Shear measurement

Shear object detections and measurements in four “counterfactual” deep coadd images with different small shear values, plus a fifth with no shear. All shear measurements are stored in the catalog Data products.

Counterfactual images

The GalSim Shear class is used to create four “counterfactual” versions of the deep coadd images with different artificial linear shears introduced, using the “reduced shear” option with a step value (g1, g2) of +0.01 or -0.01 (GalSim documentation; Sheldon et al. 2023). The application of (weak lensing) shear to an image essentially performs a transformation that adds ellipticity with a set minor-to-major axis ratio and position angle, but approximately conserves area.

The five counterfactual images and their shear directions are:

• ns: no shear

• 1p: x-direction

• 1m: y-direction

• 2p: 45 degree diagonal

• 2m: 135 degree diagonal

Detection and measurement

All objects detected with a significance of \(> 5\sigma\) in a given counterfactual image have their shapes measured.

Two approaches are used for measurement (Yamamoto et al. 2025).

1. A Gaussian forward model fit across the r,i,z band images individually (gauss). The resulting ellipticity components in the image x-y (gauss_g1) and diagonal (gauss_g2) directions are measured.

2. A specialized Fourier-space method to measure a Gaussian weighted flux for each detection on a PSF-deconvolved image (pgauss). This flux measure has the advantage of matching the effective flux apertures between different bands, which is important for the measurement of object colors and photometric redshifts.

Both gauss and pgauss measure pre-PSF properties of the object.

In addition to the measurements above, the positions, sizes, and PSF shapes are also measured.

Resulting catalog

The results of shear detection and measurement are stored in the ShearObject catalog.

If a given galaxy is detected with \(> 5\sigma\) significance in all five counterfactual images, then it will populate five rows of the ShearObject table (with five separate shearObjectId).

The process of detecting and measuring objects in the counterfactual images, and the resulting ShearObject table, is completely independent of the process that creates the Object table. These two tables are not joinable and have independent (unique and unrelated) object identifiers.