July 2006). p. So far, these constraints have imposed the design of 123 different geometries with a slightly irregular hexagonal shape which cannot be mass produced.

[0018] Document US20060050419 is related to an integrated wavefront correction module and discloses a deformable mirror built from a number of such modules. In each module, a high spatial and temporal frequency correction system is present, consisting of an array of vertical actuators which extend from a support structure to the reflective surface and can elongate or contract in a direction perpendicular to the reflective surface. A tip- tilt correction system is equally disclosed for each module which consists of linear actuators that extend from a second support structure constituting thus a second stage of actuation.

[0019] The document "The James Webb Space Telescope" by Gardner et al, Space Science Reviews, volume 123, n°4,

april 2006 (published by Springer), describes the design of the 6.6 m aperture space telescope to be launched in the early next decade. It will comprise a lightweight composite deployable structure supporting 18 hexagonal segments with 1.32 m flat-to-flat. This allows such a large aperture to fit inside the fairing of the Ariane 5 launch vehicle. The rotations, translations and the radius of curvature of each segment are actively controlled in order to compensate the surface errors stemming from fabrication, deployment, thermal gradients and ageing. These segments are constituted of beryllium, are a few dozens of millimetres thick but are micromachined as an isogrid in order to become lower weight.

Aims of the invention

[0020] The present invention aims to suggest a device and a method for fabricating such device which can be used as deformable mirror in particular in the field of astronomy which do not have the drawbacks of the state of the art.

Main characteristics of the present invention [0021] The present invention is related to a mirror as disclosed in the appended claims. The invention is thus related to a deformable mirror comprising individual units or segments, each unit comprising a continuous reflective substrate having a front and back surface, a continuous mass electrode and preferably a continuous dielectric layer on the back surface of said substrate. The unit comprises a number of horizontal, i.e. in-plane actuators, made of an electrostrictive or piezo-electric material and located between the mass electrode and individual addressing electrodes. The actuators may be embedded in said dielectric layer. The reflective substrate can for

example be a semiconductor substrate or a glass substrate. According to the preferred embodiment, each unit further comprises extra vertical linear actuators, arranged to produce tip/tilt and piston movement to the reflective substrate. The invention is equally related to a method of fabricating a mirror of the invention and to a method of producing a mirror unit according to the invention. Further preferred embodiments are described by combinations of the independent claims with one or more claims dependent thereon.

Short description of the drawings

[0022] Figure Ia is describing an example of electrode layout with increasing level of complexity according to the state of the art. Figure Ib illustrates the honeycomb electrode layout.

[0023] Figure 2 is describing the principle of unimorph/bimorph actuation. The thicknesses of the layers are exaggerated with relation to the in-plane dimension of the mirror. Moreover, the active layer can be continuous or discontinuous .

[0024] Figure 3 is describing the cross-section view of a mirror unit according to the invention. All the elements lie within the foot-print of the reflecting area. [0025] Figure 4 shows an example of a mirror consisting of 7 hexagonal mirror units, according to the invention .

[0026] Figure 5 shows a preferred embodiment of the device of the present invention wherein shape memory material being e.g. thermally expandable polymer is used in order to give the appropriate curvature to the mirror. [0027] Figure 6 shows an embodiment, wherein a metal layer is present on the front surface of the mirror unit.

[0028] Figure 7 illustrates an embodiment wherein inflatable cavity structures filled with foam are present on the back of the mirror units.

[0029] Figure 8 is describing the production steps of the screen-printing of the deformable mirror. From upper left to down right: screen-printing of common mass electrode, screen-printing of PZT actuators, PZT actuators after sintering, screen-printing of addressing electrodes. The size, shape and number of individual PZT actuators is adjusted to the particular application by proper sizing of the mask used through which the PZT paste and electrode material are screen printed. In this way, throughout all the steps of the production process all the actuators are dealt with simultaneously, and the effort and time of production is thus independent from the number of actuators. Forming thousands of actuators is as simple as less than a dozen. Screen-printing allows the production of actuators with a minimum size of 3 to 5 mm. [0030] Figure 9 is describing a fractal tessellation of large reflecting surface from a molecule of 7 hexagons arranged as a honeycomb.

