Precision optical surveying instrument manufactured by the historic German company Otto Fennel Söhne (later geoFennel). These instruments were widely respected for their accuracy in measuring horizontal and vertical angles in surveying and construction. 

Manufacturer

Otto Fennel Söhne was a German company, founded in Kassel, that became a world-renowned manufacturer of geodetic and surveying instruments. The company, which changed its name from the original in 1891 after being taken over by the sons Adolf and Otto Fennel, was known for high-quality, reliable mechanical and optical instruments. Although the original company closed in the 1970s, the production rights and legacy were continued by other firms, including the modern geoFennel. 

The Instrument

A theodolite is a precision optical instrument consisting of a movable telescope mounted on a base that can rotate around both horizontal and vertical axes. It uses internal magnifying optical systems (or digital readouts in modern versions) to measure angles with high accuracy, often down to seconds of arc. 

Key features of the Otto Fennel Söhne theodolites include:

• Optics: High-quality optics, with some models featuring internal focusing.

• Measurement: They measure both horizontal and vertical angles using graduated circles, which were read with magnifying lenses or vernier scales in older models.

• Durability: They were known for robust construction suitable for field conditions.

• Transit Capability: Many were "transit" theodolites, meaning the telescope could be revolved through a complete circle in the vertical plane, a feature useful for eliminating certain measurement errors.