Only residual local stresses in the glass arises around the bond spots. The leaf springs decouple thermal expansion of the component in the lateral direction. Usually, the thermal center is located on the optical axis. Because of the flexures, the thermal expansion difference between component and mount can be large without significant effects. If a temperature difference occurs between component and mount, the center of expansion is at Tc. The arrangement of the leaf springs determines the location of the thermal center. This is beneficial for the stability of the component under inertial loads (such as gravity). The natural frequencies of the mount are high because each degree of freedom of the optical component is constrained with high stiffness. Isostatic arrangement of three leaf springs around a component, showing the location of the thermal center. Wave front errors introduced by these parasitical forces must be checked by hand calculations or FEM analysis. ![]() In practice true kinematic constrains do not exist and bending moments due to parasitic stiffnesses can have a second order influence. Because of the isostatic design the mount cannot impose significant forces or bending moments on the optics and deform the optical component. By arranging three leaf springs around the optical component as shown in figure 3, the optical component is constrained once in each degree of freedom. Because a bond spot has high shear stiffness, but a low torsional stiffness the combination of bond spot with a leaf spring only constrains X and Y with high stiffness. A leaf spring constrains X, Y and RZ with high stiffness (in the local coordinate system in figure 2). An isostatic design means that each degree of freedom is constrained only once. Alternatively it can be manufactured using wire EDM.Ī rigid optical component has 6 degrees of freedom (DOF). The leaf spring can be manufactured with conventional inexpensive milling. When properly dimensioned, the tensile and lateral stiffness are much higher than the bending stiffness (as a rule of thumb a ratio of 1:1000 is achievable). Examples are given which are supported by analysis and test results. In this paper, a simple and predictable design approach is demonstrated which shows excellent stability and low WFE while it is still capable of surviving severe loads. Limited induced stress in the optical components to avoid stress birefringe (only critical for polarization sensitive instruments).Īs demonstrated in the stability and wave front error of mounted optics strongly depend on the mount design.For stability under changing temperature and/or changing gravity conditions a well-placed thermal center and high natural frequency are important. Important also are residual effects due to hysteresis or interface slip after vibration loads or thermal loads. This includes the temporary or permanent change in position of the optical component after initial alignment due to changing gravity direction and temperature. Stability of the optical component relative to the mount interface.Bending moments and forces on the component must be minimized to avoid deformation of the optical surfaces. Allowable wave front error (WFE) of the optics, typically in the order of tens of nanometers. ![]() Requirements for optical mount design can thus be divided between strength and performance requirements. This limits the forces and moments which can be applied by the mount on the optical component. On the other hand the mount must not damage or distort the optical components. The adhesives used to bond the optical component to the mount has nonlinear material behavior combined and a low strength level compared to metals. ![]() This results in failure stresses that are much lower than those for metals. Fracture of the glass occurs due to uncontrolled crack growth of these flaws under tensile stresses. The strength of glass depends on the random distribution of surface flaws in relation to regions under stress. This is complicated because generally optical components are made from glass. On one hand the mount with bonded optics must be robust and strong to survive the launch loads and space environment. Optical mount design for harsh environments is demanding because of conflicting requirements.
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