Metrology apparatus

An apparatus comprising first and second relatively reorientable members and an indexer arrangement which is configured to provide a plurality of angularly indexed lockable positions of the first and second members about a first axis, in which the indexer arrangement comprises:

• i) a series of features provided on the first member, the series of features extending annularly around the first axis;
• and
• ii) on the second member, at at least three discrete locations annularly-spaced around the first axis, an engagement feature is provided which is configured to intermesh with a subset of the features on the first member when in the locked state thereby providing, at each indexed position, a stable relative rest position of the first and second members;
• in which the apparatus further comprises at least a first non-contact sensor mounted to the second member, the first non-contact sensor being configured to sense a region on the first member and thereby provide a signal dependent on the spatial configuration of the first and second members when in their locked state.