¹²CO spectra of the observation toward Orion KL using the OPU 1.85 m radio telescope. (a) ¹²CO spectrum obtained using a conventional receiver system (2SB SIS mixers), and (b) its leakage into the image band. (c) ¹²CO spectrum obtained using the 2SBF-Rx with the 220/230 GHz FSF, and (d) its leakage into the image band.