Access to an internal calibration reference system during flight is an important requirement for contemporary remote sensing missions. L-3 Communications SSG-Tinsley has designed, built, and tested a novel internal calibration source based on ribbon sources. Via a flip-in mirror, the source assembly couples light through the field stop of an off-axis reimaging telescope to provide a reliable test of the following optics and electronics. Non-imaging illumination design principles assure uniform illumination of the sensor focal plane at levels adjustable over a very wide dynamic range. The source assembly can be packaged into a compact, lightweight, and efficient flight unit for convenient installation at an accessible location on the telescope. While the prototype source was specifically designed to match the GIFTS telescope as a representative example of an off-axis re-imaging telescope, the design principles are flexible to allow optimization for any comparable telescope system.