[0031] Figure 10 is describing a cross-sectional view of a honeycomb sub-assembly constituted by 7 individual mirror units. [0032] Figure 11 is describing a simultaneous application of modal control and fractal zonal control for correcting errors with different wavelengths. [0033] Figure 12 shows an example of a mirror consisting of 7 hexagonal mirror units, according to the invention, interfacing a telescope flexible support structure by means of a hexapod mechanism.

Description of a preferred embodiment of the invention

[0034] According to figure 2, the principle of in- plane unimorph/bimorph actuation consists of having one or more active layers solidary with a passive substrate. According to some authors, the term ^λ bimorph' relates only to the case where two active layers are present, while ^λ unimorph' relates to the case with only one active layer attached to one passive substrate. According to other authors, ^λ bimorph' also includes the latter version with one active layer and one passive substrate. In the context of the present description, both types of actuators can be used, as well as actuators with more than two active layers. All these types will be denominated in the context of this description by the term "in-plane actuators". An electrical field applied across the active layer and parallel to the direction of polarization induces a transversal localized elongation, i.e. in the direction of the plane of the actuator. The magnitude of this transversal elongation depends on the intensity of the applied electrical field and on the piezo-electric coefficient ds _± . Displacement compatibility at the interface to the substrate results in an out-of-plane bending deformation. As stated above, in the context of this description, this type of actuator will also be referred to as an in-plane actuator, as opposed to the vertical actuators disclosed for example in US20060050419. [0035] A schematic view of deformable mirror unit 1 according to a preferred embodiment of the invention is shown in figure 3. A mirror of the invention comprises a plurality of such units, preferably mutually identical units formed in a regular array. An example of a mirror consisting of 7 hexagon shaped units 1 arranged in a honeycomb pattern is shown in figure 4. A mirror may consist of several groups of 7 units to form a large- surface mirror. The invention is characterized by a

modular mirror as shown in figure 4, i.e. composed from a plurality of preferably identical mirror units, wherein each unit comprises a plurality of in-plane actuators working according to the abovementioned unimorph/bimorph principle. The shape of the units may be hexagonal or otherwise .

[0036] Turning back to figure 3, the mirror unit 1 as such is now described in detail. Features shown in the drawings and described hereafter are not necessarily limiting to the scope of the present invention. Said scope is only defined by the appended claims. ^λ Front' and ^λ back' are used in the context of this description to refer respectively to the side of the mirror which receives incoming light, and the opposite side thereof. [0037] The mirror unit of figure 3 comprises a reflective substrate 2, which can be a semiconductor or a glass substrate. On the back surface of the substrate, a continuous mass electrode 3 is provided, e.g. a Au electrode, and in contact with said electrode, a number of electrostrictive or piezo-electric unimorph/bimorph actuators 4, formed in this embodiment of separate regions of electrostrictive or piezo-electric material. All the actuators in the array are preferably applied simultaneously which makes the deposition process to a large extent independent from the complexity of the array. Addressing electrodes 5 are arranged on the back surface of said actuators, one electrode for each actuator. Said actuators and the addressing electrodes are embedded in a dielectric layer 6. The addressing electrodes are connected by connectors 7, which pass through holes in the dielectric layer, to a flexi-circuit 8, connected to a voltage amplification circuit 9 (see figure 4), arranged within the footprint of the mirror unit. An electronic connector 10 is present to connect the unit to external

