Interferometric plural-dimensional displacement measuring system

A method of setting up a measuring system having a bed (10), a first stage (12) movable in a first direction (Y) relative to the bed, a second stage (18) movable in a second direction (X), generally at right angles to the first direction, relative to the first stage, and a plural-dimensional displacement measuring system employing a first interferometer system (FIG. 5A) for measuring linear displacement generally in the first direction between a first reference reflector (66) and a first target reflector (70) fixed relative to the first stage and producing a corresponding first output signal, a second interferometer system (FIG. 5B) for measuring linear displacement generally in the second direction between a second reference reflector (72) and a second target reflector (82) fixed relative to the second stage and producing a corresponding second output signal, and means (64) for processing the output signals to determine linear displacement of the second stage relative to the bed in the first and second directions, the second interferometer system including a intermediate reflector (74) fixed relative to the first stage and arranged to produce a beam (98,100) that extends generally in the first direction from a position fixed relative to the bed to the intermediate reflector and that, upon reflection thereby, extends generally in the second direction from the intermediate reflector to the second target reflector, the method comprising the steps of:

• adjusting the orientation of the first interferometer system so that the measuring direction thereof is parallel to the direction of movement of the first stage relative to the bed;
• and adjusting the orientation of the first stage at a reference position so that the direction of movement of the second stage relative to the first stage at that reference position is parallel to the measuring direction of the second interferometer system.