A rotary stage is a component of a motion system used to restrict an object to a single axis of rotation. The terms rotary table or rotation stage are often used interchangeably with rotary stage. All rotary stages consist of a platform and a base, joined by some form of guide in such a way that the platform is restricted to rotation about a single axis with respect to the base. In common usage, the term rotary stage may or may not also include the mechanism by which the angular position of the platform is controlled relative to the base. Principle of operation In three-dimensional space, an object may either rotate about, or translate along, any of three axes. Thus, the object is said to have six degrees of freedom (3 rotational and 3 translational). A rotary stage exhibits only one degree of freedom (rotation about one axis). In other words, rotary stages operate by physically restricting 3 axes of translation and 2 axes of rotation. Bearing types Rotary stages consist of a platform that moves relative to a base. The platform and base are joined by some form of bearing which restricts motion of the platform to rotation about a single axis. A variety of different styles of bearings are used, each with benefits and drawbacks, making them more appropriate for some applications than for others. Plain bearing A plain bearing is simply two surfaces sliding against each other. Typically, a circular step on the platform mates snugly with a circular depression in the base allowing free rotation while minimizing side to side motion. A rotary stage built with this type of bearing is usually only used for coarse positioning and is adjusted manually simply by turning the platform. Index marks on either the base or the platform are often provided, allowing for somewhat repeatable positioning of the platform relative to the base. Rolling-element bearing This type of rotary stage includes ball bearing stages, crossed roller bearing stages, and possibly others. Any of a number of different rolling-element bearings may be employed. Typically, a pair of bearings is used and they are preloaded to take up any slack which could result in the stage platform lifting relative to the base. Position control methods Manual direct Some rotary stages are operated simply by turning the platform by hand. The platform may have index marks for setting different angular positions relative