dsf.covariance.ingredients.noise module

Noise terms for projected DeltaSigma covariance calculations.

This module defines the discreteness and shape-noise terms that enter simple DeltaSigma covariance models. The functions distinguish between lens shot noise, which comes from the finite number of lens galaxies, and source shape noise, which comes from the intrinsic ellipticity scatter of source galaxies.

The returned quantities are intended to be combined with survey geometry, redshift-window factors, and matter power spectra when building covariance blocks for galaxy-matter lensing and related projected observables.

dsf.covariance.ingredients.noise.angular_shape_noise(sigma_e, n_eff_arcmin2)

Return source shape noise for an angular shear field.

Shape noise describes the variance from the intrinsic ellipticity scatter of source galaxies. This is the angular form used when the source density is treated as a number per solid angle.

Parameters:

• sigma_e (float) – Per-component intrinsic ellipticity dispersion.

• n_eff_arcmin2 (float) – Effective angular source galaxy density in arcmin^-2.

Returns:

Angular shape-noise amplitude in steradian units.

Return type:

float

dsf.covariance.ingredients.noise.projected_shape_noise(sigma_e, n_eff_arcmin2, chi_eff)

Return source shape noise in projected comoving coordinates.

This is the shape-noise contribution expressed on the transverse comoving plane at an effective lens distance. It is useful for DeltaSigma covariance calculations written in projected physical or comoving coordinates rather than purely angular coordinates.

Parameters:

• sigma_e (float) – Per-component intrinsic ellipticity dispersion.

• n_eff_arcmin2 (float) – Effective angular source galaxy density in arcmin^-2.

• chi_eff (float) – Effective comoving lens distance in Mpc / h.

Returns:

Projected shape-noise amplitude in (Mpc / h)^2.

Return type:

float

dsf.covariance.ingredients.noise.shot_noise(n_gal)

Return lens-galaxy shot noise from a three-dimensional number density.

Shot noise describes the discreteness noise from representing the lens galaxy field with a finite number of galaxies. A denser lens sample has a smaller shot-noise contribution, while a sparse lens sample has a larger one.

Parameters:

n_gal (float) – Comoving three-dimensional lens galaxy number density in (h / Mpc)^3.

Returns:

Poisson shot-noise amplitude in (Mpc / h)^3.

Return type:

float