Abstract:
The invention relates to an optical system comprising at least one first and second optical element, whereby the optical elements are arranged at a predetermined distance from one another, using a mounting. The mounting comprises compensation elements for modifying the predetermined distance between a first optical element and a second optical element, according to the temperature. The optical system is a telescope and the distance between the primary mirror and the secondary mirror is modified according to the temperature.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
         [0001]    Not applicable.  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    The invention relates to an optical system with at least a first and second optical element, the optical elements being arranged at a predetermined distance from each other by means of a mounting.  
         TECHNICAL FIELD  
         [0004]    A mirror telescope with a primary mirror and a secondary mirror, which are arranged spaced from each other by means of a mounting, is known from German Patent Document DE 39 40 924 A1, for example. The mounting includes a telescope tube of Zerodur®. Likewise, a securing star of Zerodur® is provided for the mounting of the secondary mirror, and is connected to the telescope tube.  
           [0005]    The material Zerodur® is selected because of its low thermal expansion coefficient. An athermal behavior, particularly in the temperature range from 20° C. to −50° C., is desirable in telescopes for optical telecommunication which are used in space, since in such uses readjustment during use is practically impossible.  
           [0006]    In particular, deformations of the mirror are disadvantageous, since a displacement of the focal point is associated with them. Also, such a displacement of the focal point results in a defocusing. A nearly athermal behavior is obtained by the use of Zerodur; however, it is disadvantageous that this ceramic material is very brittle and only be handled or loaded to a small extent.  
           [0007]    Furthermore, invar is used as a material in telescopes. However, this material has a considerable thermal expansion coefficient, so that the telescope has a temperature-dependent behavior.  
           [0008]    The production of a mirror blank by a casting technique is known from German Patent Document DE 43 26 762 A. Silicon carbide is intended as the material.  
           [0009]    It is known from U.S. Pat. No. 5,579,333 to use ceramics of silicon nitride (Si 3 N 4 ) for the production of industrial mirrors.  
           [0010]    Undesirable thermal effects also occur in objectives for semiconductor lithography. The optical properties of the respective components, such as mirrors and lenses, change due to heating; in particular, the focal length changes.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention has as its object to provide an optical system which has at least two optical components and which has a nearly athermal behavior, at reduced costs.  
           [0012]    A further object of the invention is to provide an optical system, particularly a telescope, which has increased mechanical loading capacity with the smallest possible weight.  
           [0013]    By the measure that a mounting includes compensation elements for a temperature-dependent change of a predetermined distance between a first and a second optical element, it is possible to compensate for a change of the position of the focal points of the optical elements due to thermal deformation by means of the mounting, in particular by means of the compensation elements. It is possible by means of the compensation elements to adapt the position of the second optical element to the new focal length of the first optical element, and vice versa. The optical system is thereby always optimally focused, independent of temperature.  
           [0014]    It has been found to be advantageous to arrange the compensation elements parallel to an optical axis defined by the optical elements. The greatest possible length change of the position of the focal points of the optical elements in relation to the length extension of the compensation elements in the optical axis direction, per temperature interval, can thereby be attained.  
           [0015]    The material used for the compensation elements is to be selected in dependence on the length of the compensation elements in the axial direction and in dependence on the focal point displacement per temperature interval, so that the length change of the compensation elements compensates for the displacement of the focal point.  
           [0016]    In particular, it has been found to be advantageous to designate a material for the compensation elements which has a greater thermal expansion coefficient than the material of the mounting. It is thereby possible to attain a large length change in dependence on the temperature change.  
           [0017]    It has been found to be advantageous to arrange the compensation elements in the region of the first optical element, in particular in the region of a primary mirror of a telescope, so that there is no, or nearly no, temperature difference between the first optical element, particularly the mirror member, and the compensation elements. Thereby the compensation elements undergo approximately the same temperature change as the first optical element.  
