W. Herschel’s 7-foot Reflector Bought: 1806 Company: Herschel, London

In 1666 Isaac Newton recommended that instead of lenses mirrors should be used in telescopes because the focus of the mirror is the same for all wavelengths. Newton presented his telescope in the Royal Astronomical Society in 1672. Later on it was perfected by Laurent Cassegrain and John Hadley. In spite of everything mirror telescopes could not compete with lens telescopes – until William Herschel succeeded in creating a cheap and practical telescope with a metal mirror. Today it is known as the 7-foot Herschel reflector.

The complex 7-foot reflector in the working position. Pay attention to the reasonable position of handles and oculars.

The telescope is suitable for a single observer: The focal length of seven feet (210 cm) meant in the Newtonian system that in a traditional observation (the object’s height 20-70 degrees the ocular was at the height of 100-150 cm. The telescope was aimed at the object with the help of the leading binoculars; in watching the object the screw driving gear was used. The diameter of the mirror 160 mm gave sufficient power of light for visual observations. Most of their observations both William and Caroline Herschel made namely with the 7-foot telescopes.

Together with his brother Albert, William Herschel made 200 7-foot telescopes in 1778-1820, out of them 21 telescopes have survived. The Herschel telescope in Tartu was mainly used for the observing the occultation of stars by the Moon.

The 7-foot telescope of Tartu