In the mid-twentieth century a map showing the position of the world’s large telescopes would have looked very different to that of today. At that time almost all of the world’s large telescopes were in the northern hemisphere close to the centres of population in the USA and Europe.
Today there are a number of new observatories with large telescopes in remote desert regions of the world where sky conditions are best suited to astronomy. These include the Hawaian Islands, the Canary Islands, Chile, Northern China, South Africa and Australia.
The heavy concentration of telescopes in the first half of this century in the northern hemisphere, had led to a striking imbalance in observational astronomy. Several of the most interesting astronomical objects can only be seen from the southern hemisphere, notably:
- The centre of our galaxy (the Milky Way) which is in the constellation of Sagattarius
- The nearest external galaxies to our own: the two Magellanic Clouds.
The plan to set up a large telescope, committed to surveying southern skies was intended, both to fill this gap in astronomical knowledge and to give Irish astronomers, north and south, access, once more, to world class equipment. The telescope, to be named the Armagh-Dunsink-Harvard telescope (ADH), was to be set up at the Harvard Southern Station near Bloemfontein, South Africa, where clear skies were guaranteed. Following the participation of the two Irish observatories several other European countries including Belgium, Sweden and West Germany joined with Ireland and the USA to form the first international observatory Boyden Observatory, South Africa.
The Armagh-Dunsink-Harvard Telescope was built by the Perkin Elmer Corporation of the USA and financed by a £5,000 grant from each of the two Irish observatories and by Harvard College. It was a telescope of unique design, which was capable of giving first class images of stars over a wide field, (circa 10 degrees). The design, by James Baker of Harvard, was based on the principles of the Schmidt telescope design developed in Hamburg, Germany by the brilliant German optician Bernhard Schmidt. This design avoided most of the optical abberations which had previously afflicted telescopes; namely, astigmatism, chromatic abberation and coma. Baker’s modification was to use a secondary mirror to form the image behind the primary mirror. This was a distinct advantage over the classical Schmidt design which formed its image inside the main telescope tube above the primary mirror.
The telescope, which was almost entirely used for photography of stars and galaxies, made a valuable contribution to the study of the southern Milky Way and the Magellanic Clouds. In 1951 it was fitted with a large objective prism; at that time the largest glass prism in the world. This additional element caused the normally point-like stellar image to become extended into a short spectrum for each star. With the ADH, plus objective prism combination, it was possible to record the spectra of up to a thousand stars simultaneously; a very cost effective method of observing. The ADH was finally superseded in the 1960’s by two large southern classical Schmidt Telescopes, one in Australia (the UK Schmidt Telescope) and the other in Chile (European Southern Observatory).