A collimator unit includes an on-axis collimator tube fixedly attached to at least two off-axis collimator tubes. The off-axis collimator tubes are angled relative to the on-axis collimator tube. Each collimator tube can include a lens, a light source, and a target placed at the lens' focal point. The collimator tubes can be positioned within a housing such that the on-axis collimator tube is coaxial and centered with respect to the housing.