Controls how multiple AST masks are combined AST.ZERO_UNION
"
ast.zero_lowhits"
and parameter
"
ast.zero_snr"
), then they are combined into a single mask. If ast.zero_union is true (i.e. non-zero), then the
source region in the combined mask is the union of the source regions in the individual masks. If
ast.zero_union is false (i.e. zero), then the source region in the combined mask is the intersection of the
source regions in the individual masks. [1]