The Petzval astrograph Bought: 1911, rebuilt in Tartu, 1927 Company: Zeiss, Jena

Photography which became fashionable in the 1830s soon reached also astronomy. The first photo of the Moon was made by John William Draper in 1840. Five years later Louis Fizeau and Lèon Foucault photographed the spots of the Sun, by 1880 the astronomers had also photographed nebulae. The advantages of photography – documentation and the ability to accumulate light – compensated the inconveniences connected with photographing and processing of photos. At the beginning of the 20th century photography became the sole governer of astronomy. All the most important observations like the measurement of the location and the movement of celestial bodies, photometry (the measurement of brightness) and the spectral observations were carried out with the help of photography. The only field, where photography was less important than visual observation, was the study of the details of the surface of planets.

Josef Petzval and his objectives. It must immediately be said that the popular title “Petzval astrograph” is absolutely erroneous. The name gives an impression as if the camera has been made by Petzval. In fact, both the camera and the objective have been made in the Zeiss Company and the instrument’s full name is “The astrograph with the Petzval-type objective” - this has given the shortened name.

Josef Petzval (1807–1891) was a mathematician from Hungary, the professor of the Buda Technology Institute and later the professor of Vienna University. His hobby was photography and film making. In the 1840s the main problem of photography was insufficient light sensitivity of photo materials and this is why exposition times were minutes and even hours. Especially big inconveniences happened in taking portrait photos when the person had to sit without any movements and even blinking of eyes could fully spoil the result. The only possibility to shorten the exposition time was to enlarge the diameter of the objective, but it also brought along the increase of the focal length, it made the field of view smaller. To place the object in the picture, the camera had to be moved farther from the object. It was accompanied by the decrease of light and the expected effect was not achieved.

The mathematician Petzval solved the problem geometrically. Combining different lenses (not real but their mathematical models!) and drawing and calculating the light rays in such systems, he invented the objective consisting of four lenses which allowed to decrease the proportions between the distance of the focus and the diameter of the objective from the earlier 16 to 3.6. As a result, the time of making a portrait photo decreased from earlier 10 minutes to 15-30 seconds.

The Petzval portrait objective from 1856. Between the two double lenses there is the diaphragm with the changeable aperture – an absolutely contemporary solution.

Also, in astrophotography there were similar problems. One does not take portraits of celestial bodies but in their case the focal length determines the size of the field of the picture. The smaller the focal length, the bigger is the field and the more celestial bodies are in one picture. Photographing planets and the Moon, this problem does not exist because it is enough to have the field of view of half a degree or even smaller. In all the cases the camera`s limit of ability is determined by the diameter of the objective.

The Zeiss objective of the Petzval type with the diameter of 16 cm and the focal length 78 cm was of excellent quality and the largest in its time. It is sad that the best years of the instrument were the war years when it was not used.

The Petzval astrograph, as it was called in the observatory, was a combined instrument, the years of the construction of separate parts were different – even more than one hundred and fifty years. The main element was the Zeiss 16 cm camera from 1911, the leading telescope – the Troughton refractor from 1806 and the guide – the telescope from 1959 which was used for observing the satellites of the Earth. The instrument was built and placed to the pavilion of the zenith telescope located in the yard of the observatory.

The observationsVery quickly the Petzval astrograph became the most often used instrument in the observatory in the Estonian Republic before the World War II. Separate areas of the Milky Way were photographed to find the variable stars there (Simberg and Muischneek 1927–1932); the planet Mars was photographed (Livländer 1926 and 1928); the comets Baade (1922) and Wilk(1930) were observed, also the solar eclipse on 9 June 1936 (Kipper, Keres, Simberg). Aksel Kipper made the objective prism for the astrograph which allowed using the instrument for spectrographic research. In the years 1935-1940 astronomers photographed the spectra of stars (Kipper and Simberg 1935–1937); the spectrum of ξ Aurigae in the period of its minimal brightness (Kipper 1935) and the spectrum of the bright nova in the constellation of Hercules (Kipper and Simberg 1937–1940).

The most important observation in the Tartu Observatory in the post-war period was the program of patrol observations of the Milky Way started by G. Kusmin. During fourteen years (1950–1963) four areas in the belt of the Milky Way were photographed, all in all for 891 times. With one-hour exposition time on the photo plate of glass base 9 x 12 cm the images of stars were obtained up to the 15th magnitude. Every plate covered the area of the sky with the dimensions 6.5 x 8.5 degrees.

The obtained collection of plates, known as the Tartu phototeque, has offered much work for the researchers of variable stars. The first Candidate thesis was written by Heino Albo in 1964. He was followed by Peep Kalv, Voldemar Harvig and others. The value of the plate collection in good condition is increasing every year.

The four control fields of the Tartu phototeque located in the map of the sky.