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Paper: Optimization by Smoothed Bandpass Calibration in Radio Spectroscopy
Volume: 476, New Trends in Radio Astronomy in the ALMA Era
Page: 407
Authors: Kameno, S.; Yamaki, H.; Mizuno, I.; Beppu, H.; Imai, H.; Kuno, N.; Akashi, T.
Abstract: 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, SAM45, on the Nobeyama 45-m telescope. The optimal smoothing windows were determined as 501, 28, 18, and 4 ch for bandwidths of 2000, 500, 125, and 15.6 MHz, respectively. The optimal scan pattern was designed as sequences of [60-s ON + 10-s OFF] and [48-s ON + 16-s OFF] scan pairs for narrow and wide bandwidth, respectively. The noise level with the SBC was reduced by factors of 1.8 and 1.3 compared with the conventional method. We also found sporadic instability (bursts) in the total power and spectra in IF during the test observations. Although the bursts significantly distorts the resultant spectra, we found a solution to cancel the spectral variation by decomposing them into static and burst components. This solution can be a hint to ease spectral flagging processes and to bring better efficiency in integration time.
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