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: high dimensional stability (no creep), and low thermal expansion (Table 6.3).
The mass of the mirror (the property we wish to minimize) is
Where is the density of the material of the disk. The elastic deflection, , of the center of a horizontal disk due to its own weight is given.
The quantity g in this equation is the acceleration due to gravity: 9.81 m/s2; E, as before, is Young’s modulus. We require that this deflection be less than (say) 10 µm. The diameter 2R of the disk is specified by the telescope design, but the thickness t is a free variable. Solving for t and substituting this into the first equation gives
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
|