Abstract: A densitometer providing simultaneous sensing of the density in precisely adjacent portions of plural adjacent tracks. The densitometer preferably operates with stellar a pattern in which two reference tracks have been photographically recorded either side of a central specimen track of a star. The simultaneous detection of the three tracks insures a high degree of alignment between the reference and star tracks so that the correlation of their relative positions is a more accurate indication of the shift between the reference and star tracks. Alignment controls and the ability to microscopically view the radiation path through the densitometer facilitates set-up of the system to the high accuracy of results that it can achieve.