data and power sources. The circuit 9 comprises control electronic means as known in the art, for controlling the actuators and thereby the actuator-controlled deformation of the reflective substrate. Preferably, the circuit 9 comprises high voltage amplifiers. In figure 4 it can be seen that the actuators 4 may have a hexagonal shape. However, within the scope of the present invention the shape of the actuators 4 is not limited to a hexagonal shape : the actuators may be hexagonal or otherwise. Moreover, the actuator layout is not limited to a honeycomb. The actuators 4 may be arranged as a honeycomb or otherwise. Also the shape of the individual unit 1 may have any suitable shape, other than the hexagonal shape shown in the drawings . [0038] According to an embodiment, in stead of the separate regions of active material 4, a continuous electrostrictive or piezo-electric layer is provided between the mass electrode and an plurality of addressing electrodes. In this case, it can still be said that a plurality of actuators is present, defined by the position of the addressing electrodes on the surface of the continuous active layer (the 'actuator' is then the part of the continuous active layer which is contacted by an addressing electrode) . [0039] Each mirror unit is preferably equipped with a set of linear piezo-electric or electrostrictive vertical actuators 15, arranged to actuate a piston, tip and tilt movement of the mirror unit, with respect to a holder structure 16, which is in turn attached to the telescope structure 17 via precision screws 18. With Vertical' is meant that the elongation/contraction of these actuators takes place in a direction perpendicular to the substrate surface or at least not parallel to the substrate surface. Piston, tip and tilt actuations are required to properly

align and co-phase the segments in order to smooth the shapes generated by the assembly of deformable mirrors. The piston/tip/tilt actuators may consist of the active bipod kinematic mounts illustrated in figure 4. These mounts are very compliant in the in-plane radial directions mitigating out-of-plane deformations of the optical surface resulting from thermal variations. These mounts can also provide vertical actuation following the inclusion of active elements along their legs. [0040] A damping layer 19, preferably consisting of a visco-elastic foam, may be present in the space between the holder and the dielectric layer. This damping layer is applied for reasons of mitigation of the resonant response to actuation and external vibration disturbances. [0041] According to a preferred embodiment of the invention, the mirror unit may be equipped with a means to give a shape with a desired curvature to the reflective substrate, without actuating the horizontal and vertical actuators, i.e. a ^coarse' curvature is given to the substrate surface. This coarse deformation is static or has a low frequency of change, and is further finetuned by the action of the actuators 4/15. In this way, the mirror unit of the invention can be produced as a standard product, without requiring polishing steps to produce a desired curvature with high accuracy. This approach allows to produce lighter weight mirror units, which can be produced in a fast and simplified way with respect to existing mirror units. [0042] According to a first embodiment, the coarse curvature is obtained by selecting the materials of the different components (reflective substrate, dielectric layer, horizontal actuators and mass electrode) to have differential thermoelastic distortions. When changing from the temperature of deposition to the temperature of

operation, the different layers will undergo differential thermo-elastic distortions resulting in the desired curvature at the temperature of operation.

[0043] According to a second embodiment, the dielectric layer 6 may be a shape memory polymer layer, as illustrated in figure 5. Also, a shape memory polymer layer 20 may be applied in contact with the dielectric layer. The shape memory polymer induces the required curvature following a phase change induced by appropriate heating or cooling.

[0044] According to a third embodiment, a metallic coating 21 is applied on the front surface of the reflective substrate, as illustrated in figure 6. In this case, the metal layer's surface becomes the reflective surface of the mirror unit. The metal layer can be for example a gold, silver or aluminium layer. In this embodiment, the Reflecting substrate' is formed by the combination substrate/metal layer. The material of the substrate on which the metal layer is deposited may thus be other than a reflecting material. The metal layer introduces a pre-stress on the substrate, resulting in the coarse curvature deformation. Also, the metal may be chosen to obtain a curvature as consequence of differential thermo-elastic distortions (in relation to the deposition temperature) between the metal layer and other components of the unit.

