Data Preparation

We need a lens and a source catalog to calculate the galaxy-galaxy lensing signal. Several publicly available datasets are supported, as described in the Applications section. Once we have obtained the data, we need to convert it into a format understandable by dsigma.

Generally, dsigma expects all lens, source, and calibration catalogs to be astropy tables with data stored in specific, pre-defined columns.

Lens Catalog

The following columns are required for lens catalogs.

  • ra: right ascension in degrees

  • dec: declination in degrees

  • z: best-fit redshift

  • w_sys: systematic weight \(w_{\mathrm{sys}}\)

The weight \(w_{\mathrm{sys}}\) is often used to mitigate systematics in the lens selection.

Source Catalog

The following columns are required for source catalogs.

  • ra: right ascension in degrees

  • dec: declination in degrees

  • z: best-fit photometric redshift

  • w: inverse variance weight for galaxy shape

  • e_1: + component of ellipticity

  • e_2: x component of ellipticity

Additionally, the following columns may be used in the analysis.

  • m: multiplicative shear bias

  • m_sel: selection bias

  • e_rms: root mean square ellipticity

  • R_2: HSC resolution factor (0=unresolved, 1=resolved)

  • R_11, R_22, R_12, R_21: METACALIBRATION shear response

  • z_bin: tomographic redshift bin, non-negative and starts at 0

  • z_l_max: maximum lens redshift used for lens-source pairs

Redshift Distributions

For most modern weak lensing surveys, sources are divided into tomographic bins. Each tomographic bins has a redshift distribution \(n(z)\). These can be specified as follows:

  • z: redshift grid

  • n: \(n(z)\), must be two-dimensional, i.e., the value for each bin at redshift \(z\)

Calibration Catalog

As an alternative to redshift distributions, photometric redshifts can be corrected using the \(f_{\rm bias}\) correction, which requires a calibration catalog.

  • z: best-fit photometric redshift

  • z_true: “true” redshift

  • w: inverse variance weight for galaxy shape

  • w_sys: systematic weight \(w_{\mathrm{sys}}\)

The weight \(w_{\mathrm{sys}}\) is used to offset, for example, color differences between the source and the calibration catalog.