A new contact type of on-machine measuring system is developed in order to measure aspherical optical parts with steep surface angle for large numerical aperture (NA). A ceramic air slider made of SIALON is adopted for the measurement probe because of its low thermal expansion coefficient, high rigidity and light weight. A high accuracy glass scale is employed to reduce the thermal drift of the displacement gauge. The air slider or the measuring probe is tilted for 45° against the aspherical workpiece axis, and the probe is scanned over the workpiece surface, so as to keep the contact angle between the probe axis and the contact surface constant in order to reduce the change in the probe friction force. © 2008.