Rotating furnace
Encyclopedia
A rotating furnace is a device used to create lenses
or primary mirror
s for optical telescope
s. It was known already to Newton that the centrifugal-force-induced shape of a spinning liquid is a paraboloid, the same as the shape of a telescope's primary focusing mirror. So glass
is placed in the furnace, the furnace is spun up and heated, the top surface of the melted glass takes on a concave shape, and the glass is cooled to solidification while still spinning. If the shape does not exactly represent a telescope mirror, it is then corrected by computer-controlled grinding machines. The net amount of grinding done, and the net mass of glass material used, is less than would have been required without spinning. Once solidified, the concave top surface can be silvered
and used as a focusing mirror. (If centrifugal force & gravity are the main shaping forces, this would tend to create a concave top-surface of the lens, and the bottom surface would be shaped by the liquid's container much like a mold; only the top surface would be shaped by non-contact forces.)
The paraboloid
of revolution is the natural concave shape of a spun-up liquid.
Reflective telescopes use primary mirrors which are parabolids,
to focus collimated light from the far field onto a nearby point.
Large-diameter focusing mirrors are also made this way with the liquid elemental metal mercury
, such as the Large Zenith Telescope
. The rotation is continuous and the metal remains liquid; once all waves and imbalances die out, the telescope is then used.
The technology is used for the Giant Magellan Telescope
, with the first of its seven planned mirrors coming out casting in late 2005.
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
or primary mirror
Primary mirror
A primary mirror is the principal light-gathering surface of a reflecting telescope.-Description:The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective metal , or in later telescopes, glass or other material coated with a reflective layer...
s for optical telescope
Optical telescope
An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....
s. It was known already to Newton that the centrifugal-force-induced shape of a spinning liquid is a paraboloid, the same as the shape of a telescope's primary focusing mirror. So glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
is placed in the furnace, the furnace is spun up and heated, the top surface of the melted glass takes on a concave shape, and the glass is cooled to solidification while still spinning. If the shape does not exactly represent a telescope mirror, it is then corrected by computer-controlled grinding machines. The net amount of grinding done, and the net mass of glass material used, is less than would have been required without spinning. Once solidified, the concave top surface can be silvered
Silvering
Silvering is the chemical process of coating glass with a reflective substance. When glass mirrors first gained widespread usage in Europe during the 16th century, most were made of an amalgam of tin and mercury, but by the 19th century mirrors were commonly made through a process by which silver...
and used as a focusing mirror. (If centrifugal force & gravity are the main shaping forces, this would tend to create a concave top-surface of the lens, and the bottom surface would be shaped by the liquid's container much like a mold; only the top surface would be shaped by non-contact forces.)
The paraboloid
Paraboloid
In mathematics, a paraboloid is a quadric surface of special kind. There are two kinds of paraboloids: elliptic and hyperbolic. The elliptic paraboloid is shaped like an oval cup and can have a maximum or minimum point....
of revolution is the natural concave shape of a spun-up liquid.
Reflective telescopes use primary mirrors which are parabolids,
to focus collimated light from the far field onto a nearby point.
Large-diameter focusing mirrors are also made this way with the liquid elemental metal mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
, such as the Large Zenith Telescope
Large Zenith Telescope
The Large Zenith Telescope is a 6.0 m diameter liquid mirror telescope located in the University of British Columbia's Malcolm Knapp Research Forest, about east of Vancouver...
. The rotation is continuous and the metal remains liquid; once all waves and imbalances die out, the telescope is then used.
The technology is used for the Giant Magellan Telescope
Giant Magellan Telescope
The Giant Magellan Telescope is a ground-based extremely large telescope planned for completion in 2018. It will consist of seven diameter primary segments, with the resolving power of a primary mirror and collecting area equivalent to one...
, with the first of its seven planned mirrors coming out casting in late 2005.
See also
- Liquid mirror telescope
- List of largest optical reflecting telescopes