We developed, characterized, and verified an alignment procedure for the DESHIMA 2.0 instrument, an ultra-wide-band spectrometer operating between 200 and 400 GHz, at the ASTE telescope. To this end, we mounted the warm optics, consisting of a modified Dragonian dual reflector system, on a motor-controlled hexapod. Crucial in the alignment procedure is our sky chopper, which allows fast beam switching. It has a small entrance and exit aperture coupling to the (cold) sky, which creates a measurable signal with respect to the warm cabin environment. By scanning the instrument beam across the entrance aperture of the sky chopper using the hexapod, we found the hexapod configuration that produced the lowest signal on our detectors, implying that the beam is coupled fully to the cold sky and not the warm cabin. We first characterized the alignment procedure in the laboratory, where we used a vat containing liquid nitrogen as the cold source behind the sky chopper. Then, we applied the alignment procedure to DESHIMA 2.0 at ASTE. We found that the alignment procedure significantly improved the aperture efficiency compared with previously reported values of the aperture efficiency of DESHIMA at ASTE, which indicates the veracity of the alignment procedure.

We present a phase and amplitude beam pattern measurement technique using harmonic mixers. This allows a simultaneous multi-frequency phase sensitive characterization of a low resolution and wideband (220-420GHz) on-chip spectrometer using microwave kinetic inductance detectors. We investigate the beam quality, in particular the beam pointing and inferred telescope coupling and hence aperture efficiency. The measurements match the goal requirements for the DESHIMA-2 instrument for the ASTE telescope. The technique would be of interest for any (direct detector) spectrometer with a wide instantaneous bandwidth, particularly ones with dispersive components.