Metal mirror

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Metal mirrors came into use in astronomy from around 1650 with the invention of the various mirror telescopes . At that time, grinding metal surfaces was much easier than grinding glass mirrors , and the problem of streaks in larger glass molds was still unsolved.

As a mirror material usually served copper - tin - alloys which have been polished or to increase the reflectivity coated thin. Yet they only reflected about 50% of the starlight as it hit. Therefore, from around 1900 (when the casting technique was developed) they were largely replaced by glass mirrors, and from around 1980 by glass ceramics .

Today metal mirrors are mainly used for physical measurement technology ( e.g. spectrometers ) under extreme environmental conditions (space, aviation). In addition, because of the simple integration of a cooling system, they offer advantages in laser technology as resonator mirrors for high-power lasers.

Telescope mirrors by John Mudge and James Short

The doctor and amateur astronomer John Mudge (1721–1793) was one of the first to examine various metal alloys for their suitability for telescope mirrors. In 1777 he received the Copley Medal of the Royal Society for his Directions for making the best Composition for the Metals for reflecting Telescopes; together with a description of the Process for Grinding, Polishing, and giving the great Speculum the true Parabolic Curve , which were published in the Philosophical Transactions . Mudge may have had contact with Isaac Newton about methods of mirror grinding and also shared his experience in telescope construction with the mathematician and optician James Short .

The most suitable alloy he received was copper - tin bronze in a ratio of 2: 1, while Newton recommended 3: 1. For the physicist John Michell , he built a particularly bright, short reflector telescope with an aperture ratio of 1: 4 and a focal length of 10 feet, whose operation (in contrast to the much longer Herschel instruments with 1:10 to 1:20, see below) required no help.

The then master of telescope construction James Short (1710–1768) initially used spherical glass mirrors for his Gregory telescopes . In order to be able to grind the main mirrors in the optically better paraboloid shape, he later switched to metal mirrors made of tin bronze (speculum). As a result, he finally succeeded in giving the secondary mirrors the ideal ellipsoidal shape. His unmatched sharp telescopes made him known throughout Europe; he produced almost 1,400 telescopes, 110 of which have survived to this day.

William Herschel's mirror workshop

The Schiefspiegler and Newtonian telescopes first established themselves as telescope types , but the diameters initially remained rather modest. The development of larger metal mirrors began with the technician and (initially) amateur astronomer Wilhelm Herschel . As a German emigrant to England from around 1770 he found a source of income in the cut of astronomical mirrors, of which he sold several hundred.

Wilhelm Herschel's 48-inch reflector telescope with a focal length of 40 feet

Herschel cast the mirrors from soft metal alloys that were easy to grind. Then he polished them bright, but they tarnished again in use and they often had to be polished. ( Hieronymus Schröter (1745–1816) was the first to work with coatings , see below). Herschel discovered the planet Uranus with a six inch (15 cm), most of his nebula objects with a 47 cm mirror and 6 m focal length (built in 1783). His largest telescope from 1789 (see picture) measured 1.22 m × 12 m and remained the largest in the world until a storm in 1839.

The mirrors and coverings by Hieronymus Schröter

Herschel equipped u. a. the private observatory Lilienthal of the chief magistrate and lunar researcher Schröter with some metal mirrors that got bigger and bigger over time. The well-off official began observing the moon in 1779 with a pure 6 cm lens telescope from Dollond , but Herschel's discovery of Uranus (1781) prompted him to order a 12 cm reflector telescope with a 122 cm focal length . Soon a Newton telescope 17/214 cm followed, for which he built a two-story observatory near Bremen. In addition to moon and solar observations - for which relatively matt metal mirrors have always been suitable - the light intensity of the new acquisition could now also be used to observe planets , double stars and clusters of nebulae . The instrument became the basis for Schröter's excellent moon observations, which he published in 1791 - still as an amateur - under the title selenotopographical fragments .

In 1792 Schröter and the Kiel chemistry professor Johann Gottlieb Friedrich Schrader developed a whitish, brittle copper-tin alloy with a little arsenic. To increase the reflectivity (initially hardly 20%), they vaporized an additional layer of arsenic , which solved Herschel's polishing problem. The alloys were easy to grind into the hollow form, but were already hard enough to keep their good imaging performance for a long time.

Schroeter completed his actual " giant telescope " in 1794 - it had a 51 cm opening and an octagonal, 9 m long tube. The special arsenic coating was made by his gardener Harm Gefken , who later founded an optical workshop. However, he died of arsenic fumes at the age of 55.

The shiny half-meter mirror was on a par with the much larger but dull one by Herschel, so that Schröter could even examine the night side of the moon, discover numerous star clusters and nebulae and make daytime observations of the bright planets. With this telescope, three of the first four minor planets were discovered between 1802 and 1807 and the assistant Friedrich Wilhelm Bessel, who was appointed by Schröter as an observer , trained as an astronomer.

Lord Rosse's one and two meter mirrors

The Lilienthal telescope was soon surpassed by a 36-inch reflector telescope from Lord Rosse (Ireland), with whose light intensity the Earl was able to explore many foggy objects . But Rosse built an even larger telescope from 1842–1845, which was soon named " Leviathan of Parsonstown ". It had an aperture of 183 cm, a focal length of 16 m and was mounted between two massive, 15 m high walls - it could be swiveled slightly using pulleys (see picture). With this telescope, Rosse has made a decisive contribution to cosmology and a. recognized the spiral nature of galaxies , but had to shut down temporarily due to the Irish famine.

The “Leviathan” by Lord Rosse (1848) with a 1.8 m metal mirror

The mirror consisted of " speculum metal " ( speculum metal ), a bronze-like alloy, weighing nearly 4 tons alone. A special, very careful casting technique was necessary for production. If the block had cooled down too quickly, it would have cracked. In fact, the 1st attempt failed, and for the 2nd attempt the Lord had additional heating units installed and the cooling process carefully checked. How long the process ultimately took is not known (the cooling of the 5-meter glass mirror on Mount Palomar took over a year). The heavy 3800 kg bronze mirror had against the deflection are stored on 27 columns, made even increase later to 81 the number Rosse.

A separate, steam-driven machine was constructed for grinding into the required paraboloid shape and for the subsequent polishing . Nevertheless, the huge mirror kept going blind and had to be repolished and parabolized twice a year. In order to bridge these weeks of time, Lord Rosse finally had a second, identical mirror made.

End of the metal mirror

At the turn of the century 1900, the production of glass was so advanced that it also appeared suitable for large telescope mirrors. In addition to the higher degree of reflection , the significantly lower volume weight was also advantageous .

At the Mount Wilson Observatory , the 2.5 m Hooker mirror went into operation in 1917 , and served as a model for all large telescopes until the 1970s. At 5 m ( Mount Palomar ) and 6 m ( Selentschuk ), however, the limit was reached and one went over to segmented mirrors . From around 1980, glass ceramics from the Mainz Schott works (especially Zerodur ) were increasingly used instead of glass , because they have very low thermal expansion .

Metal mirrors in physics and technology

Today metal mirrors are used in many areas of technology and physics. Some examples are:

High energy applications

Light and microwaves

literature