1. A method, comprising; conditioning a radiation beam using an illumination system; imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam using a reticle comprising a pattern image area and at least one reticle mark adjacent to the pattern image area; projecting an image of the patterned radiation beam onto a target portion of a substrate by a projection system, the substrate being held on a substrate table; illuminating the at least one reticle mark by the radiation beam to generate an aerial image of the at least one reticle mark; projecting the aerial image on an image sensor, the image sensor being arranged on the substrate table; collecting image data of the projected aerial image from the image sensor; obtaining, from the image data, positional parameters of the projected aerial image; and correcting any deviation of the positional parameters from a required position of the aerial image, wherein the correction comprises compensating an illumination beam induced thermal expansion of the reticle by an estimated magnification correction of magnification settings of the projection system, the estimated magnification correction being calculated from a prediction of a temporal thermal expansion of the reticle.