Details

Type: Story

Status: Done

Resolution: Done

Fix Version/s: None

Component/s: None

Labels:None

Team:SQuaRE
Description
It seems that there are two commonly used conventions to define the “ellipticity”, which Bosch et al. 2017 refer to as the “distortion” and the “shear” (see Appendix 3 of https://arxiv.org/abs/1705.06766 , particularly footnote 8 ). This difference in definition is also described in the wikipedia article https://en.wikipedia.org/wiki/Gravitational_lensing_formalism#Measures_of_ellipticity . It seems that the LSST pipeline is using the “distortion” definition based on the discussion in Bosch et al. 2017, and this appears to be confirmed by Equation 2 of Mandelbaum et al. 2017 https://arxiv.org/abs/1705.06745 . The formula currently in `validate_drp` looks to be using the “shear” definition, e.g., slide 8 of this presentation by Catherine Heymans http://www.roe.ac.uk/~heymans/KSBf90_for_DUEL/Download_Lecture_Notes_files/Lecture1_CH.pdf . That same formula for the “shear” case also appears in the wikipedia article with the epsilon symbol. Meanwhile, it seems that the SRD is using the “distortion” definition (equations 10 and 11, https://docushare.lsst.org/docushare/dsweb/Get/LPM17 ). Therefore, it seems that for consistency, that we would also want to use the “distortion” convention in `validate_drp`. In that case, I think we would have
e = (I_xx  I_yy + 2j*I_xy) / (I_xx + I_yy) 
e1 = np.real(e) 
e2 = np.imag(e) 
Here are the relevant line in validate_drp:
Jim Bosch Could you please verify that the convention to be used for defining the ellipticity is the "distortion" and that the suggested implementation above for the ellipticity components in validate_drp is correct? Thanks!