Speculum metal
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Speculum metal is a very hard white alloy of roughly 75-80% copper and 20-25% tin[1]; some compositions contained 1–2% of arsenic. Composition with 45% tin has more resistance to tarnishing.
For reflection, the composition needs to be copper 68.21 per cent, tin 31.7, with more copper making it more yellow and more tin more blue reflection.[2]
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[edit] Speculum and Telescopes
An early composition of speculum metal was used in the first reflecting telescope, Newton's Reflector of 1669 (33-mm (1.3-inch) aperture).[4] The composition of speculum metal was further refined and went on to be used in the 1700s and 1800s in many designs including those by Robert Hooke, William Herschel, and Lord Rosse. The ability to grind parabolic mirrors led to increased use of speculum although advances in the design of refractor telescopes (which use glass lenses), during the same period, resulted in an ongoing competition.
Speculum metal mirrors allowed for very large telescopes; William Herschel's 126-cm (49.5-inch) reflecting telescope of 1789 was larger than the largest practical glass refracting telescope a century later, the 102-cm (40-inch) Yerkes (at Yerkes Observatory) of 1895. [5] [6]
In 1845 Lord Rosse used the alloy for the 183-cm (72-inch) mirror of his "Leviathan of Parsonstown" telescope. The metal has the unfortunate property of tarnishing in open air with a sensitivity to humidity, requiring periodic re-polishing to maintain its usefulness for reflective telescopes.
In 1835 a German chemist named Justus von Liebig developed a process for silvering mirrors. This process gained wide acceptance after further improvement by von Liebig in 1856.[7][8] Use of solid speculum metal for mirrors began to decline after 1859 when Leon Foucault published his results on silvered glass mirrors and built a 33-cm (13-inch) silvered glass telescope in 1860. Foucault's work was an extension of the work of von Liebig and even earlier work on mercury-glass mirrors (see Mercury silvering).
Nevertheless, the last large speculum metal mirror telescope of the era, the Great Melbourne Telescope with a 122-cm (48-inch) aperture was finished in 1867.
In the 1900s, speculum metal for reflecting telescopes was replaced by metal coated glass mirrors. This was aided by the success of the Crossley telescope and the desire for greater aperture at lower cost than refractors. The metallic layer on front-coated optics (silver early on and later aluminum in modern telescopes) needs to be re-finished periodically.
The telescopes of the early period used thick, cast metal blanks that were ground and polished. Speculum metal can also be used as the metallic coating on glass mirrors (as opposed to silver or aluminum) giving a reflectivity of 68% at 6000 angstroms when evaporated onto the surface.[9]
[edit] See also
[edit] External links
[edit] References
- ^ [1]
- ^ http://chestofbooks.com/reference/Henley-s-20th-Century-Formulas-Recipes-Processes-Vol1/Speculum-Metal.html
- ^ "Original mirror for William Herschel's forty-foot telescope, 1785". Science Museum. http://www.sciencemuseum.org.uk/objects/astronomy/1932-567.aspx. Retrieved on 2008-11-23.
- ^ The History of the Telescope By Henry C. King, Page 74
- ^ http://amazing-space.stsci.edu/resources/explorations/groundup//lesson/scopes/yerkes/index.php
- ^ http://amazing-space.stsci.edu/resources/explorations//groundup/lesson/scopes/herschel/index.php
- ^ Justus Liebig (1835). "Ueber die Producte der Oxydation des Alkohols". Annalen der Chemie 14 (2): 133. doi:.
- ^ Justus Liebig (1856). "Ueber Versilberung und Vergoldung von Glas". Annalen der Chemie und Pharmacie 98 (1): 132–139. doi:.
- ^ http://www.iop.org/EJ/abstract/0950-7671/24/9/308
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