This is the octagonal tube of William Herschel's 20ft (reflector) telescope and portions of its mounting. The tube is made of wood, painted dark grey and is in good condition. It is reinforced with metal bands.

The original specula (mirror) was in position when the telescope was first brought to the Museum but has since been catalogued as a separate object AST0786. The hinged cover for the mirror end of the tube opens and is screwed to the main structure. The portions of mounting consist of oak beams and iron rack bars. The metal framework painted light grey for suspending the eye-end of the instrument, complete with chain and hooks, was made by Department of the Environment (around 1970s) and is not original. There are also 7 cogwheels of varying sizes, a large hook and bolts.

The 20ft was used chiefly for the discovery and examination of nebulae and star clusters with low powers and was William Herschel's favourite observing instrument. When it was first constructed, it was used with a flat, i.e. Newtonian mirror arrangement, but Herschel soon discarded the small mirror to gain more light, & 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. It was made jointly by William and John Herschel. This tube could be the replacement made by John and William Herschel in 1820 using the parts of the original telescope that had not decayed.