This quadrant was given to William Herschel by fellow-astronomer Alexander Aubert in 1786. The quadrant was fixed to the large 20ft telescope by William on 24 June 1789 and was later used with his 40ft telescope.

Like other astronomical quadrants, this instrument is made up of a minimum of two radial bars joined together at one end at right angles to each other, and a scale, covering a quarter of a circle's circumference, joining the two bars at their other ends. The scale is marked out in degrees from 0 to 90. Pivoted at the point at which the two bars meet (at the centre of the circle) is a moveable sighting bar.

In this quadrant the sighting bar is a telescope and there is a level across one of the radial bars. To use this quadrant you would first hold or mount the quadrant so the level shows that bar to be in the horizontal. You would then move the telescope to point at your chosen celestial body. Next you read the scale at the point at which the telescope crosses it and this gives you the angular height of the celestial body.

This quadrant is mounted on a mahogany board, complete with telescope and level and is in very good condition except for a small brass fitting for attaching the slow motion to the arm carrying the telescope.