Liquid mirror

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Large Zenith Telescope (Canada) with 6 m mercury-based mirror

A liquid mirror is a concave mirror whose shape is formed by the rotational deformation of a reflecting liquid. The most commonly used liquid is mercury , but other liquids are also possible (e.g. 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 can therefore be used to build a mirror telescope . However, he was not able to actually build one because he did not have the technical means to stabilize the rotation speed. The concept was further developed by Ernesto Capocci of the Capodimonte Observatory in Naples (1850), but it was not until 1872 that Henry Skey in Dunedin ( New Zealand ) constructed the first working liquid mirror on a laboratory scale. In 1908 Robert Wood of Johns Hopkins University developed a 50 cm mirror made of liquid mercury.

Liquid mirror telescopes can only observe the sky at the zenith and are therefore unsuitable for observations in which the telescope has to be pointed at the same celestial object for a long time.

Conventional liquid mirror telescopes (earth)

These consist of a liquid in a container in the approximate shape of a paraboloid of revolution , made of composite materials , such as. B. Glass fiber reinforced plastic . The container is set in a rotary motion around its perpendicular main axis until it reaches a few revolutions per minute. The liquid surface gradually forms a paraboloid of revolution. The surface of the mirror is very precise and unaffected by small irregularities in the shape of the container. The quantities of mercury required are very small because the liquid layer is less than a millimeter thick because the container is roughly the shape of the liquid surface. The resulting focal length depends only on the acceleration due to gravity and the speed. At a speed of about 21 revolutions per minute, a focal length of one meter is set.

advantages

The biggest advantage of liquid mirror telescopes is the low cost of the mirror, which is about 1% of the cost of a conventional mirror. This reduces the cost of the entire telescope by more than 95%. The Large Zenith Telescope of the University of British Columbia costs with its diameter of 6 meters, only one-tenth of the price of a conventional telescope with a mirror of glass.

disadvantage

The mirror can only be directed vertically upwards. If it is inclined, it loses its shape. The field of view of the telescope is constantly changing and over the course of a day it sweeps over a narrow strip of sky with constant declination , corresponding to the latitude at which the telescope is set up. It is not possible to track certain objects over a longer period of time, although a limited extension of the observability can be achieved by means of deflecting mirrors or electronically. The latter is done by applying a voltage to the CCD sensor that causes the electrons on it to move at the same speed as the image, creating a sharp image. But there are also areas of research in astronomy that do not depend on this, for example long-term programs to monitor the starry sky and to search for supernovae and other temporary phenomena. Since it is assumed that the universe is isotropic and homogeneous ( cosmological principle ), full-sky observations are not absolutely necessary in order to study its structure, so that liquid mirror telescopes can also be used here.

Liquid mirror telescopes on the moon

Ionic liquids with a low melting point (below 130 Kelvin ) have been suggested as a possible base for extremely large diameter liquid mirror telescopes to be installed on the moon. The low temperatures are advantageous if you want to image very long-wave infrared light that is produced by extreme redshifts and comes from the most distant parts of the visible universe. Such a liquid base would be covered with a thin, metallic film which forms the reflective surface.

Web links

Individual evidence

  1. ↑ David Leverington: Observatories and telescopes of modern times - ground-based optical and radio astronomy facilities since 1945. Cambridge University Press, Cambridge 2017, ISBN 978-0-521-89993-2 ; P. 90
  2. ^ Paul Hickson, Brad K. Gibson, David W. Hogg: Large astronomical liquid mirrors , bibcode : 1993PASP..105..501H
  3. ↑ govertschilling.nl: Alles Over Sterrenkunde
  4. ↑ Ermanno F. Borra, Omar Seddiki, Roger Angel, Daniel Eisenstein, Paul Hickson, Kenneth R. Seddon, Simon P. Worden: Deposition of metal films on an ionic liquid as a basis for a lunar telescope. In: Nature. 447, 2007, pp. 979-981, doi : 10.1038 / nature05909 .