The Fraunhofer Refractor – the most famous telescope of the Tartu Observatory Bought: 1824 Company: Utzschneider and Fraunhofer, Munich

The technology of the beginning of the 19th century allowed taking up bigger work. Already the 40-foot reflector with the mirror diameter 122 cm made by Herschel was a wonder. The Irish nobleman, the Count of Rosse William Parsons had even a bigger instrument. His “Leviathan” with the mirror diameter 180cm would even find a respectable place in the priority list of present-day telescopes. The big telescope makes the number of the observable objects even larger; it is also simpler to get big magnification.

For perfect work the telescope should have high quality optics and an exact and firm guiding system. Because of the rotating Earth in the case of big magnifications there was a danger “to lose” the star from the field of view.

The problem had a simple solution: the telescope had to be placed on a revolving base, the axis of which was parallel to the Earth’s axis (directed to the North Pole in the sky). Such an instrument was built in 1795 by E. Troughton for the Observatory of Armagh, but it was inconvenient.

When Fraunhofer began to design the Tartu telescope, he had already found the technical solution. The revolving platform carrying the telescope was fastened to one end of the polar axis, the clock mechanism turning it to the other end. The axis in its turn was attached with two bearings to the slanting log, the angle of which in relation to the horizontal plane was equal to the degree of Tartu’s latitude. To the platform another axis, transverse to the polar axis, was attached. In the one end the telescope was fastened, to the other the counterweight and the so-called declination circle for turning the telescope. The solution was so reasonable that it immediately became known all over the world as the “German mounting” and with smaller improvements was generally used during more than one hundred years.

The polar axis. It is not Fraunhofer’s invention but it was he who found the right solution for the transverse axes. The telescope must not be placed in the crossing of the axes but may well be placed rather far in order not to be hindered by the constructions of the column when being turned. Certainly, the system must be balanced but it can be achieved easily with attaching counterweight to the other end of the declination axis. When operating such a telescope, it is necessary to follow fixed rules because the telescope tube can come up against the construction elements. This is a small inconvenience when compared with other, rather clumsy in comparison with the clumsy horseshoe or yoke mountings.

The equatorial design has one more concealed privilege which becomes evident during measurements, especially during photographing: in comparison with all other ways of directing it is the only one when the field of view of the telescope preserves its orientation.

Four legs. The telescope is supported by four massive wooden legs at the ends of which there are screws which allow putting both crossed logs “perpendicular” in spite of the uneven surface.

The slanting column. To direct the polar axis exactly to the pole of the sky, Fraunhofer built a slanting column on the vertical column. It carried the polar axis and the clock mechanism was attached to it. The slant of column was selected in conformity with the degree of latitude of Tartu (58°26’).

Bringing the centre of gravity down with counterweight. The drawback of the refractor telescopes is the long tube: as the magnification depends on the distance of the focus, for a large magnification it is necessary to have a long telescope but the heavy objective of the big telescope moves the centre of the mass in the direction of the upper end of the tube. Fraunhofer solved this problem in a complex manner: with the tube there are parallel rods in the lower part of which there is a brass counterweight. The rods support the tube in three points decreasing it deformation under the influence of its weight. Both the rods and the counterweights are in the plane perpendicular to the declination axis and do not disturb the operation of the telescope.

The guiding rods are meant for control of the telescope. In Herschel’s 7-foot telescope this function was performed with two cranks which were near the ocular. The inclination axes of the big refractor are placed far from the ocular and this is why it was necessary to use the guiding rods with the length of two and a half meters. The right solution – to fasten the leading rods to the telescope tube – was found by Fraunhofer already after the Tartu refractor started working. His following great apparatuses – the big refractor for the Berlin Observatory (the twin of the Tartu telescope) and the heliometer for Köningsberg – are operated with the help of the guiding rods brought close to the ocular.

The clockwork mechanism is a real masterpiece. The turning of the telescope demands much bigger strength than moving the hands of the clock – consequently it is necessary to have a more powerful mechanism. All the 18th – 19th century clocks were regulated with the help of the gravitational or the spring pendulum which stopped the leading gearwheel for a moment. This is why the hands of the clock do not move evenly but by jerks. In the case of a clock it does not matter but it is not allowed in the telescope. In a strange manner both problems are solved by one and the same technical approach. The pendulum must be replaced by a centrifugal regulator. The regulators, which were used in steam engines, function evenly. They were very well worked out at the time of Fraunhofer. The accuracy of the movement of the telescope was surprising. Struve wrote about the case when the thread cross of the ocular directed to the Jupiter’s satellite had divided the satellite into four equal parts during more than a quarter of an hour.

Here it is: the gear wheels with the transmission coefficient of 100000 and the heart of the clock – the brass ball sliding along the wall of the case.

Observations Measurements of binary stars. Struve started measuring binary stars in 1814 with the Troughton telescope, with the big refractor it was the main task. In 1827 the new catalogue of binary stars (Catalogus Novus Stellarum Duplicium) was published. Struve had presented in it the data of 3,112 binary stars he had discovered during three years (the result of observing 120,000 stars in the Northern sky). In 1837 “The micrometric measurements of binary and multiple stars” (Stellarum duplicium et multiplicium mensurae micrometricae)was published. The position angles and the distances between the components of 2,714 binary stars were measured.

Measurements of the star parallax. The determination of the distances of the stars through the annual parallax (the shift of the star because of the rotation of the Earth) was a favorite problem at the beginning of the 19th century. The measurement demanded accurate instruments, The Fraunhofer telescope with the ocular micrometer was an accurate instrument. Struve measured the shift of the bright star Vega which did not set in Tartu with respect to the neighboring weak star and published the result in his paper of the micrometric measurements of binary stars in a separate chapter titled “About the parallax of the fixed star”. Struve’s initial result – one eighth of the second of the arc – was at the limit of the resolving power of the telescope (and by chance almost absolutely right result). Later, after repeated observations Struve corrected his result to one fourth of the second of the arc (it was wrong, but it showed that it was not possible to rely on the authenticity of the observations).

Mädler’s observations of the Mars. The possibility to observe planets with one of the best telescopes of the world was definitely one of the reasons why Mädler had come to Tartu. But the good qualities of the telescope were nullified by the bad climate of Tartu and its location too far in the North. Mädler was drawing Mars during the great opposition of the Mars in 1845.

Modeling of the surface of the Moon. After unsuccessful observation of the Mars Mädler turned back to studying the mountains of the Moon. The idea was to make three-dimensional gypsum models of the observable objects and when lighting them at different angels to compare with the picture seen in the telescope. Mädler’s wife Minna (nèe Witte) and her mother who had made one of the most exact Moon globes of the time helped Mädler to make gypsum models.

The observations of the Sun in 1874–1911. The last observations with the Fraunhofer telescope were made in 1915-1921 when many instruments of the observatory were evacuated to Russia (among them the Zeiss refractor). It is said that Ernst Öpik observed the Mars opposition in 1939 with the telescope in the Eastern hall which was the last observation with the historical telescope.