The ast.zero_snr parameter will mask the map after each iteration based on the the signal to
noise ratio within each map pixel. For example, if it is set to 5, after each iteration all map pixels with an SNR
below this threshold will be forced to zero. the mask is re-evaluated on each iteration. An ast.zero_snr value of
zero means no SNR mask is used. See also parameter
"ast.zero_snr_ffclean" . [0.0]