RETROCAM was designed for the unused auxiliary instrument port of the MDM 2.4m Hiltner Telescope's Multi-Instrument System (MIS).  We modified the calibration pickoff mirror in the MIS to redirect light from the telescope to the MIS side-port when the other side of the pickoff was directing calibration light into the main instrument (like a spectrograph).  RETROCAM had a 5-position filter wheel and a 5x3 arcminute field of view. It was designed to allow the 2.4m to conduct optical monitoring projects requiring long time baselines but using only a small fraction of a night.  It is permanently mounted on the 2.4m and so co-exists seamlessly with the direct Cassegrain focus instrument, making the 2.4m a 2-instrument telescope that can switch between the main instrument and RETROCAM in about 1 minute.  It was used to conduct a number of synoptic studies, including multi-year photometric monitoring of lensed quasars, from 2004 until its retirement in 2020.