Liquid mirror telescope
A liquid mirror is a concave mirror, whose shape is formed by the rotational deformation of a reflective liquid. The liquid mercury is most commonly used, but other fluids are possible ( for example, low-melting alloys of gallium ). Liquid mirrors can be an inexpensive alternative to conventional large telescopes.
Isaac Newton noticed that the free surface of a rotating liquid forms a paraboloid of revolution and therefore can be used to build a reflecting telescope. However, he was not able to actually build such because he did not have the technical capabilities to stabilize the rotation speed. The concept was further developed by Ernesto Capocci of the Astronomical Observatory of Capodimonte in Naples ( 1850), but only constructed in 1872 Henry Skey in Dunedin (New Zealand) the first working liquid levels in the laboratory scale.
Liquid mirror telescopes can only observe the sky at the zenith and are therefore unsuitable for observations where the telescope must be directed to the same celestial object over time.
Conventional liquid mirror telescopes (Earth)
These consist of a liquid in a container is in the approximate shape of a paraboloid of revolution, consisting of composite materials, such as glass fiber reinforced plastic. The container is set into a rotary motion about its vertically oriented major axis, until it reaches a few revolutions per minute. The liquid surface formed thereby gradually a paraboloid of revolution. The surface of the mirror is very precise and not affected by small irregularities in the shape of the container. The required amounts of mercury are very small, because the liquid layer is less than one millimeter thick, because the container has approximately the shape of the liquid surface.
Benefits
The biggest advantage of liquid mirror telescopes are the low cost of the mirror, which account for about 1 % of the cost of a conventional mirror. This reduces the cost of the whole telescope by more than 95 %. The Large Zenith Telescope of the University of British Columbia costs, with a diameter of 6 meters only a tenth of the price of a conventional telescope with a mirror of glass.
Disadvantages
The mirror can be directed vertically upwards. If it is tilted, it loses its shape. The field of view of the telescope that is constantly changing and sweeps over the course of a day a narrow strip of sky at constant declination, corresponding to the latitude at which the telescope is set up. It is not possible to monitor specific objects over time, with a limited extension of observability can be achieved by deflecting or electronically. The latter is realized by applying a voltage to the CCD sensor, which causes the electrons to move it at the same speed as the image, whereby a sharp image. There are also branches of research in astronomy, which are not dependent on this, for example, long-term monitoring programs of the starry sky and the search for supernovae and other transient phenomena. Since we assume that the universe is isotropic and homogeneous ( cosmological principle ), full sky observations are not absolutely necessary to study its structure, so that liquid can mirror telescopes are also used here.
Liquid Mirror Telescopes on the Moon
Ionic liquids with low melting point ( below 130 Kelvin) have been proposed as a possible basis for liquid mirror telescopes with extremely large diameter to be installed on the moon. The low temperatures bring benefits if you want to represent very long-wavelength infrared light, which is caused by extreme redshift and dates from the remotest parts of the visible universe. Such a liquid base would be covered with a thin metallic film that forms the reflective surface.