Primary mirror
The main mirror of the light-collecting mirror of a telescope is called. He is usually cut as a paraboloid, with special optics as the Schmidt camera as a mirror ball. In telescopes for optical wavelengths main mirrors are now exclusively made of glass or glass-ceramic.
History
In the early years of the invented by Newton mirror telescope mirrors were made of metal mirror. Because the metal but quickly oxidized, these mirrors often had to be re-polished. It is worsened slightly the smooth surface produced and altered the exact surface shape. Therefore, it came to glass as a carrier, which is mirror-coated with silver. Today's telescope mirrors are vapor-deposited in a high vacuum with a thin aluminum layer and also often provided to protect against rapid loss of sight with a protective quartz layer.
Ray tracing and aberration
The mirror telescopes are very small concave mirror in the form of pure spherical mirror. A spherical mirror collects parallel light beams and not just in one point but in a spatial extent of the focal spot along the longitudinal axis (so-called " firing line "). For larger mirrors therefore a paraboloid of revolution is made, that really gathers the light rays at one point. Very large telescopes today are mostly built as a Ritchey -Chrétien telescope where the primary mirror is deformed hyperbolic - the secondary mirror the way also, in addition to the hyperbola, which he needs for his job in Cassegrainsystem anyway.
Production
As a material for amateur mirror borosilicate glass is usually used, which has a very low coefficient of expansion. The glass blanks were previously formed by pressing or casting in metal molds. Today, borosilicate float glass is produced (eg Borofloat ) with 25 mm thickness, from which the glass blanks are cut.
The large mirror in astronomical research are mainly made against it from the glass ceramic. In special kilns the mirror blanks are equal to melted glass break in shape. While the furnace is rotated at the defined speed at which the parabolic shape of said mirror is achieved. During cooling of the molten glass, the temperature profile is controlled so that by ceramic crystallization a mixture of 60 percent ceramic and 40 percent glass is formed. The negative expansion coefficient of the ceramic is canceled out with the positive of the glass, so that virtually no thermal expansion occurs at all. In order to achieve freedom and power to crystallize the ceramic content, the cooling process takes a correspondingly long.
If the level is completed cooled, the final shape can be sanded and polished. When polishing a surface accuracy must Lambda / 2 (half the wavelength to be observed in the later ), but usually better lambda / 8, can be achieved. Professionally inset mirrors are manufactured up to 20 nanometer precision.
Small primary mirror, with a ratio of diameter to thickness of 10: 1 can be produced, are dimensionally stable by itself. From 50 cm diameter such mirrors are quite heavy. If you compare the mirror but thinner ago, so they bend with change in position by its own weight by itself
Larger levels were drilled earlier to reduce the weight from the rear. Today, the same levels are molded with a honeycomb structure at the rear.
Nevertheless, levels were more than 8 m in diameter due to the deformation by gravity no longer manufactured. Therefore mirrors were first produced to ten meters in diameter of segments. These segments were positioned by the holder ( static) so the result is a flawless image. Today active overlays have been developed that dynamically support the mirror at many contact points and thus compensate for the bending under its own weight or installation errors. In addition, the adaptive optics system has been developed to compensate for interfering influences by the atmosphere.