This thesis describes the opto-and electro-mechanical concept of a modern astronomical spectrograph with imaging capabilities as it is realised in the ultraviolet-visual FOcal Reducer / low dispersion Spectrograph -Fors. The focal reducing optics reimages the telescopic focal plane onto the CCD detector with a demagnification that is required to match the detector's pixel size with the natural stellar seeing disk. This focal reducer is converted from an imaging photometer into a spectrometer by inserting dispersive optical elements between collimator and camera optics into the parallel beam.Fors became the very first instrument at the optical 16-m 'Very Large Telescope' of the European Southern Observatory. The four 8-m telescopes of the VLT have turned meanwhile into a world-class observatory on the 2 600 m high Paranal mountain in the coastal range of Northern Chile. The Fors imager and spectrograph is realised in two copies Fors1 and -2 for the Cassegrain foci of the VLT unit telescopes Antu (UT1) and Kueyen (UT2) which offer the deepest look from earth into space due to its optics layout, the focus position in lack of a prime focus and the supreme site conditions.The Fors imager and spectrographs are multi-mode instruments offering up to ten different observing modes in numerous configurations. Direct imaging as well as spectroscopy are the prime modes. Additionally, the Fors instruments -located at the Cassegrain foci -offer the only opportunity at the VLT for linear and circular polarimetry as well as spectro-polarimetry. Due to the multi-purpose observational applications and functions, the imager and spectrographs have become a 3 m large instrument of 2 metric tonnes. Their position at the Cassegrain focus requires special solutions in structural stiffness due to the slew range and aspect angle change in the gravitational field during tracking on celestial objects.The thesis will give the scientific rationale for building such instruments. It describes the conceptual design in order to solve the specific problems of this kind of imager and spectrograph. It covers the construction, realization and the results of testing thoroughly the instrument mechanics. Those were performed with the aid of a telescope and star simulator and later confirmed through its performance at the real telescope.The thesis addresses as well the scientific aims and programmes that followed the 1st Scientific Light of the ESO-VLT in the night of September 15th, 1998. One science programme, called the Fors Deep Field, became common to all three partner institutions of the 'VLT Instrument Consortium'. The program was performed during the 'Guaranteed Observing Time' period to get answers on the tantalizing question; how the universe looked like when it was still very young during the period of galaxy formation which probably could be answered at least partially by the combination of the new powerful telescopes with high-performance instruments. Both machines appeared adequate in performing such a programme by taking very deep images an...
