List of largest optical telescopes in the 18th century includes various refractors and reflectors that were active some time between about 1699 to 1801. It is oriented towards astronomy, not terrestrial telescopes (e.g., spyglasses).
Many of the largest were metal mirror reflectors, some of which had substational apertures even for the 20th century. One problem was that many instrument makers. including William Herschel, did not pass on their mirror-making craft, and by the next century reflectors had largely been passed over in favour of small achromats (2-lens refractors). It was not until the 20th century that really large reflectors would predominate once again.
The major breakthrough in the 18th century was the perfection of two and even three lens refractor telescopes, as well as the increased number of reflectors using polished metal mirrors rather than metal-coated glass, which was not developed until the mid-19th century.
The main telescope technologies during this period were refractors with non-achromatic objectives (single lens), speculum metal reflectors, refractors with achromatic doublets objective (doublet lens), and apochromatic triplets (after 1760s) objectives. The list is inherently limited by what examples and records have survived.
| Name(s) | Aperture cm (in) | Significance | Location then/Original Site | Extant* | |
|---|---|---|---|---|---|
| Herschel 40-foot (1.26 m diameter)[1] | 126 cm(49.5") | World's largest 1789; Mimas & Enceladus discovery telescope | 1789–1815 | ||
| Reverend John Michell's Gregorian reflector[2] | 75 cm (29.5") | World's largest 1780 | 1780–1789 | ||
| Herschel "X Feet" or "Large 10 Feet"[3] | 61 cm (24") | England | 1800 | ||
| Father Noel's Gregorian reflector | 60 cm (23.5") | World's largest 1761; 1796 Newtonian conversion | 1761-1841[4] | ||
| James Short Gregorian reflector | 50 cm (19.5") | World's largest 1750 | 1750 | ||
| Schröter 27 foot Newtonian[5] | 47 cm (18.5") | Lilienthal, Lower Saxony (Germany) | 1793–1813 | ||
| Herschel 20-foot[6] [7] | 47 cm (18.5") | Observatory House | 1782 | ||
| 46 cm (18.1") | Reflector – Gregorian | Spain | 1752 | ||
| James Short's Gregorian reflector | 38 cm (14") | World's largest 1734 | 1734 | ||
| Huygens aerial for Royal Society of London[8] | 19 cm (7.5") | London, England | 1691–1786[9] | ||
| William Herschel 7-foot | 16 cm (6.3") | Used to discover Georgium Sidus (Uranus) | England | 1776–1783 | |
| Hadley's Reflector[10] | 15 cm (6") | First parabolic newtonian | England | 1721 | |
| Van Deyl of Amsterdam telescope[11] | 11.4 cm (4.5") | achromat | England | 1781 | |
| James Short 4.5 inch reflector | 11.4 cm (4.5") | reflector | England | 1737[12] | |
| 10 cm (4.1") | achromat | First large equatorial[13] | Warwickshire, England | 1791–1923 | |
| Dollond Apochromatic Triplet[14] | 9.53 cm (3.75") | apochromat | First apochromatic triplet | England | 1763 |
| Dollond Triplet for Wollaston[15] | 9.02 cm | triple lens | RAS No. 16 | England | 1771[16] |
| Francesco Bianchini's aerial telescope[17] | 6.6 cm (2.6") | Rome, Italy | 1726 | ||
| Chester Moore Hall's Doublet | 6.4 cm (2.5") | achromat | First achromatic doublet | England | 1733 |
| Troughton Equatorial Telescope[18] | 5.08 cm (2") | achromat | Equatorial mount | Armagh Observatory, Ireland | 1795 |
| Newton's reflector[19] (1st) | 3.3 cm (1.3") | First reflecting telescope | England | 1668–1704 | |
* (First light or Build Completion to Inactive or Deconstruction)