Hilger Watts Theodolite Manual Link

Use a soft, clean camel-hair brush to remove dust. Clean lenses with photographic lens cleaner and lens tissue, never with abrasive cloth [3].

These were smaller, less precise instruments (often 20-60 arc-second), designed for construction and mining. hilger watts theodolite manual

A "one-second" high-precision instrument used for primary triangulation and geodetic surveying. Use a soft, clean camel-hair brush to remove dust