Optimization by Smoothed Bandpass Calibration in Radio Spectroscopy

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, 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.