A carriage printer includes a printhead; a print region; a carriage configured to move the printhead back and forth in a carriage scan direction across the print region, the carriage including: a first outer wall; a second outer wall that is separated from the first outer wall by a carriage width along the carriage scan direction; and an optical sensor assembly disposed on the carriage at a first distance from the first outer wall and at a second distance from the second outer wall, wherein the first distance and the second distance are both less than the carriage width.