[0045] According to a fourth embodiment, the coarse deformation is obtained by introducing a uniform pressure load on the mirror unit, by providing one or more inflatable cavity structures on the backside of the reflective substrate, preferably underneath the dielectric layer. By controlling the pressure inside each of said cavities, the coarse deformation of the reflective surface is controlled. By applying a pressure in the order of a

few percent of atmospheric pressure and combining with the piezoelectric correction it is possible to induce a radius of curvature in the order of 10m and with a precision matching typical optical requirements. Figure 7 illustrates this embodiment, wherein a single compartment 50 is provided on the back of the mirror. The compartment is obtained by attaching a membrane 51 to the edges of the dielectric layer. Suitable means (not shown) are provided to allow a pressure fluid to be introduced into the cavity or removed therefrom, to thereby regulate the pressure inside the cavity and thus the curvature of the mirror. [0046] According to a further preferred embodiment, compartment 50 is filled with a foam, preferably a visco- elasctic foam, which can be inflated or deflated by introduction or removal of a fluid into the pores of the foam and enables the application of pressures below the atmospheric pressure (thereby obtaining a deformation of the substrate by suction) . This provides a combined means for vibration, resonant response mitigation to actuation and curvature control.

[0047] The deformable mirror proposed according to a preferred embodiment consists of a Si wafer commercially available about 600 - 750 μm thick for wafers with a diameter of 150 mm and which might be thicker for larger diameters. The wafers are covered at the back with a PZT honeycomb actuator array, about 80 μm thick but which can be thicker or thinner (other electrostrictive materials like PMN could also be employed) . It has been produced by the Division of Ceramic Technology of the Fraunhofer Institute (IKTS) employing a technique called Screen Printing. First of all, a thin layer of gold, about 10 μm thick, is deposited on the back of the wafer in order to form a continuous mass electrode (similarly, a layer of other high conductive material could also be employed

instead) . Then, a mask with the shape of the intended actuator layout is placed over the gold electrode, and the PZT layer is deposited adhering to the mirror at the holes of the mask. The set is then placed inside an oven for sintering the piezoelectric actuators. After the sintering step, the individual addressing electrodes are finally deposited on the top of the PZT actuators. Each of these phases is depicted in figure 8. Preferably, a SiO2 layer is applied to the back of the silicon wafer, to prevent a reaction of the gold electrode with the Si. The SiO2 is thus present between the silicon wafer and the gold electrode .

[0048] The connection of the independent electrodes at the back of the mirror to the voltage amplification electronic circuit comprises two main components as illustrated in figure 3:

(a) A layer 6 of an insulating material, like epoxy or another polymer, with a low dielectric constant with little holes at the locations of the center of the addressing electrodes.

(b) A Flex-circuit 8 connecting the little holes to the electric connectors 10 at the back of the mirror and further on to the voltage amplifiers of the control electronics 9. [0049] The back of the mirror is provided with a layer 19 of viscoelastic material, or other high damping flexible material like Sorbothane to damp the flexible modes of the mirror without restricting the quasi-static flexible deformations as described in the figure 3. [0050] The mirror is supported at 3 points by linear vertical actuators 15 constituted of PZT or other electrostrictive material, with typical strokes in the order of 50 μm, and which take care of the tip-tilt and piston deformations.

[0051] The mirror, plus its PZT 4 and electrode 5 array plus the flexi-circuit 8 plus the visco-elastic layer 19 plus the PZT linear vertical actuators 15, plus the control electronics 9, plus the electric connectors 10 form a unit 1 fully contained within the foot-print of the mirror and ready to be assembled on the supporting structure .

[0052] The unit may contain 91 electrodes distributed in a honeycomb array over a hexagonal region with a width of 10cm making it suitable for applications in existing adaptive optics system , or it may contain from 91 to 2600 electrodes distributed in a honeycomb array over a hexagonal region with a width of 15 to 30cm, compatible with existing Si wafers. [0053] The silicon wafer shall undergo a precision cutting following or not the boundaries of the honeycomb pattern of actuators in order to allow minimum gaps between each unit when an assembly of several units is formed. [0054] The unit consists typically of a flat mirror (when the coarse curvature means is not present or not active) but might also be a curved mirror, obtained with or without a polishing step, by the coarse curvature described above, in combination with the actuated deformation [0055] If the thermal expansion coefficients of the dielectric layer and the passive substrate are judiciously selected it is possible to skip the polishing step. Following a correct choice of materials, even if the mirror is produced with a plane surface, the two layers will undergo a differential thermoelastic distortion into a spherical one during operation, due to the difference of temperature during these two phases.

