The invention provides a stage apparatus, comprising an object support comprising a ring shaped protrusion having an outer radius in a first plane, and configured to support an object with a radius in the first plane larger than the outer radius of the ring shaped protrusion. The stage apparatus further comprises a sensor module configured to detect the object support, and the object when it is arranged on the object support. The stage apparatus further comprises a processing unit configured to receive one or more signals from the sensor module, and to determine, based on said one or more signals, a position of the object relative to the ring shaped protrusion when the object is arranged on the object support. The processing unit is further configured to determine, based on said position of the object, an offset value representing the position of the object relative to the ring shaped protrusion.