Abstract: A prealigner includes a light source and light detector that detects the position of the periphery of a substrate as a rotary chuck rotates the substrate. The prealigner includes a processor that receives and records the peripheral position of the substrate relative to the angular position of the substrate. The prealigner determines the center of the substrate by fitting a curve to the recorded data set, after eliminating any spikes in the data, e.g., caused by the notch or parts of the chuck. The center may then be easily and accurately determined using the amplitude and phase of the fitted curve. The position of the notch is determined by fitting a curve to the data set for the notch and determining the derivative of the curve. The mid-point of the notch and the offset error of the apex of the notch from the mid-point are calculated based on the derivative.