Question on psf error map

[LyneVdV]

• psfr version: latest
• Python version: 3.9
• Operating System: Linux

Description

In the psf error map calculation, the expected noise level (i.e. sqrt(error_map_list[i])) is subtracted from the residuals (lines 358-360). Here are the lines of code I am talking about:

        # subtract expected noise level
        if error_map_list[i] is not None:
            residuals -= np.sqrt(error_map_list[i])

If I understand correctly, the goal is to avoid double counting the errors, i.e. separate the "expected gaussian+poisson noise" to the one which comes from a true difference between the star and the estimated psf, the latter being the only one which should be taken into account in the psf_error_map. Is it correct ? If so, why are we subtracting simply instead of quadratically substracting ? I am not sure of the right answer, I just wanted to be sure we are consistent.