The focal spot measurement jig consists of a cassette holder which can be set at any angle up to 20° above the horizontal.
 Mounted on the cassette holder is a tower which holds a 1° star pattern, supplied with the jig, giving a magnification factor of approximately 2.5x for an F.F.D. of 600mm.
At the base of the tower is a centre-marker (a circular ring) which, when concentric with the star pattern image, indicates that the jig is correctly aligned to the central beam axis.