The 13 foot x 17.5 foot aperture end of the 120-foot long Calibration Horn Antenna, otherwise known as the “Little Big Horn,” at Green Bank. It was built in 1959 to observe the sky’s strongest non-solar radio source, Casseopeia A, and measure its total power output at a frequency of 1.4 GHz (L-band, 20cm wavelength). It was also used to map Cas A’s spectrum and take absolute temperature measurements of the background sky.