The new infrared sensor technology (NIRST) is an infrared radiometer on board the SAC-D/Aquarius mission, launched on June 10, 2011. It is composed of middle wave infrared (MWIR) and a long wave infrared (LWIR) cameras, with three arrays of 512 microbolometers each. The three MWIR camera arrays operate in the spectral range, whereas LWIR camera arrays 1 and 2 operate in the spectral range and array 3, in the spectral range. NIRST also has a pointing Be mirror that covers the range of [30 deg, 60 deg], where 45 deg corresponds to nadir pointing. The aim of this work is to present the on-ground NIRST mirror’s characterization. The characterization was performed in an ambient environment by acquiring data at three different pointing angles, , at several blackbody sources’ temperatures for both cameras. The on-ground mirror characterization revealed that the laboratory blackbody sources used present a temperature gradient on their emitting surfaces. The results also reveal the system response to be affected by the pointing angle, i.e., the data at 60 deg was noisier that at 30 deg.