The model. At its simplest, the mirror is a circular disk, of diameter 2R and mean thickness t, simply supported at its periphery (Figure 6.3). When horizontal, it will deflect under its own weight m; when vertical it will not deflect significantly. This distortion (which changes the focal length and introduces aberrations) must be small enough that it does not interfere with performance; in practice, this means that the deflection of the midpoint of the mirror must be less than the wavelength of light. Additional requirements are: highdimensional stability (no creep), and low thermal expansion