ABSTRACT
In Chapter 9 of Volume 1, we consider a variety of ways in which to mount mirrors in the size range up to about 20 in. (51 cm). A few of those techniques are applicable, within limits, to larger elements but totally inadequate for really large mirrors. The important criteria for mounting suitability are allowable deflection due to gravity, thermal effects, and performance level required. When the aperture is modest, the thickness and material choice conducive to high stiffness, and the performance requirements low, the optic can be considered a rigid body and mounted semikinematically or with a multiple point mount, such as the Hindle mount. Larger mirrors are more flexible and so require more careful consideration. In any practical application, large mirrors are susceptible to deformations due to temperature gradients, thermal property inhomogeneities, internal stresses, and imposed forces such as acceleration and gravity. Changes in orientation of the optic during use within the Earth’s gravitational field are also of significance, as are variations in thermal loading. Release of gravitational effects must be considered in the case of mirrors fabricated, tested, and mounted on Earth and then launched into space. The largest gravitationally caused deformations of the mounted mirror occur when gravity acts along the optic axis. Back supports are used in astronomical and similar instruments that point upward or downward from the horizontal to counter these effects. That situation is the subject of Chapter 4. The more general case of mounting mirrors for variable axis orientation is discussed in Chapter 5. Design details for metallic mirrors and the unique aspects of mountings for those mirrors are discussed separately in Chapter 6.
