A sensor apparatus includes an illumination system, a detector system, and a processor. The illumination system is con-figured to transmit an illumination beam along an illumination path and includes an adjustable optic. The adjustable optic is configured to transmit the illumination beam toward a diffraction target on a substrate that is disposed adjacent to the illumination system. The transmitting generates a fringe pattern on the diffraction target. A signal beam includes diffraction order sub-beams that are diffracted by the diffraction target. The detector system is configured to collect the signal beam. The processor is configured to measure a char-acteristic of the diffraction target based on the signal beam. The adjustable optic is configured to adjust an angle of incidence of the illumination beam on the diffraction target to adjust a periodicity of the fringe pattern to match a periodicity of the diffraction target.