The Merz refractor telescope (1862)


Giovanni Virginio Schiaparelli, director of the Astronomical Observatory of Brera from August 1862, commissioned it to the German constructor Georg Merz in November 1862.

The constructor was well known to Schiaparelli, who could learn of his ability during the period when he studied in Pulkovo, under the guide of Friedrich August Winnecke and Otto Sturve.

In Pulkovo, Schiaparelli actually learnt the techniques of observation by the use of his own 15 inches refractor built by Georg Merz and Joseph Mahler in 1839 for the Russian Imperial Observatory.

Schiaparelli needed a new telescope because of the inadequacy of the instruments then available in the Observatory. His request was quickly accepted by the Italian government, thanks to the interest of Quintino Sella; the astronomer's discovery of the new planetoid Esperia, happened in Brera on the night of April 29th 1861, was a recent credit to him, and threw its weigh on the approval of the purchase.

The telescope was delivered in the summer 1865, but only at the end of 1874 it was put in the new dome opportunely projected and built on the north-eastern tower of the Observatory, where there was an Amici reflector before. Regular observations started in February 1875, after a preliminary period devoted to verify the properties of the instrument.

At the beginning, Schiaparelli wanted to use the telescope to observe comets, double stars, and planetoids, a field in which he had already obtained very good results. But, with the aim to investigate the optical qualities of the instrument, Schiaparelli began, quite casually, a long series of planetary observation that would have strengthened his image of an astronomer meticulous in his observations and cautious in his theorizations, and that would have made him known among the public.

Schiaparelli observed Mars for a long time, starting from the particularly favourable 1877 opposition, during which he identified on the planet several dark and straight lines, producing a complex system, that were called canals. The objective of the refractor has a diameter of 218 mm (8.05 French inches), and a focal length of 3.15 m (f/14.5).

The tube of the telescope, made of deal with a mahogany veneer, is sustained by an equatorial mounting: the polar axle and the declination one are perpendicular like the lines of a capital T. The equatorial mounting is a characteristic typical of the instruments built by Joseph von Fraunhofer, of whom Merz was a pupil, and is an excellent method to support very long tubes: thanks to an efficient system of counterbalances, the tube can be moved by a light pressure.

The tube is at one end of the declination axle, supported by a brass cot; the declination circle and the counterbalances are at the other end. The polar axle, fixed to the base of the telescope, points to the celestial North pole and lies in the local meridian plane; the right ascension circle and an adjustable counterbalance are at the lower end.

The axles are made of iron; the remaining elements are of brass with different composition, and thus with different resistance, depending on the mechanical function they are prefixed to.

The mounting rests on a cast iron pyramidal base, whose decorative panels are lost, built by the Tecnomasio of Milano; the inscription "PARATVM AERE PVBLICO ANNO MDCCCLXII/C. MATTEVCCI ET F. BRIOSCHI REM LITERARIAM GERENTIBVS" is fused on it. As the dome is at an altitude of about thirty meters above ground-level, the pyramidal base rests on a masonry cylinder, unconnected with the floor of the dome, in order to minimise the influence of the structural vibrations of the building on the sensitivity of the instrument. The dome itself rests on the load-bearing structures of the palace.

The micrometric regulation, both of the declination axle and of the right ascension one, can be made staying at the ocular and acting on two long sticks. Schiaparelli had at his disposal a very good micrometer with a movable thread, an annular micrometer, 7 positive oculars adaptable to the threaded micrometer (with a magnification from 87 to 690), and 6 negative ones (with a magnification from 67 to 468): the oculars are lost.

The telescope is equipped with a finder and a disposal to illuminate the field and the threads of the micrometer, made of two lamps and of a system of internal elliptical small mirrors to reflect the light. The clockwise motion is ruled by a clockwork and weight-falling device similar to Fraunhofer's one.

The tube is pointed to the object one wants to observe, one stops the declination movement and the mechanisms permits the clockwise axle to rotate slowly at the revolution speed every 24 hours such as to be able to follow the apparent movement of the stars. It seems that this disposal never worked with precision and Schiaparelli himself often complained about in his writings.

The most remarkable characteristic of the instrument is with no doubt the optical system: the objective of the refractor is a doublet (crown-flint), up to now in a very good state of conservation notwithstanding its natural ageing.

The objective presents a slight green colour that was present at Schiaparelli's time already, it turns achromatic in the red-green portion of the spectrum and causes an excess of blue. Such excess could influence the evaluation of the colours in the observations of the double stars and of the planetary details; Schiaparelli corrected the effect by the use of a deep yellow or orange filter. Because of this characteristic, the instrument came out particularly suitable to observe Mars, with its peculiar reddish surface. With the refractor, Schiaparelli also observed the other planets of the Solar System but without the very good results he had obtained for Mars.

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