Abstract:
An active mirror has a reflectively coated diaphragm, a mount, and at least one actuator, in which a second diaphragm is connected to the mount, and the actuator is operatively connected to the two diaphragms. The active mirror is symmetrical with respect to a midplane.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     Not applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an active mirror with a reflectively coated diaphragm, a mount, and at least one actuator. 
     2. Discussion of Relevant Art 
     Such mirrors are known, for example, from U.S. Pat. No. 5,204,784 and German Patent DE 35 02 024 C2. The known actuators specify that the actuator(s) is/are supported against rigid structures. A diaphragm is understood to be a thin, flat structure, the flexural strength of which is small as against its tensile strength, such as are provided as a deformable mirror element in the above-mentioned documents. 
     Such active mirrors find application, for example, as laser mirrors and as a portion of microlithography projection devices, and also, for example, in astronomical telescopes. 
     SUMMARY OF THE INVENTION 
     The invention has as its object an active mirror for the highest precision requirements, in particular as regards the constancy of the vertex position of the mirror, and thus minimizing decentering, defocusing, and tilting. 
     The solution is attained according to the invention by an active mirror of the given category having a reflectively coated diaphragm, a mount, a second diaphragm connected to the mount, and at least one actuator that is operatively connected to the reflectively coated diaphragm and the second diaphragm. By fastening the actuators to a second diaphragm as a support, the deformation of the reflectively coated diaphragm proceeds from an established fixed reference base. 
     Particularly for arrangements with several actuators, advantageous features of the invention reduce the introduction of moments or lateral thrust, which are produced by the extension or contraction of the diaphragm with a fixed support. The introduction of moments or lateral thrust are particularly minimized by the symmetry of the two diaphragms. 
     Ball joints that support the actuators on the two diaphragms are also an advantageous feature of the invention for undisturbed deformation, since the introduction of bending moments is prevented. Another advantageous feature of the invention is that the mount includes a pillar that is situated on the optical axis A of the active mirror and connects the reflectively coated diaphragm to the second diaphragm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is further described in detail with reference to the drawings, in which: 
     FIG. 1 a  shows, schematically in longitudinal section, a mirror according to the invention; 
     FIG. 1 b  shows a cross section of FIG. 1 a;    
     FIG. 2 a  shows a second embodiment, schematically in longitudinal cross section; 
     FIG. 2 b  shows a cross section of FIG. 2 a.   
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The active mirror of FIGS. 1 a  and  1   b  consists of a reflectively coated diaphragm  1  and a mount, which consists here of a central column  2  with radially arranged, outward-directed holding webs  22  and a pot-shaped protective housing  20  with a connecting flange  21 . A second diaphragm  4  is arranged opposite the reflectively coated diaphragm  1 . Actuators  31 - 36  that are respectively supported by means of ball joints  311 ,  312 , are arranged between the two diaphragms  1 ,  4 . The whole assembly has mirror symmetry with respect to the support plane M. 
     Both diaphragms  1  and  4  are like parts, usually of glass ceramic (Zerodur ®, made by Schott); the second diaphragm  4  is only lapped on the surface, while the reflectively coated diaphragm  1  is optically fine-polished and has suitable thin layers applied to it. The central column  2  of the mount is preferably also made of glass ceramic, since otherwise the effect of thermal expansion would lead to a disturbing deformation of the reflecting diaphragm  1 . 
     Piezoelectric translators are provided as the actuators  31 - 36 , and are proven elements with high resolution of the displacement path. They are all arranged perpendicularly to the support plane M. 
     If an actuator, e.g.  31 - 36 , extends, the reflectively coated diaphragm  1  is thus curved, and the distance of the pressure point (ball joint  311 ) from the optical axis A has to shorten. If, as was heretofore usual, the other end were mounted with a ball joint  312  on a rigid base, a tilting of the actuator would thereby result, and thus a lateral radial force component on the reflectively coated diaphragm  1  would arise. 
     Since however the actuators  31 - 36  are supported on a likewise elastic diaphragm  4 , which likewise curves, this effect is reduced, and in the illustrated case where the two diaphragms  1  and  4  have the same mechanical properties, the tilting is completely prevented, and only a parallel displacement takes place. By this arrangement, the displacement path of the actuators  31 - 36  indeed acts only half as a deformation of the reflecting diaphragm  1 , for which, however, the resolution is doubled and the disturbing effect of thermal expansion of the actuators  31 - 36  is halved. 
     Several advantages are likewise attained by the construction of the mount with the central column  2  as a counter-support to the actuators  31 - 36 : the position of the middle of the mirror (mirror vertex with curved mirrors) is independent of the deformation by the actuators  31 - 36 . 
     The central column  2  that defines the position of the reflecting diaphragm  1  is compact and hence not susceptible to temperature gradients. A mounting ring (see FIGS. 2 a/b ) would, in contrast, be more easily susceptible to length differences between opposing places, due to temperature differences, and thus to tilting of the reflecting diaphragm. 
     The dimensions of the reflecting diaphragm  1  are, for example, 5-20 cm. diameter with a thickness of 3-10 mm. The provided displacement paths of the actuators  31 - 36  are of the order of magnitude between 2 and 10 2  nanometers. 
     FIGS. 2 a  and  2   b  show another embodiment of the invention. The reflectively coated diaphragm  100  is embodied in this embodiment as a concave mirror, and the second diaphragm  400  is correspondingly shaped. Both are mounted at the edge to a ring  200 . Here also, the support plane M is the plane of mirror symmetry. 
     A centrally symmetrical deformation of the reflecting diaphragm  100 , the vertex of which is however then displaced axially, can already be attained in this embodiment with a single actuator situated on the optical axis A. Further actuators  310 ,  320 ,  330  are arranged, symmetrically distributed over the surface of the reflecting diaphragm  100 , parallel to the optical axis A, and perpendicular to the support plane M. 
     It can be seen that the principle of soft support of the actuators, according to the invention, can be used for maintaining their axial direction independently of their length, for diverse mirror shapes such as are necessary in optics. The associated mounting technique is not limited to the forms, which have been described, and any known translators may also be used as actuators. 
     The arrangement according to the invention finds a preferred use particularly for high precision requirements at small deformations, of the order of 10 2  nm and therebelow.