Design
The furnace includes a mechanism that rotates an open-topped container at constant speed around a vertical axis. A quantity of glass sufficient to make the mirror is placed in the container, heated until it is completely molten, and then allowed to cool while continuing to rotate until it has completely solidified. When the rotation is stopped, the glass is solid, so the paraboloidal shape of its top surface is preserved. This process is called spin casting.
The same process can be used to make a lens with a concave paraboloidal surface. The other surface is shaped by the container that holds the molten glass acting as a mould. Lenses made this way are sometimes used as objectives in refracting telescopes.
The word spin is frequently used in this context without implying that the rotation is rapid. Making a mirror with a focal length of five metres, for example, requires a rotation speed less than ten revolutions per minute. Since the speed is low, accurate dynamic balancing of the rotating components is not needed.
The axis of rotation becomes the axis of the paraboloid. It is not necessary for this axis to be in the centre of the container of glass, or even for it to pass through the container. By placing the container away from the axis, off-axis paraboloidal segments can be cast. This is done in the making of very large telescopes which have mirrors consisting of several segments.
Read more about this topic: Rotating Furnace
Famous quotes containing the word design:
“What but design of darkness to appall?—
If design govern in a thing so small.”
—Robert Frost (1874–1963)
“We find that Good and Evil happen alike to all Men on this Side of the Grave; and as the principle Design of Tragedy is to raise Commiseration and Terror in the Minds of the Audience, we shall defeat this great End, if we always make Virtue and Innocence happy and successful.”
—Joseph Addison (1672–1719)
“For I choose that my remembrances of him should be pleasing, affecting, religious. I will love him as a glorified friend, after the free way of friendship, and not pay him a stiff sign of respect, as men do to those whom they fear. A passage read from his discourses, a moving provocation to works like his, any act or meeting which tends to awaken a pure thought, a flow of love, an original design of virtue, I call a worthy, a true commemoration.”
—Ralph Waldo Emerson (1803–1882)