Laboratory holographic testbed. Left: 1/15^th-scale FYST reflector system (400 mm diameter) with the signal receiver (①) mounted at the focal plane; the reference receiver (②) is placed beside the telescope to provide a phase reference. Right: 300 GHz transmitter source (③), mounted on an XY translation stage (④), is positioned approximately 5 m from the center of the M1 reflector to illuminate the telescope. The telescope beam is measured by raster-scanning the source using the XY stage.