The backstaff has a lignum vitae frame with boxwood arcs, sight vane, and horizon vane. It has a sighting vane, horizon vane and shadow vane with a Flamsteed lens (glass is missing), 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. The sight vane has a brass pressure plate for holding it tightly in place on the sixty degree arc, and an inlaid brass pinhole sight. The shadow vane is probably of a later date.

Both arcs have decorative motifs - fleur-de-lys and stars - stamped into them. In addition, the number 23 is stamped onto the small arc, the horizon vane and the sighting vane.

The transversal scale on the thirty degree 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 correction for the Sun’s semi-diameter.