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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Liquid mirror telescope ： ウィキペディア英語版 Liquid mirror telescope Liquid mirror telescopes are telescopes with mirrors made with a reflective liquid. The most common liquid used is mercury, but other liquids will work as well (for example, low melting alloys of gallium). The liquid and its container are rotated at a constant speed around a vertical axis, which causes the surface of the liquid to assume a paraboloidal shape, suitable for use as the primary mirror of a reflecting telescope. The rotating liquid assumes the paraboloidal shape regardless of the container's shape. To reduce the amount of liquid metal needed, and thus weight, a rotating mercury mirror uses a container that is as close to the necessary parabolic shape as possible. Liquid mirrors can be a low cost alternative to conventional large telescopes. Compared to a solid glass mirror that must be cast, ground, and polished, a rotating liquid metal mirror is much less expensive to manufacture. Isaac Newton noted that the free surface of a rotating liquid forms a circular paraboloid and can therefore be used as a telescope, but he could not actually build one because he had no way to stabilize the speed of rotation. The concept was further developed by Ernesto Capocci of the Naples Observatory (1850), but it was not until 1872 that Henry Skey of Dunedin, New Zealand constructed the first working laboratory liquid mirror telescope. Another difficulty is that a liquid metal mirror can only be used in zenith telescopes, i.e., that look straight up, so it is not suitable for investigations where the telescope must remain pointing at the same location of inertial space (a possible exception to this rule may exist for a mercury mirror space telescope, where the effect of Earth's gravity is replaced by artificial gravity, perhaps by rotating the telescope on a very long tether, or propelling it gently forward with rockets). Only a telescope located at the North Pole or South Pole would offer a relatively static view of the sky, although the freezing point of mercury and the remoteness of the location would need to be considered. A very large telescope already exists at the South Pole, but the North Pole is located in the Arctic Ocean. Currently, the mercury mirror of the Large Zenith Telescope in Canada is the largest liquid metal mirror in operation. It has a diameter of six meters, and rotates at a rate of about 8.5 revolutions per minute. == Explanation of the equilibrium == In the following discussion, $g$ represents the acceleration due to gravity, $\backslash omega$ represents the angular speed of the liquid's rotation, in radians per second, $m$ is the mass of an infinitesimal parcel of liquid material on the surface of the liquid, $r$ is the distance of the parcel from the axis of rotation, and $h$ is the height of the parcel above a zero to be defined in the calculation. The force diagram (right) represents a snapshot of the forces acting on the parcel, in a non-rotating frame of reference. The direction of each arrow shows the direction of a force, and the length of the arrow shows the force's strength. The red arrow represents the weight of the parcel, caused by gravity and directed vertically downward. The green arrow shows the buoyancy force exerted on the parcel by the bulk of the liquid. Since, in equilibrium, the liquid cannot exert a force parallel with its surface, the green arrow must be perpendicular to the surface. The short blue arrow shows the net force on the parcel. It is the vector sum of the forces of weight and buoyancy, and acts horizontally toward the axis of rotation. (It must be horizontal, since the parcel has no vertical acceleration.) It is the centripetal force that constantly accelerates the parcel toward the axis, keeping it in circular motion as the liquid rotates. The buoyancy force (green arrow) has a vertical component which must equal the weight of the parcel (red arrow), which is $mg$, and the horizontal component of the buoyancy force must equal the centripetal force (blue arrow), which is $m\; \backslash omega^2\; r$. Therefore, the green arrow is tilted from the vertical by an angle whose tangent is the quotient of these forces. Since the green arrow is perpendicular to the surface of the liquid, the slope of the surface must be the same quotient of the forces: :$\backslash frac\; =\; \backslash frac$ Cancelling the $m$'s, integrating, and setting $h=0$ when $r=0$ leads to :$h\; =\; \backslash frac\; \backslash omega^2\; r^2$ This is of the form $h=kr^2$, where $k$ is a constant, showing that the surface is, by definition, a paraboloid. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Liquid mirror telescope」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.