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The corrugated horn is a high-performance feed often used in radio telescopes. There has been a growing demand for wideband optics and corrugated horns in millimeter- and submillimeter-wave receivers as they improve observation efficiency and allow us to observe important emission lines such as CO in multiple excited states simultaneously. However, in the millimeter/submillimeter band, it has been challenging to create a conical corrugated horn with a fractional bandwidth of ∼60% because the wavelength is very short, making it difficult to make narrow corrugations. In this study we designed a conical corrugated horn with good return loss, low cross-polarization, and symmetric beam pattern in the 210–375 GHz band (56% fractional bandwidth) by optimizing the dimensions of the corrugations. The corrugated horn was installed on the Osaka 1.85 m mm–submm telescope with matched frequency-independent optics, and simultaneous observations of 12CO, 13CO, and C18O (J = 2–1, 3–2) were successfully made. We describe the new design of the corrugated horn and report the performance evaluation results including the optics.

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We have developed a wideband receiver system for simultaneous observations in CO lines of J = 2–1 and J = 3–2 transitions using the Osaka 1.85 m mm–submm telescope. As a frequency separation system, we developed multiplexers that connect three types of diplexers, each consisting of branch-line couplers and high-pass filters. The radio frequency (RF) signal is eventually distributed into four frequency bands, each of which is fed to a superconductor–insulator–superconductor (SIS) mixer. The RF signal from the horn is divided into two frequency bands by a wideband diplexer with a fractional bandwidth of $56\%$, and then each frequency band is further divided into two bands by each diplexer. The developed multiplexers were designed, fabricated, and characterized using a vector network analyzer. The measurement results showed good agreement with the simulation. The receiver noise temperature was measured by connecting the SIS-mixers, one of which has a wideband 4–21 GHz intermediate frequency (IF) output. The receiver noise temperatures were measured to be ∼70 K in the 220 GHz band, ∼100 K in the 230 GHz band, 110–175 K in the 330 GHz band, and 150–250 K in the 345 GHz band. This receiver system has been installed on the 1.85 m telescope at the Nobeyama Radio Observatory. We succeeded in simultaneous observations of six CO isotopologue lines with the transitions of J = 2–1 and J = 3–2 toward the Orion KL as well as on-the-fly mappings toward the Orion KL and W 51.