In 1611 Johannes Kepler suggested a telescope comprising two convex lenses, the principle of which is still used today as the “classic refractor” or “Kepler telescope.” The first convex lens acts as the objective and produces a realistic image in the focal plane. This image is viewed with the second convex lens which acts as the eyepiece. The image is reversed. In other words it is back to front and upside down, which really does not matter as far as astronomers are concerned.
The First Reflecting Telescopes
The first workable reflecting telescope was built by Isaac Newton in 1668. A concave main mirror focused the incoming light into a conical bundle, which was reflected to the ocular lens, or eyepiece, by a flat mirror positioned at a 45° angle to the optical axis of the main mirror. In theory there were several more designs for reflecting telescopes in existence. These included the Cassegrain reflector in which a small convex mirror reflects the rays coming from the main mirror back through a hole in its center. However, it was not possible to produce an actual telescope of this type until 1905 because of the hyperbolic contour of the smaller mirror.
Further practical development of Newton’s design of the reflecting telescope had to wait until the 18th century when German-born William Frederick Herschel made a considerable contribution. Within a three-year period he built and sold 60 telescopes with an excellent image quality. In 1781 he discovered the planet Uranus and a year later he was appointed “King’s Astronomer” at Greenwich Observatory.
To increase the degree of enlargement, bigger and bigger reflecting telescopes were constructed, for the bigger the area of the main mirror, the more light it can collect. But the efficiency of the mirror is dependent not only on its diameter but also on the accuracy of its shape, its surface quality and its reflective coating.
Progress in Mirror Materials
Glass Ceramic Mirror Blanks
Aluminum coating has proved successful up to the present day, but there have been some revolutionary new developments concerning the basic material for the mirror blanks. A drawback with glass or metals is that it expands when heated up and, as a result, “bends” the contour of the mirror. And where wide variations in temperature occur the images produced cannot be used. Attempts were made to develop a material with almost zero thermal expansion. SCHOTT GLAS succeeded in doing this with its “ZERODUR®” glass ceramic, which has been the preferred material for use in telescope and satellite mirror blanks since 1970.
Milestones in Material Development
up to 1835: Polished metal telescope mirrors
1835: Justus Liebig invents a process for depositing a layer of silver on glass
1856: Silver coated glass telescope mirrors
1930 onwards: Aluminum used instead of silver for the reflective coating
1970 onwards: “ZERODUR®” glass ceramic mirror blanks
1991-93: SCHOTT GLAS makes the biggest cast piece of “ZERODUR®” to date for the Very Large Telescope (VLT) with a diameter of 8.2 meters