Development of a new wideband heterodyne receiver system for the Osaka 1.85-m mm-submm telescope -- Receiver development & the first light of simultaneous observation in 230GHz and 345GHz bands with an SIS-mixer with 4-21GHz IF output

Abstract

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 superconductorinsulator-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 1 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 the 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 the on-the-fly (OTF) mappings toward the Orion KL and W 51.

Development of a new wideband heterodyne receiver system for the Osaka 1.85-m mm-submm telescope -- Corrugated horn & Optics covering 210-375 GHz band

“…The peak antenna temperature in 12 CO(J = 2-1) toward Orion KL observed by the present system is ∼45 K (Masui et al 2021), which is the same as Nishimura et al (2015) with a fluctuation of ∼a few percent. The OTF mapping results in 12 CO, 13 CO, and C 18 O(J = 2-1) are also consistent, including the temperature scale with the fluctuation.…”

“…On the other hand, if the diameter is large, the cutoff frequency of higher-order modes shifts to a lower value, causing higher-order modes on the high-frequency side. It is also necessary to match with the waveguide diplexer (Masui et al 2020(Masui et al , 2021 connected immediately after the horn. As a result of the optimization based on the above, the diameter of the circular waveguide was set to 0.91 mm.…”

“…We installed the horn/optical system developed in this study on the 1.85-m telescope and realized the simultaneous observation of six molecular lines (J = 2-1, 3-2) of CO isotopologues. The spectra toward Orion-KL and the OTF mapping results are shown in Masui et al (2021). Here, we derive the beam characteristics of the present system from the results of the commissioning observations from December 2020 to January 2021 with the remote operation.…”

“…For this purpose, we are developing a wideband receiver that covers the frequency coverage of 210-375 GHz (56% of the fractional bandwidth), where the above emission lines can be obtained simultaneously. The overall receiver development is described in Masui et al (2021). This paper focuses on the horn and the optics and presents the performance when installed on the telescope.…”