Stand of the 20ft (reflector) telescope and portions of its mounting. Two fifteen foot spars of black wood and cogs. The tube of this telescope is catalogued separately as AST0950 as is the original specula mirror (AST0786) that was in position in the tube when those parts were first brought to the Museum.

The 20ft was used chiefly for the discovery and examination of nebulae and star clusters with low powers. When it was first constructed, it was used with a flat, i.e. Newtonian mirror arrangement, but William Herschel soon discarded the small mirror to gain more light and never reverted to the other form, though it was retained in all his smaller instruments. This new type of set up made it a Herschelian rather than a Newtonian reflector telescope. The mirror was cast in 1817 and completed in 1818. Although the original 20ft was made by William alone, it was rebuilt, with new mirrors jointly by William and John Herschel. This stand could be the replacement made by John and William Herschel in 1820 using the parts of the original telescope that had not decayed.