Self aligning raster input scanner

A system for calibrating a raster input scanning device for detecting an image on a preselected surface, including a linear photoelectric sensor array comprising a plurality of discrete photosensitive elements arranged along a first axis, for sensing and generating electrical representations of an image on a surface; a carriage assembly movable along a second axis, transverse to said first axis; and supporting a sensor detectable target image for movement along the second axis. The sensor detectable target image includes an alignment feature for use during alignment operations. A drive system moves the carriage assembly carrying the target image from a starting position along said second axis, and the distance traveled by the carriage assembly from said starting point until the alignment feature on said target image is sensed is measured. A y-axis comparator compares the measured distance with a y-axis reference to determine an y-axis offset value which is stored in a y-axis memory as an y-axis offset value. Magnification and focus calibration are provided in similar fashion.