https://github.com/zxdawn/HyperGas/blob/a7d2c9d67aa7d0a8400a5702b9a1facbe3467363/hypergas/a_priori_mask.py#L89
Sometimes the plume is weak and the background is noisy, we need to adjust the thresholds from [2std, 3std] to a lower one (e.g., [1std, 2std]).