[0056] Shape memory polymers as described in figure 5 constitute an alternative to differential thermoelastic distortion for the obtention of a spherical shape, or

another coarse high curvature shape. Preference will be given to solutions that minimize thermoelastic sensitivity at the temperatures of operation. Another possibility would be to immediately apply a shape memory polymer material on the back of the reflective semi-conductor substrate and before applying the continuous electrodes and the actuators .

[0057] A conic shape may also be obtained by active means, i.e. further finetuned with respect to the coarse curvature. In the case of a spherical shape, it can be obtained by applying a constant voltage on the electrodes. Typically, a radius of curvature of 60 m is achieved with a constant voltage near 10 V. Here, one must take care of the hysteresis of the PZT under voltage excitation. If a curvature sensor is available, the constant voltage may be controlled in closed-loop. Another alternative is to control the PZT in charge, which considerably reduces hysteresis. If the curved shape is obtained via curvature control of the flat mirror, a full telescope might be built by the assembly of flat mirrors almost uniformly distributed over a paraboloid.

[0058] Whether spherical or flat, the units may be used as building blocks for various mirrors of a telescope, including the primary mirror of a large telescope of about 30 m. Simulations have shown that it is possible to uniformly distribute units with constant shape with about δ = 10 cm diameter over the surface of a sphere and maintaining an average gap between segments δ below 1 mm. This configuration results on a parameter δ/δ in the order of 0.0134 and a Strehl ratio in the order of S = 0.95. The gap between segments, δ, accounts for different contributions stemming from the distribution of the segments over a spherical surface, the manufacturing tolerances and the thermal expansion.

[0059] Such a large reflecting surface can be tessellated by fractally repeating a simple molecule constituted by 7 hexagons arranged in a regular honeycomb as it is illustrated in figure 9. A 30 m diameter telescope would require about 81600 mirrors measuring 10 cm straight- to-straight or 12200 mirrors measuring 26 cm straight-to- straight. The number of degrees-of-freedom to be controlled amounts to the hundreds of thousands or millions . [0060] The complexity of assembling such a high number of mirrors can be reduced if these are grouped in honeycomb sets of 7 of these mirrors as illustrated in figure 10. This sub-assembling considerably facilitates the transportation of the mirrors and the final assembly of the primary mirrors. A primary mirror with a 30 m aperture would only require about 1000 such sub-assemblies, which is in the order of the number of segments in the conventional designs of the future TMT and E-ELT. [0061] Future adaptive optics systems will require the control of tens of thousands of degrees of freedom for compensating the effects of atmospheric turbulence on very large apertures. Maintaining the shape of very large segmented primary mirrors also requires the control of at least several thousands of degrees-of-freedom. Neither case, however, can be achieved by implementing classical centralized control approaches.

[0062] An alternative for controlling such large systems passes by applying hierarchical approaches that divide the active aperture in different sub-domains and performs the simultaneous but independent shape control of these domains according to the same control law. The size of each sub-domain can then be increased, but still applying the same control law, to correct the errors of lower spatial frequency following a fractal pattern.

