mast is a science and technology museum located in the grounds and architectural complex belonging to the former national observatory in rio de janeiro. soon after the museum was created, the historical heritage existing there, relating to a significant period in the history of science in brazil, was listed by the brazilian national heritage institute (iphan) and the rio de janeiro state cultural heritage institute (inepac) in 1986 and 1987, respectively. the studies that preceded the restoration of a theodolite from the mast collection started with an appraisal of the state of conservation of the institution's scientific instruments collection. next, a set of pre-established criteria were used to select this object, which was in a critical state of conservation. after this, historical research was undertaken of the instrument, the composition of its main parts was determined, a study was made into its working, and finally the intervention per se was undertaken on the object. the restoration procedure involved three stages: the conservation of parts, the restoration of parts, and the replacement of missing parts. the whole process was photographed exhaustively. the procedure adopted in this study has been replicated in further restoration projects. studies were undertaken into the conservation of scientific instruments and research was done into the construction of trajectories of objects and collections by the archive preservation group, based at mast, resulting in the identification of restoration as a singular moment in these trajectories.