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sweverett / Balrog-GalSim

Modules for GalSim that will be useful for DES images in conjunction with Balrog.
MIT License
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Truth tables aren't saving updated ellipticities for rotated objects #76

Closed sweverett closed 5 years ago

sweverett commented 5 years ago

Balrog currently leaves the truth table unmodified except for the updated positions - any other changes are expected as additional columns. However this is a problem for shapes; if random rotations are used, the final ellipticity axes are uncorrelated with the truth table!

This is best left for update_columns() for each BalObject, but we should save the rotation angle in the truth table as well so the new shape can be constructed with an afterburner if not yet implemented in update_columns().

sweverett commented 5 years ago

The needed rotation is a standard spinor rotation matrix, as outlined here: https://github.com/esheldon/ngmix/blob/master/ngmix/shape.py#L97. We do not need to update g_cov as (1) we are rendering truth objects exactly from the parametrizations, and (2) we do not even save g_cov to the truth table because of (1).