Context. The Deep Spectroscopic HIgh-redshift MApper (DESHIMA) is a broadband integrated superconducting spectrometer for millimeter (mm) and submillimeter (submm) wave astronomy based on kinetic inductance detectors (KIDs). The first generation, DESHIMA 1.0, was successfully tested in 2017. This paper describes the upgrade to DESHIMA 2.0 and presents its characterization in laboratory settings. Aims. We enhanced the performance by increasing the band coverage and efficiency while developing a fast sky-position chopper for the efficient removal of atmospheric fluctuations. Methods. The instrument features NbTiN superconducting microstrip (MS) filters with a low-loss a-SiC:H dielectric and an ultra-wideband leaky-wave antenna. A laboratory setup was designed, incorporating the cryostat housing cryogenic optics and integrated superconducting spectrometer chip comprising 339 KIDs connected to MS filters tuned for (sub)mm wave frequencies. Room-temperature mirrors on a hexapod stage allowed us to precisely position and align the optical elements. The sky-position chopper was positioned on a motor-controlled stage for a fine-tuned control over its position and alignment. The multiplexing capability of KIDs enabled us to simultaneously measure multiple performance metrics across the entire frequency range. Results. DESHIMA 2.0 achieved significant improvements in the performance compared to its predecessor: the measured instantaneous frequency coverage was 200–400 GHz with a mean filter Q_filter of 340 ± 50, the instrument efficiency reached ~8%, indicating a four times wider band coverage and a four times higher sensitivity. The yield rate for the MS filters exceeded 98%. The data loss caused by the sky-position chopper was limited to <20% without beam truncation. The estimated aperture efficiency from measured beam patterns agreed well with the designed value of approximately 70%. The telescope far-field beam patterns calculated from the measured patterns also agree well with the design specifications. We also demonstrated that a new method of absolute frequency calibration is valid using the data from the beam pattern measurement.

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.

Context. Integrated superconducting spectrometers (ISSs) for wide-band submillimeter (submm) astronomy use quasi-optical systems for coupling radiation from the telescope to the instrument. Misalignment in these systems is detrimental to the system performance. The common method of using an optical laser to align the quasi-optical components requires an accurate alignment of the laser to the submm beam from the instrument, which is not always guaranteed to a sufficient accuracy. Aims. We develop an alignment strategy for wide-band ISSs that directly uses the submm beam of the wide-band ISS. The strategy should be applicable in both telescope and laboratory environments. Moreover, the strategy should deliver similar quality of the alignment across the spectral range of the wide-band ISS. Methods. We measured the misalignment in a quasi-optical system operating at submm wavelengths using a novel phase and amplitude measurement scheme that is capable of simultaneously measuring the complex beam patterns of a direct-detecting ISS across a harmonic range of frequencies. The direct detection nature of the microwave kinetic inductance detectors in our device-under-test, DESHIMA 2.0, necessitates the use of this measurement scheme. Using geometrical optics, the measured misalignment, a mechanical hexapod, and an optimisation algorithm, we followed a numerical approach to optimise the positioning of corrective optics with respect to a given cost function. Laboratory measurements of the complex beam patterns were taken across a harmonic range between 205 and 391 GHz and were simulated through a model of the ASTE telescope in order to assess the performance of the optimisation at the ASTE telescope. Results. Laboratory measurements show that the optimised optical setup corrects for tilts and offsets of the submm beam. Moreover, we find that the simulated telescope aperture efficiency is increased across the frequency range of the ISS after the optimisation.