A new off-point-less observing method for millimeter and submillimeter spectroscopy with a frequency-modulating local oscillator (FMLO). (arXiv:1911.02574v1 [astro-ph.IM])
<a href="http://arxiv.org/find/astro-ph/1/au:+Taniguchi_A/0/1/0/all/0/1">Akio Taniguchi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tamura_Y/0/1/0/all/0/1">Yoichi Tamura</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kohno_K/0/1/0/all/0/1">Kotaro Kohno</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Takahashi_S/0/1/0/all/0/1">Shigeru Takahashi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Horigome_O/0/1/0/all/0/1">Osamu Horigome</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Maekawa_J/0/1/0/all/0/1">Jun Maekawa</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sakai_T/0/1/0/all/0/1">Takeshi Sakai</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kuno_N/0/1/0/all/0/1">Nario Kuno</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Minamidani_T/0/1/0/all/0/1">Tetsuhiro Minamidani</a>

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.

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.

http://arxiv.org/icons/sfx.gif