Chinnici - Merz Telescopes A global heritage worth preserving

CNR, Fondazione Scienza E Tecnica, Florence, Italy

Paolo Brenni

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It is generally accepted that the beginning of the saga of the great 19th century refractors coincides with the realization of the one made by Joseph von Fraunhofer (17871826) for the Observatory of Dorpat (today Tartu , Estonia ). In 1824, Friedrich Georg Wilhelm von Struve (17931864), the director of the observatory, while admiring the assembled telescope, wrote:

Moreover, the optics of the instrument fully satisfied Struve, who added: So I absolutely consider our achromatic lens the most perfect masterpiece of optics that has ever existed (Struve ).

This telescope certainly was the mechanical and optical marvel of the time, and today is rightly considered the father of all the large 19th century refractors (Fig. ). The instrument has several peculiar and important characteristics:

Certainly, a number of these elements were not new and had been included in other, older, instruments, but, for the first time, they were combined together in a remarkably large, very efficient and powerful refractor. The instrument proved to be successful and, soon, other similar and larger instruments were constructed.

Fraunhofer started to build another refractor, similar to the one at Dorpat, for the Berlin Royal Observator but, in 1826, his premature death, at the age of 39 years, put a sudden stop to his brilliant career. However, Fraunhofers legacy was not lost. First, Georg Merz (17931867) and Joseph Mahler (17951845), and then Georgs sons and grandsons, not only successfully continued the production at the famous optical institute of Munich , but also increased the importance and enlarged the market of the firm, up to the beginning of the 20th century.), Harvard (1847, aperture 38 cm), and Moscow (1859, aperture 49 cm), as well as the objectives for the refractors of Strasburg (1880, aperture 49 cm), Milan (1881, aperture 49 cm), and Manila (1893, aperture 51 cm). Certainly, the golden era of large Merz refractors covers the period between the end of the 1830s and the early 1880s. After that time, the firm built a few more instruments of this kind or supplied large objectives to others.

With the increasing focal length and aperture of the large equatorials, the structure of the instruments became more complex and required technical solutions which an optician and instrument-maker could no longer master. This is one of the reasons why, in the second half of the 19th century, the Merz firm, which also refused to hire people outside Bavaria , lost its leading position in the construction of large telescopes. Indeed, it continued to supply large and excellent objectives, but for the realization of the mechanical parts of refractors, it was necessary to turn to specialized engineering firms. Late 19th century refractors were heavy machines and, at the same time, delicate instruments. The massive mountings, the big counterweights, the robust axles and their bearings, the long metallic tubes, all were typical products of heavy mechanical engineering. For example, the steel polar axe of the Yerkes Observatorys main telescope was more than 4 m long and weighed 3.5 tons, while the declination axe was about 335 cm long and weighed 1.5 tons (Fig. ). Nevertheless, the large divided circles and their reading microscopes, the sophisticated driving mechanism that compensated for the movement of the Earth and kept the observed object in the field of vision of the telescope so that good astronomical photographic plates could be taken, were elements requiring the precision work typical of the high quality instrument industry. Moreover, good and very large achromatic objectives could be ground and polished only by a few very skilled and very specialized opticians who had to order the best glass blanks (often weighing several hundred kilograms) from the only three or four glassmaking companies capable of supplying them. Therefore, while the Merz firm had, for a good number of years, been able to produce the optical glass and fashion the large lenses, as well as the mechanical parts of the telescope, in the second half of the 19th century, very large refractors were often made thanks to the collaboration of different actors (engineers and instrument-makers, opticians and glassmakers). Among them, a few names have particularly to be recalled.