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Topic Review
Liquid Crystal Waveguide Structures
Liquid crystal materials can be used to make either a core, in which light beams can be confined, or a cladding of optical waveguides.
  • 835
  • 02 Nov 2021
Topic Review
Liquid Crystal-Tuned Planar Optics in Terahertz Range
Terahertz waves of higher frequencies compared to microwave and radio frequency have shown great potential in radar detection and high-speed wireless communication. To spatially control the wavefront of terahertz beams, various novel components, such as terahertz filters, polarization converters and lenses, have been investigated. Metamaterials and metasurfaces have become the most promising technique for the free manipulation of terahertz waves. Metadevices integrated with liquid crystals have been widely used in active terahertz devices.
  • 467
  • 17 Feb 2023
Topic Review
Liquid Mirror Telescope
Liquid mirror telescopes are telescopes with mirrors made with a reflective liquid. The most common liquid used is mercury, but other liquids will work as well (for example, low melting alloys of gallium). The liquid and its container are rotated at a constant speed around a vertical axis, which causes the surface of the liquid to assume a paraboloidal shape, suitable for use as the primary mirror of a reflecting telescope. The rotating liquid assumes the paraboloidal shape regardless of the container's shape. To reduce the amount of liquid metal needed, and thus weight, a rotating mercury mirror uses a container that is as close to the necessary parabolic shape as possible. Liquid mirrors can be a low cost alternative to conventional large telescopes. Compared to a solid glass mirror that must be cast, ground, and polished, a rotating liquid metal mirror is much less expensive to manufacture. Isaac Newton noted that the free surface of a rotating liquid forms a circular paraboloid and can therefore be used as a telescope, but he could not actually build one because he had no way to stabilize the speed of rotation. The concept was further developed by Ernesto Capocci of the Naples Observatory (1850), but it was not until 1872 that Henry Skey of Dunedin, New Zealand constructed the first working laboratory liquid mirror telescope. Another difficulty is that a liquid metal mirror can only be used in zenith telescopes, i.e., that look straight up, so it is not suitable for investigations where the telescope must remain pointing at the same location of inertial space (a possible exception to this rule may exist for a mercury mirror space telescope, where the effect of Earth's gravity is replaced by artificial gravity, perhaps by rotating the telescope on a very long tether, or propelling it gently forward with rockets). Only a telescope located at the North Pole or South Pole would offer a relatively static view of the sky, although the freezing point of mercury and the remoteness of the location would need to be considered. A very large telescope already exists at the South Pole, but the North Pole is located in the Arctic Ocean. The mercury mirror of the Large Zenith Telescope in Canada was the largest liquid metal mirror ever built. It had a diameter of six meters, and rotated at a rate of about 8.5 revolutions per minute. It is now decommissioned. This mirror was a test, built for $1 million but it was not suitable for astronomy because of the test site's weather. They are now planning to build a larger 8 meter liquid mirror telescope ALPACA for astronomical use and a larger project called LAMA with 66 individual 6.15 meter telescopes with a total collecting power equal to a 55 meter telescope, resolving power of a 70 meter scope.
  • 430
  • 08 Oct 2022
Topic Review
List of 19th-Century Lunar Eclipses
See also: List of lunar eclipses, List of 18th-century lunar eclipses, and List of 20th-century lunar eclipses
  • 455
  • 25 Oct 2022
Topic Review
List of Asteroids in Astrology
Asteroids are relatively new to astrology, having only been discovered in the 19th century. However, some of them (especially the largest of them), are believed by some astrologers to influence human affairs. Still though, they are often ignored within mainstream systems of astrology, especially in more traditional astrology systems like Vedic astrology or Hellenistic astrology. Their use has become significant to a few Western astrologers yet still only a minority of astrologers use the asteroids in chart interpretation.
  • 4.7K
  • 22 Nov 2022
Topic Review
List of Conjunctions
This is a list of the Solar System's recent and forthcoming planetary conjunctions (in other words, when two planets look close together). In astronomy, a conjunction is an event, defined only when using either an equatorial or an ecliptic celestial coordinate system, in which any two astronomical objects (e.g. asteroids, moons, planets, stars) have the same celestial longitude, normally as when observed from the Earth (geocentric). In the case of a geocentric conjunction of two of the Solar System's planets, since the Solar System's planets appear to travel "along the same line" (the ecliptic), the two planets appear to an Earthbound observer as being near one another in the sky around the time of the conjunction. The list below presents instances during the period 2005–2020 in which two Solar System's planets are in conjunction according to the equatorial coordinate system (in which the celestial longitude is termed right ascension).
  • 414
  • 27 Oct 2022
Topic Review
List of Delta IV Medium Launches
Since November 2002, rockets from the Delta 4 Medium family have been launched 29 times, all of which were successful. Its last flight was with a 3rd generation GPS satellite in August 2019.
  • 403
  • 18 Oct 2022
Topic Review
List of Exoplanetary Host Stars
The following is a list of exoplanetary host stars. The table contains information about the coordinates, spectral and physical properties, and number of confirmed planets. The two most important stellar properties are mass and metallicity because they determine how these planetary systems form. Stars of higher mass and metallicity tend to have more numerous and more massive planets. Most of the stars below are solar-type, mainly in the spectral classes F, G, and K, because astronomers tend to look for planets around stars similar to the Sun. Others are giants, which have used up all the hydrogen in their cores. Finding planets around giant stars gives clues as to how planetary systems evolve and how the properties of planets change with the evolution of the stars. As of April 2018, there are 2,816 stars with at least one confirmed planet, of which 628 stars have two or more confirmed planets; that is, 22% of all confirmed exoplanetary host stars have two or more confirmed planets. As of 2018, the star with the most confirmed planets is Kepler-90, with eight planets, although HD 10180 may have nine (two are unconfirmed). The most massive exoplanetary host star is Omicron Ursae Majoris (3.09 M☉), while the least massive is 2M J044144 (0.021 M☉). The most metal-rich star is NN Serpentis (1.744, 55.5 × Sun), while the most metal-poor is BD+20°2457 (−0.999, 0.100 × Sun). The nearest exoplanetary host star is Proxima Centauri (4.25 ly), while the most distant is NY Virginis (26940 ly). Visually, the brightest exoplanetary host star seen from Earth is Pollux (1.15), while the faintest is OGLE-2005-BLG-390L (27.98).
  • 312
  • 04 Oct 2022
Topic Review
List of Jupiter Trojans (Trojan Camp) (1–100000)
This is a partial list of Jupiter's L5 trojans (60° behind Jupiter) with numbers 1–100000 (also see main page).
  • 547
  • 05 Dec 2022
Topic Review
List of Jupiter Trojans (Trojan Camp) (300001–400000)
This is a partial list of Jupiter's L5 trojans (60° behind Jupiter) with numbers 300001–400000 (also see main page).
  • 230
  • 24 Nov 2022
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