We investigate the 1/f noise of the Five-hundred-meter Aperture Spherical Telescope (FAST) receiver system using drift-scan data from an intensity mapping pilot survey. All the 19 beams have 1/f fluctuations with similar structures. Both the temporal and the 2D power spectrum densities are estimated. The correlations directly seen in the time series data at low frequency f are associated with the sky signal, perhaps due to a coupling between the foreground and the system response. We use Singular Value Decomposition (SVD) to subtract the foreground. By removing the strongest components, the measured 1/f noise power can be reduced significantly. With 20 modes subtraction, the knee frequency of the 1/f noise in a 10 MHz band is reduced to $1.8 \times 10^{-3}\Hz$, well below the thermal noise over 500-seconds time scale. The 2D power spectra show that the 1/f-type variations are restricted to a small region in the time-frequency space and the correlations in frequency can be suppressed with SVD modes subtraction. The residual 1/f noise after the SVD mode subtraction is uncorrelated in frequency, and a simple noise diode frequency-independent calibration of the receiver gain at 8s interval does not affect the results. The 1/f noise can be important for HI intensity mapping, we estimate that the 1/f noise has a knee frequency (fk)∼ 6 × 10−4Hz, and time and frequency correlation spectral indices (α)∼0.65, (β)∼0.8 after the SVD subtraction of 30 modes. This can bias the HI power spectrum measurement by 10 percent.
Reference:
1/f Noise Analysis for FAST HI Intensity Mapping Drift-Scan Experiment, Wenkai Hu, Yichao Li, Yougang Wang, Fengquan Wu, Bo Zhang, Ming Zhu, Shifan Zuo, Guilaine Lagache, Yinzhe Ma, Mario G. Santos, Xuelei Chen, MNRAS accepted, arXiv:2109.06447