This list of largest optical telescopes in the 19th century includes what were large optical telescopes for their time. See List of largest optical telescopes in the 20th century for later telescopes. The list includes various refractors and reflectors that were active at some time between about 1799 to 1901.
The main material used early on for reflecting telescope mirrors was speculum metal, which reflected only about two-thirds of the incident light, and which tarnished, requiring maintenance. Two-element refracting telescopes were extensively used in 19th century observatories despite their smaller apertures than metal, and later glass, mirror telescopes.
The technology for silver-coating glass mirrors, more reflective than speculum metal and not subject to tarnishing, was developed in the mid-19th century but was slow to be adopted. A major technology advance of this time was the development of photography, permitting astrophotography, and some telescopes were tailored to this application. A wide variety of scientific instruments were developed for use with telescopes, such as for spectroscopy and various astronomical measurements.