  We have developed the Smoothed Bandpass Calibration (SBC) method and the best
suitable scan pattern to optimize radio spectroscopic observations. Adequate
spectral smoothing is applied to the spectrum toward OFF-source blank sky
adjacent to a target source direction for the purpose of bandpass correction.
Because the smoothing process reduces noise, the integration time for
OFF-source scans can be reduced keeping the signal-to-noise ratio. Since the
smoothing is not applied to ON-source scans, the spectral resolution for line
features is kept. An optimal smoothing window is determined by bandpass
flatness evaluated by Spectral Allan Variance (SAV). An efficient scan pattern
is designed to the OFF-source scans within the bandpass stability timescale
estimated by Time-based Allan Variance (TAV). We have tested the SBC using the
digital spectrometer, VESPA, on the VERA Iriki station. For the targeted noise
level of 5e-4 as a ratio to the system noise, the optimal smoothing window was
32 - 60 ch in the whole bandwidth of 1024 ch, and the optimal scan pattern was
designed as a sequence of 70-s ON + 10-s OFF scan pairs. The noise level with
the SBC was reduced by a factor of 1.74 compared with the conventional method.
The total telescope time to achieve the goal with the SBC was 400 s, which was
1/3 of 1200 s required by the conventional way. Improvement in telescope time
efficiency with the SBC was calculated as 3x, 2x and 1.3x for single-beam,
dual-beam, and on-the-fly (OTF) scans, respectively. The SBC works to optimize
scan patterns for observations from now, and also works to improve
signal-to-noise ratios of archival data if ON- and OFF-source spectra are
individually recorded, though the efficiency depends on the spectral stability
of the receiving system.
