Using flt.zero_lowhits causes samples to be excluded
from the filtering performed by the FLT model if they fall in regions of the map where the number of samples
falling in each pixel is higher than flt.zero_lowhits times the mean number of samples per pixel,
averaged over the map. A value of zero means that no masking of low hits regions is performed.
The mask is updated on each iteration. 
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East Asian Observatory