           [0018]    It has been found to be advantageous to use for the mounting a material with a sufficient thermal conductance and very small expansion coefficients, so that when the mounting or the telescope tube is exposed to unilateral or unequal irradiation, a more rapid temperature equalization takes place and the deformations due to a temperature gradient remain small. In this manner, stresses in the mounting itself, and warping resulting from temperature gradients, due to a local expansion of the mounting and the components fixedly connected to the mounting, are avoided.  
           [0019]    In particular, with a seating constituted in the shape of a star for a secondary mirror in a telescope, the result of a temperature gradient in the region of the seating of the secondary mirror is a bending of the seating, giving rise to defocusing.  
           [0020]    In objectives or objective systems in semiconductor lithography, large troublesome effects arise from the smallest departure from adjustment, since extremely small structures are imaged. A system-specific adjustment of the compensation elements by the use of a material with a very small expansion coefficient for the mounting of the optical elements is facilitated, or even made possible for the first time, since then primarily only the influence of the material of the optical elements themselves has to be considered.  
           [0021]    The production costs can be minimized by the measure, in optical systems with at least one mirror, of producing the mirror members from SiN; this is of particular interest for production in large numbers of items.  
           [0022]    In particular, a replication process can be used for mirror manufacture when SiN is used, and aspheric mirrors can also thereby be produced at a favorable cost, which is of particular interest as regards use in lithographic objectives. In mirror manufacture by the replication process, very hard materials can be used which also can be brittle and unsuitable for polishing. Ceramic materials above all are possible here; besides having low weight, they also have low expansion coefficients.  
           [0023]    It has been found to be advantageous to designate for the mounting of the material C/C SiC, the one with similar physical properties. C/C SiC is a carbon-fiber strengthened combined material that comprises silicon carbide. In particular, if the mounting includes a telescope tube, it has been found to be advantageous to make the telescope tube of C/C SiC.  
           [0024]    Further advantageous measures are described in further dependent claims. As an embodiment example, a telescope and a schematically shown optical system are described. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 shows a telescope with a primary mirror produced by polishing technique and a mirror carrier of SiN;  
         [0026]    [0026]FIG. 2 shows a telescope with a mirror member of SiN and a primary mirror produced by replication technique, and  
         [0027]    [0027]FIG. 3 shows an optical system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    The principal structure of a telescope  1  is first described with reference to FIG. 1.  
         [0029]    The telescope  101  shown has a primary mirror  103  and a secondary mirror  127 , the mirror faces  107 ,  128  of which are arranged facing each other. An optical axis  102  is defined by these two mirrors  103 ,  127 . These two mirrors are connected together by means of a mounting  115  [and compensation element  119 ], and are arranged at a predetermined distance  129  from each other.  
         [0030]    In the embodiment example shown, the mounting  115  includes a telescope tube  117  arranged coaxially of the optical axis  102 , and a seating  122  in the form of a holding star  123  for mounting the secondary mirror  127 . The holding star  123  and the telescope tube  117  preferably consist of the identical material, to avoid stresses due to differing expansion coefficients of the materials. In the embodiment example shown, C/C SiC is provided as the material, and has a sufficient thermal conductance and very small expansion coefficients, so that in the mounting  115 , temperature gradients and deformations can occur only briefly, if at all, due to a unilateral irradiation. A large quotient formed by dividing the thermal conductivity by the expansion coefficient is to be sought.  
         [0031]    A mirror seating  125  for the secondary mirror  127  is connected to the holding star  123 . Compensation elements  119  in the form of three feet  121 , arranged at an angular spacing of 120°, are provided on the end of the telescope tube  117  remote from the secondary mirror  127 . These feet  121  engage at one end around the end of the telescope tube  117  and at the other end are connected to a mirror mounting  111  of the primary mirror  103 . A ring could also be provided as a compensation element, of a material which has a thermal expansion coefficient other than that of the mounting. It is crucial that the compensation element(s) has/have an extension in the direction of the optical axis  102 .  
