Riding the wave

Astronomical Theodolite

The horizontal shaft for the rotation of the telescope, perpendicular to the optical axis, rests on two supports shaped like an "A" staying over an horizontal ring: on this instrument the engraving says "The Filotecnica Eng.A.Salmoiraghi, Milan 41691". (The Filotecnica was founded in Milan in 1864 by the astronomer Ignazio Porro and was then acquired by Eng.Salmoiraghi, who gave his name to the Company.) The shaft may be raised from its supports to exchange the ends, reversing the instrument.

One of the supports may be precisely moved vertically by means of a screw. In the horizontal ring there is a disc graded at intervals of 30' (engraved figures every 10 degrees): the divisions are read through two openings protected by a glass, placed 180 degrees apart.

At each opening there is a vernier divided into 50 parts which allows a minimum reading of 36" and is read by two microscopes.

On the shaft, on eyepiece side, there is a disc divided into intervals of 30' with engraved figures every 10°; two opposing verniers allow a minimum reading of 36". The readings are done through two microscopes. The two movements, horizontal and vertical, are micro regulated.

The weight of the objective of the telescope is compensated by a counterweight.

The eyepiece is at one end of the shaft and protrudes enough from it. It is fastened to a plate that runs perpendicular to the optical axis, so that the full image is easily observable. The grid is engraved on a glass plate.

The horizontal ring is supported by a rod with three arms ending with leveling screws. To check the position of the instrument there are two spirit levels, arranged perpendicular to each other. For the verification of the accurate position of the shaft, a spirit level with a frame is placed on supports at the ends of the shaft. On the supporting rod an arm carries a cylinder containing a magnetic needle: its position is observed through a small eyepiece.

Teodolite Astronomico

The tool is part of the collection of the Hydrographic Institute of Navy and was placed in its observatory.
By Paola Presciuttini
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