The Kensington Telescope (circa 1888)

The Kensington telescope was built by Thomas Cooke & Sons for the Solar Physics Observatory at South Kensington when Norman Lockyer became the world's first professor of astronomical physics.  The Kensington telescope was set up at South Kensington in 1884 to apply Lockyer’s spectrographic methods to the measurement of the temperature of stars. The aim was to conduct investigations on the stars similar to those he had already made for the Sun. (See Lockyer 6¼-inch refractor & Mond dome) 

The instrument marks the start of the research in astronomical physics and is, therefore, a telescope of world heritage significance. 

Today, it is used by the public on clear nights to view the planets and brighter night sky objects. On sunny days it is used to show Fraunhofer lines in the solar spectrum. 

The instrument is fitted with its original equipment, including a photographic tube and objective prism. The telescope comprises a 10-inch main viewing tube and a 9-inch photographic tube with a 30° objective prism. Although the two tubes are mounted parallel, the angle between their directions of view is fixed and, after finding the desired star field visually, the setting circles permit easy transfer of this to the photographic instrument. The telescope is mounted on a German equatorial mount and driven by a mechanical clock, controlled by a centrifugal governor, turning the instrument at constant speed about the polar axis. The image of the stars on the photographic plate forms a series of spectra. Lockyer and his staff scanned these spectra by a photometric microscope (micro-photometer) - a device that produced a graphic indication of light intensity along the spectrum. The telescope is in good optical condition and regularly used for public viewing. 

Lockyer was able to classify the stars by their spectral type, initially dividing them into 13 classes. He related these classes to stellar temperature and arranged the stars in order of their temperatures. Later he realised that any given star could have reached its temperature either by warming or cooling and he divided his list into two - those stars he thought were warming and those he believed were cooling. Eventually he produced a paper that suggested the two lists were linked at the very hottest stars. He devised an arc along which he believed the stars could be arranged. The sketch from his original paper is shown below. Thus Lockyer laid out the basic order leading to the theory of stellar evolution.

Lockyer led the world in the ideas of stellar classification by spectral temperature and, although most observatories followed his lead, the Kensington telescope was the first and leading instrument in the field. It is almost in its original condition, and has avoided 'cannibalisation' and modification. Throughout its period at the Observatory, it has been preserved by a dispute over ownership that persisted from Lockyer's death in 1920 up to 1991. Most contenders to ownership left the field when the instrument and dome became "Grade I listed" for preservation. Ownership was passed by the Royal Astronomical Society to the Observatory.

An interactive exhibition shows how the spectrum is used to find the composition of a star’s atmosphere and measure its temperature. The Kensington dome is a Grade II listed building. In 1997, new rails and running gear replaced severely rusted parts so that the dome could be turned with ease.