  The present paper describes the construction, the installation and the
operation of the Optical Imager Galileo (OIG), a scientific instrument
dedicated to the 'imaging' in the visible. OIG was the first instrument
installed on the focal plane of the Telescopio Nazionale Galileo (TNG) and it
has been extensively used for the functional verification of several parts of
the telescope (as an example the optical quality, the rejection of spurious
light, the active optics and the tracking), in the same way also several parts
of the TNG informatics system (instrument commanding, telemetry and data
archiving) have been verified making extensive use of OIG. This paper provides
also a frame of work for a further development of the imaging dedicated
instrumentation inside TNG. OIG, coupled with the first near-IR camera
(ARNICA), has been the 'workhorse instrument' during the first period of
telescope experimental and scientific scheduling.
