Astronomy

Liquid Mirror Telescopes

moon in liquid mirror telescope

A new of astronomical imaging can emerge, given the success of liquid mirror telescope that uses mercury instead of glass for its primary mirror.

Conventional astronomical telescope have glass mirrors, finely polished to an accuracy of a few tens of nano-meters . The mirrors need costly and complex support systems such as adaptive optics to counteract the distortion caused by the atmospheric turbulence. A 6-m diameter mirror costs more than $10 million ( Rs 66 Crore).

An alternative at one –tenth of the cost is now possible. The primary mirror will have a rotating dish covered with mercury. It is based on an elementary principle viz. Any variation of potential energy ion the surface of a liquid in equilibrium would lead to a force. If the liquid is rotated at a constant angular speed about a vertical axis , the surface takes on the shape of a parabloid, which is ideal to focus light as in a conventional telescope.

Even though Ernesto Capooci in Naples first described this idea in 1850, it became a reality only in recent times (1980s), when CCDs came into use. Canadian physicist Emanon F. Borra and his team built mercury mirrors, 1.5-m in diameter. Subsequently they improved the quality of digital imagery.

Liquid mirrors have, however, one limitation: inability to look in any direction other than the zenith,. But this is not considered a serious hindrance as distant galaxies can be studied in a fixed pointing mode. One other problem faced by the liquid mirror was the diffuse halo of light surrounding a star. This was solved by suspending a very thin film of transparent Mylar plastic just above the mirror.

An international liquid mirror telescope, a 4-m instrument is proposed to be built on a Chilean mountain top. Another one that has an 8m diameter mirror is also planned in Chile. A novel idea is to place a liquid mirror telescope on the moon. As there is no air on the moon, it is proposed to make the mirror float like a train on magnetic levitation and use low temperature ionic liquids that will not freeze.

As more and more windows in the electromagnetic spectrum open to reveal new phenomena in the skies and as an increasing number of satellites and ground–based telescopes are creating a deluge of data; a transformation in imaging is under way from optical processing to electronic processing. The history of imaging has been mostly marked by the progress in optical materials. In recent years, signal processing in optical and electronic areas has started to give a new dimension to imaging.

Lucky camera

A recent innovation illustrates this trend. Progress in light detectors has given a ground – based telescope unprecedented clarity – double that of the space-based Hubble. One such detector is combined with the Hale telescope in the United States to snap as many as 20 images a second , when by chance at least one image would escape the air turbulence and a computer can lock on to it and combine the images electronically . The device is aptly called Lucky camera! Though the idea was first suggested in 1978, electronic detector technology was not developed to the extent needed to outperform Hubble.

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