next previous
Up: Development of a

3. Realization of the prototype

As seen in Fig. 2 (click here), in the cat's eye system the VCM is located between the input and ouput beams of the delay-line and its diameter is then limited by these two beams. The overall diameter of the system cannot exceed 30 mm, and this fact led us to realize a small mirror with a 16 mm diameter. Starting the curvature variation with a plane mirror to achieve a maximal curvature of (84 mm)-1, this range corresponds to an f ratio varying from tex2html_wrap_inline1095 to f/2.6 and the maximal central flexion achieved is then 380tex2html_wrap_inline1029m. Due to the large bending of the mirror, the full domain of curvature is not achievable with a classical optical material as vitroceramic glass (Zerodur). Metal alloys having 100 times higher flexibility than glasses, a stainless steel substrate (AISI 420) has been chosen for this reason and for the fact that deviations to Hooke's law (stress-strain relation) are small with such materials.

The required pressure for the maximal deformation has to be easily achievable with industrial components and this technical constraint led us to choose a very thin meniscus for the active part of the mirror. As displayed in Fig. 4, with a fixed material (i.e. E and tex2html_wrap_inline1085 fixed), in order to reduce the load necessary to achieve a given deflection W0 we have to reduce the thickness tex2html_wrap_inline1107. But the flexion ratio tex2html_wrap_inline1083 increases so that it is not possible to obtain a quasi-linear relation all along the variation range. A compromise between a small load q and a linear load/curvature relation lead us to chose a meniscus with a 300tex2html_wrap_inline1029m central thickness. With these conditions (tex2html_wrap_inline1113 and tex2html_wrap_inline1115m) the maximal required load q is lower than 10 daN/cm2 when tex2html_wrap_inline1121 84 mm and the maximal stress tex2html_wrap_inline1123 reach 55 daN/ mm2 well below the elastical limit strength of the material tex2html_wrap_inline1127 daN/mm2.

Remembering the amplitude of the deformation achieved at the maximal curvature (W0 = 380tex2html_wrap_inline1029m), the reason why we developed the large deformations theory of elasticity is obvious: The flexion ratio tex2html_wrap_inline1083 is larger than 1.25, very far from the 1/3 limit of the small deformations approximation.

The realization of the VCM uses an holosteric solution where the edge of the central active meniscus is linked with an outer rigid ring via a very thin collar having a 50tex2html_wrap_inline1029m thickness (Fig. 5 (click here)), and an air pressure is applied on the back side of the meniscus. This simplifies the mounting and avoids air leak by having the mirror and its outer ring in the same sample.

  figure375
Figure 5: Holosteric concept developed for the VCM


next previous
Up: Development of a

Copyright by the European Southern Observatory (ESO)