The Scale Model of the 1,5 m Telescope of the Tartu Observatory at Tõravere Bought: 1976 Company: the Leningrad Company of Optics-mechanics (ЛОМО)

The Astronomical Council of the Academy of Sciences of the Soviet Union (Astrosovet) passed in 1953 a decision to found a contemporary astronomical observatory in Estonia. For the main telescope of the observatory it was planned to have a telescope with the 1.5-m mirror which had to be built by the leading producer of optical instruments at that time – the Leningrad Company of optics-mechanics (LOMO). The decision seemed to be somewhat strange because at that time in Leningrad the 2.6-m telescope was being constructed for the Crimean Astrophysical Observatory. It would have been logical to produce a telescope of the same type (one such was made; a similar telescope for the Bjurakan Observatory in Armenia was completed in the same year as the 1.5-m Tõravere telescope).

All the big telescopes of the middle of the 20th century were mirror telescopes of universal use. By that time astronomers had learnt to change the distance of the focus placing suitable intermediary elements to it (mirrors, lenses, etc.). When in the earlier telescopes the distance of the focus was determined by the qualities of the objective, in the new telescopes it was possible to use many different foci.

The universal telescope of the 1950s

P1 – the main paraboloid mirror; its diameter determines the “size” of the telescope.
F1 – the main focus. If no additional mirrors are placed, the image of the starry sky which can be photographed is formed here. In the main focus the telescope has the largest field of view and the smallest magnification.
P2 – the first secondary mirror, the so-called Cassegrain mirror. The convex mirror decreases the convergence of the rays thus increasing the focal length. The image is formed behind the main mirror, for the passage of the rays a hole has been made in the main mirror. The result is the “classical telescope” with the help of which stars are observed in the “natural” direction, only the tube is four-five times shorter.
F2 – the Cassegrain focus. It is used in visual observation and in photographing single objects, in measuring their brightness and for registering the spectra with low resolution.
P3 – the mirror for the side view with the help of which the light is directed out along the declination axis. It is analogical to the mirror of the zenith telescope.
F3 – the Nashmith focus. It is used in large telescopes with the azimuthal mounting.
P4 – the coudé mirror. The convex mirror directs the light down along the cavity inside the polar axis.
F4 – the coudé focus. It has the smallest field of view and the largest focal lenght. It is used for registering high resolution spectra of the stars.

The 1.5-meter telescope AZT-12 named after Aksel Kipper at Tõravere. The length of the telescope tube is seven meters – only twice the size of the Zeiss telescope in spite of the fact that its diameter is seven and a half times larger. The telescope has three “traditional” foci: the main focal length is 6 m, the Cassegrain focal length is 24 m (almost four times longer than the tube!) and the coudé focus located one floor downwards has the focal length of 52.5 m. The telescope has a classical German mounting (until the year 1980 it was the largest telescope with German mounting in the world), the total mass of the moving parts is 13.5 tonnes, the diameter of the dome is 15 meters. The telescope and the dome are controlled by the computer; for observing the objects there is a photo guide. It is also used for observing planets at open public observation evenings. The telescope has been named after the first director of the Institute of Physics and Astronomy academician Aksel Kipper (1907–1984).

During more than 30 years this telescope has been the most important instrument for the Estonian astronomers who mainly observe variable stars, the main attention is given to spectral observations. It is the largest telescope in Northern Europe; its privilege is the possibility to have long observation series at winter nights. The telescope’s opportunities for use are limited by a bad astroclimate and a small number of clear nights. The telescope is a good practice base for the students of astronomy at Tartu University.

In the museum there is an electrical scale model 1:10 which is one of the two models used in the factory during designing the telescope.