The McClean Telescope actually consists of several refractors mounted together: the main ones are: a 24-inch (61 cm) for photography, an 18-inch (46 cm) for visual work and a guider of 8 inches (20 cm). ("refractor" means that they collect light via a lens rather than by a mirror) The telescope was built by Howard Grubb of Dublin and completed 1899-1901. The 24-inch was used originally with a spectrograph and later for taking glass photographic plates to measure the distances of the stars by parallax. The visual refractor was used for photography, photometry and measuring double stars. The observer usually adjusted the telescope's tracking via the guide telescope during long exposures but in later years an autoguider (shown) was used. Spectroscopy was also done with the Giant Objective Prism (see link below).