Abstract: An optical image (54) of an astronomical scene is focussed by a telescope (16) onto a charge-coupled-device (CCD) photosensor array (50) to produce successive electronic pixel images (92) of the scene which are registered and co-added to produce an accumulated image (96) which is substantially free of streaking caused by rotation of the earth. A current image (92) is registered with an accumulated image (94) by sensing the coordinates of a selected bright star (90) in both images (92,94), computing the offset .DELTA.I between the coordinates, and shifting the current image (92) by the offset .DELTA.I prior to addition to the accumulated image. The telescope (16) may have a clock drive (24) to which the offset .DELTA.I is also applied to correct for mechanical tracking errors.