The DESHIMA 2.0 spectrometer, installed on the ASTE telescope, is designed to enable broadband, high-sensitivity observations in the submillimetre wavelength range. This design allows simultaneous observation across a wide frequency range, making it possible to detect multiple emission lines in a single measurement. This is particularly useful for identifying emission lines from distant galaxies with unknown redshifts, allowing redshift determination in a single observation. This study evaluates the sensitivity and noise limitations of DESHIMA 2.0, addressing whether its on-sky performance aligns with a photon-noise model and lab-measured instrument characteristics.

First, the sensitivity of DESHIMA 2.0, integrated with the ASTE telescope, is assessed under fixed tele- scope conditions by observing the atmospheric signal without nodding the telescope or beam-switching. Observations confirm that the instrument operates near the photon-noise limit, consistent with theoret- ical predictions and laboratory measurements.

Next, a combined ABBA chopping-and-nodding technique, which integrates beam-switching with nod- ding of the telescope, is implemented to mitigate atmospheric noise during observations. This study demonstrates that this approach effectively filters out atmospheric noise over integration times of up to 3000 seconds, achieving a standard deviation of the noise level on the order of 10−4 K in the 250 to 300 GHz range. These findings confirm that the instrument maintains photon-noise-limited perfor- mance under varying atmospheric conditions.

Finally, the observed spectrum of Mars is compared to its known spectral characteristics to validate the sensitivity and calibration of DESHIMA 2.0 for bright astronomical sources. Although the observed spectrum closely resembles the known spectrum of Mars, the remaining channel-to-channel deviations exceed what the photon-noise model predicts. This highlights the presence of systematic calibration er- rors and deviations in noise behaviour. These results underscore the need for further refinement of the calibration process and the ABBA chopping analysis to enhance performance for fainter astronomical sources.

Overall, this study establishes that DESHIMA 2.0 achieves photon-noise-limited performance under specific observational setups, marking an important milestone for high-sensitivity submillimetre astron- omy. However, further work is needed to optimize the system for detecting weaker signals from distant sources, such as dusty star-forming galaxies, where signal strengths are much weaker.