[0063] The mechanical decoupling existing between each unit of the bimorph mirror presented herein makes the implementation of such distributed control strategies straightforward. The domains of increasing size can be defined by fractally repeating in circular molecule of 7 hexagons according to figure 11. The control of each segment can however be performed globally within the segment by applying classical modal approaches. [0064] The concept of the present invention consists of the utilization of multiple, preferably identical unimorph/bimorph deformable mirrors as elemental modules for generating larger and more complex segmented active reflecting systems. [0065] The cross-section of the proposed elemental module is depicted in figure 3. A single such module already has corrective capabilities on its own that make it suitable to equip adaptive optics systems of current astronomical observatories. [0066] A first application envisioned for this concept consists of employing several units of the type described above, for assembling a segmented deformable mirror with correction capabilities of up to tens of thousands degrees of freedom or more. This configuration will be capable of compensating in real-time the atmospheric induced distortions in future extremely large telescopes like the TMT or the European Extremely Large Telescope, E-ELT. Observations performed with such a telescope with 30 m diameter will require on a first phase the correction of about 5000 degrees-of-freedom. This capability of correction can be achieved by employing 49 individual mirrors forming two levels of fractal distribution of a single mirror as in figure 9 and would result in a deformable mirror with about 70 cm diameter. Unimorph and bimorph mirrors produced by screen-printing

are extremely light-weight and for instance, the proposed embodiment has an estimated surfacic density in the order of 6 to 10 kg/m ² . It is thus extremely advantageous when applied as a deformable mirror supported by light-weight structures like secondary deformable mirrors, M2, or the deformable mirror M4 of the E-ELT. Figure 12 illustrates a possible embodiment in which a deformable segmented mirror as proposed in this invention is utilized as a secondary deformable mirror M2 or as a deformable mirror M4 and interfaces the telescope support structure by means of a hexapod mechanism, 60. Control-structure interaction is very significantly mitigated when deformable mirrors are very light-weight. Therefore, the interaction with other control systems of the telescope like the primary mirror, optical delay lines, tip-tilt mirrors and other deformable mirrors as well as the production of vibrations that degrade the image on other components of the telescope will be enormously reduced. Moreover, screen-printing is an extremely simple process of depositing actuators. By spraying the in-plane actuators through a mask, the complexity and the time of production become independent of the number of actuators. This fact, together with the more compact, simple and robust configuration of the unimorph/bimorph in-plane actuators becomes a critical advantage when correcting many thousands of degrees-of- freedom in comparison with devices in which the optical surface is deformed by linear actuators. Finally, the fact that high-voltage amplifiers are part of the module makes the multiplication of modules to form the assembly straightforward avoiding the additional installation of components for electrical conditioning. Deformable mirrors with this degree of correction will also find application in domains other than astronomy such as systems employing lasers for scientific, industrial or military applications.

[0067] Another example of the concept proposed herein is the assembly of the primary mirrors of the future extremely large telescopes from the identical units of the type described above. Simulations have shown that it is possible to tessellate a spherical mirror with identical elements with inter-segment gaps small enough to prevent the introduction of distortions. The nominal curvature required for each component in order to generate the global primary mirror shape can be obtained by design, by adequately matching the thermoelastic coefficients of the substrate, electrostrictive and dielectric layers in order to produce a spherical shape without requiring a complex polishing process. During operation, active correction of the surface, enables both the disturbance rejection from wind and thermal gravity gradients and also the generation of a parabolic shaped surface which is different for every segment radial position. The active control of such a parabolic shape is considerably facilitated by the imposed nominal spherical shape. [0068] This approach enables the assembly of extremely large primary mirrors from exactly identical components that can be mass-produced, and skips the steps of dedicated polishing of different curved surfaces for each segment. Means for sensing the adequate co-phasing and alignment between segments need to be envisaged and will probably consist of optical sensors based on Hartmann test and or edge sensors.

[0069] Another example of application of the proposed embodiment is in space telescopes, in which weight is the most important factor in the selection of a structure. Actively controlled segmented unimorph/bimorph deformable mirrors with a surfacic density of 6kg/m2 have the potential for outperforming the current technology of passive and monolithic space telescope reflectors or

segmented telescope mirrors with thick segments and controlled in 7 degrees-of-freedom.