         [0032]    The mirror mounting  111  is mounted on a mirror carrier  112 , which in turn is isostatically received by the mounting elements  109 . The mirror mounting  111  and also the primary mirror  103  are coaxial to a tube  113  arranged on the optical axis  102  and in its turn including a collimator.  
         [0033]    In the embodiment shown, the primary mirror  103  includes a mirror member  105  of quartz glass, provided with a mirror surface by polishing technique. The mirror mounting  111  is of invar, and the mirror carrier  112  is of SiN. C/C SiC is provided for the mounting  115 .  
         [0034]    In this telescope  101 , the radiation striking the primary mirror is deflected to the secondary mirror, this radiation thus being focused over the tube  113  by reflection at the secondary mirror  127 .  
         [0035]    On a heating of this telescope  101 , particularly of the primary mirror  103 , the focal length of the primary mirror  103  is displaced to greater distances. The distance  129  predetermined by the mounting  115  is increased by the compensation elements  119 , which are likewise arranged in the region of the primary mirror  103 , so that no displacement of the focal point takes place.  
         [0036]    The embodiment example shown in FIG. 2 differs principally in the primary mirror  103 . In this embodiment example, the primary mirror  103  was made by replication technique with a mirror member  105  of SiN.  
         [0037]    In particular, aspheric mirror surfaces  108  can be produced at a favorable cost in replication technique. Very hard, and in some circumstances brittle, materials can also be used, which must not be polished. Such stiff materials generally have low thermal expansion coefficients. Because of the stiff material for the mirror member  105 , no separate mirror mount  111  and no mirror carrier  112  are required, as in the embodiment example according to FIG. 1. From the stresses arising in the mirror member  105  in the replication technique, only very small deformations result due to the shrinkage of the replication resin.  
         [0038]    The mirror member  105  is connected to mounting elements  109  by which it is received isostatically. The mirror member  105  is provided on its outer radius with projections  110  on which compensation elements  119 , which are again constituted as feet, are supported by their ends. A ring of a material which has a thermal expansion coefficient other than that of the mounting  115  could also be provided as the compensation elements  119 . In this embodiment example, the mounting  115  and the holding star  123  are of C/C SiC. It is crucial that the compensation element(s)  119  has/have an extension in the direction of the optical axis  102 . The material for the compensation elements  119  is to be selected in dependence on the mirror member  5  used, where the material for the compensation elements is to be selected in dependence on their extension in the axial direction at a reference temperature, and in dependence on the focal point displacement to be expected per temperature change. The length change of the mounting  115  in the axial direction in dependence on temperature is also to be considered, so that this length change plus the length change of the compensation elements  119  gives the displacement of the focal point.  
         [0039]    An optical system is shown in FIG. 3. This optical system includes a first optical element  3 , here a mirror, which is mounted by a mirror mount  11 , and a second optical element  27 , here a lens, which is mounted by a mount  22 . The mount  22  is in its turn fixedly supported. This lens could however also be movably supported. It is crucial that the optical system  1  formed by the optical elements  27  and  5  is almost athermalized. The mount  9 ″ is connected to the mount  22  via compensation elements  19  and a mounting  15 . The changes in the optical properties, particularly the change of the focal length, are compensated by means of the compensation elements  19 , as already described for the telescope.  
                                                 List of Reference Numerals                                 1   optical system   115   mounting        2   optical axis   117   telescope tube        3   first optical element   119   compensation element        5   mirror member   121   feet        9   mounting element   122   seating        11   mirror mount   123   holding star        15   mounting   125   mirror seating        19   compensation element   127   secondary mirror        27   second optical element   128   mirror surface        29   predetermined distance   129   predetermined distance       101   telescope       102   optical axis       103   primary mirror       105   mirror member       107   mirror (surface)       108   aspheric mirror       109   mounting element       110   projections       111   mirror mount (polishing technique)       112   mirror carrier       113   tube with collimator