A new off-point-less observing method for millimeter and submillimeter spectroscopy with a frequency-modulating local oscillator
| Autor: | Shigeru Takahashi, Nario Kuno, Kotaro Kohno, Akio Taniguchi, Yoichi Tamura, Jun Maekawa, Tetsuhiro Minamidani, Osamu Horigome, Takeshi Sakai |
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| Rok vydání: | 2019 |
| Předmět: |
Local oscillator
Superheterodyne receiver FOS: Physical sciences 01 natural sciences law.invention Optics law 0103 physical sciences Spectroscopy Instrumentation and Methods for Astrophysics (astro-ph.IM) 010303 astronomy & astrophysics Physics Sideband 010308 nuclear & particles physics business.industry Astronomy and Astrophysics methods: data analysis techniques: imaging spectroscopy Space and Planetary Science Modulation Millimeter Radio frequency methods: observational Astrophysics - Instrumentation and Methods for Astrophysics business Frequency modulation techniques: spectroscopic atmospheric effects |
| Zdroj: | Publications of the Astronomical Society of Japan. 72 |
| ISSN: | 2053-051X 0004-6264 |
| Popis: | We propose a new observing method for single-dish millimeter and submillimeter spectroscopy using a heterodyne receiver equipped with a frequency-modulating local oscillator (FMLO). Unlike conventional switching methods, which extract astronomical signals by subtracting the reference spectra of off-sources from those of on-sources, the FMLO method does not need to obtain any off-source spectra; rather, it estimates them from the on-source spectra themselves. The principle is a high dump-rate (10 Hz) spectroscopy with radio frequency modulation (FM) achieved by fast sweeping of a local oscillator (LO) of a heterodyne receiver: Because sky emission (i.e., off-source) fluctuates as $1/f$-type and is spectrally correlated, it can be estimated and subtracted from time-series spectra (a timestream) by principal component analysis. Meanwhile astronomical signals remain in the timestream since they are modulated to a higher time-frequency domain. The FMLO method therefore achieves (1) a remarkably high observation efficiency, (2) reduced spectral baseline wiggles, and (3) software-based sideband separation. We developed an FMLO system for the Nobeyama 45-m telescope and a data reduction procedure for it. Frequency modulation was realized by a tunable and programmable first local oscillator. With observations of Galactic sources, we demonstrate that the observation efficiency of the FMLO method is dramatically improved compared to conventional switching methods. Specifically, we find that the time to achieve the same noise level is reduced by a factor of 3.0 in single-pointed observations and by a factor of 1.2 in mapping observations. The FMLO method can be applied to observations of fainter ($\sim$mK) spectral lines and larger ($\sim$deg$^{2}$) mapping. It would offer much more efficient and baseline-stable observations compared to conventional switching methods. 22 pages, 15 figures, 2 tables, accepted for publication in Publications of the Astronomical Society of Japan |
| Databáze: | OpenAIRE |
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