List of largest optical telescopes in the 19th century
List of largest optical telescopes in the 19th century, are listings of what were, for the time period of the 19th century large optical telescopes. See List of largest optical telescopes in the 20th century for the 1900s. The list includes various refractor and reflector that were active some time between about 1799 to 1901. The main reflecting technology early on, speculum metal reflected some 2/3 of light, and also had higher maintenance due to tarnishing. Another technology were the 2-element refractors, which were extensively used in 19th century observatories  despite their small apertures compared to the largest metal mirror, and later glass telescopes. The technology for silver-coated glass mirrors was developed in the mid-19th century, but was slow to catch on. A major technology advance of this time was the development of astrophotography, and some telescopes were tailored to this application. Also, a wide variety of scientific instruments were developed, such as for spectroscopy and various astronomical measurements. Reflectors & refractors. Early reflectors using speculum metal had some of the record-breaking apertures of the day, but not necessarily high performance. Starting in the 1860s metal coated glass ('Silver  on glass') reflector telescopes proved more durable, for example the Crossley Reflector, which continued to be used and upgraded even into the 21st century. Telescopes with lenses, especially achromatic doublets were popular in the 19th century. (see also Great Refractor) <nowiki>*</nowiki> (First light or Build Completion to Inactive (Retired) or Deconstruction)