The backstaff is made from a mahogany frame with boxwood arcs and an inlaid boxwood plate in the main strut, with brass rivets. It has three boxwood vanes; a sight vane, a shadow vane and a second shadow vane with a Flamsteed lens. This lens and its wooden frame may be an old replacement, or may have been taken from another backstaff. The shadow vanes have brass pressure plates for holding them tightly in place on the sixty degree arc, and the sight vane had an inlaid brass pinhole sight. Decorative fleurs-de-lis are stamped in the frame.

The transversal scale on the thirtydegree arc is from 0° to 25° by 5 arcminutes, and reads to 1 arcminute along the fiducial edge of the sight vane. The graduation on the sixty degree arc is from 0° to 65° by 1°, and on the outer edge has a scale for correcting for the Sun’s sixty degree arc 190 mm. Overall length of the instrument is 631 mm.