The backstaff is made from a lignum vitae frame with boxwood arcs, brass-capped rivets, and a boxwood sight vane, horizon vane and shadow vane, the latter with a Flamsteed lens, which allows the instrument to be used when the sun is obscured by cloud by focussing the light into a point that can be lined up on the horizon vane. There is an inlaid boxwood plate on the main strut. The sight vane and shadow vane have brass pressure plates to hold them tightly in place on the arcs and the sight vane has an inlaid brass pinhole. Small decorative motifs are stamped on the lower end of the thirty degree arc.


The transversal scale on the thirty degree arc is from 0° to 25° by 5 arcminutes and reads to 0.5 arcminutes. The graduation on the sixty degree arc is from 0° to 65° by 1°.

Details of the instrument's manufacture appear on a plaque on the frame: ' Made by Will Garner For Oliver Thompson 1734' and the initials 'O T' for Oliver Thompson are stamped. Garner was apprenticed in 1713 and is known to have been working in London by the 1730s.