Namaster

NaMaster — pseudo-Cl power spectra for masked fields

You're helping a user write code with NaMaster (pymaster), DESC's pseudo-Cl angular power spectrum estimator (Alonso et al. 2019). NaMaster is the Fourier-space workhorse of TXPipe: TXTwoPointFourier and its subclasses use it for all 3×2pt Cl measurements, and TXFourierNamasterCovariance uses it for Gaussian covariance estimation.

Overview

Four core objects (import as import pymaster as nmt):

Minimal workflow:

import pymaster as nmt
import healpy as hp

nside = 512
mask = hp.read_map("mask.fits")
mask = nmt.mask_apodization(mask, aposize=1.0, apotype="Smooth")

# Spin-0 (density/convergence): one map
f0 = nmt.NmtField(mask, [delta_map], n_iter=0)

# Spin-2 (shear): two maps [g1, g2]
f2 = nmt.NmtField(mask, [g1, g2], n_iter=0, lmax=lmax)

# Binning
b = nmt.NmtBin.from_nside_linear(nside, nlb=40)   # linear, width-40 bands
# OR: b = nmt.NmtBin.from_edges(ell_lo, ell_hi)

# Compute coupling matrix once, reuse across tomographic bins
w = nmt.NmtWorkspace.from_fields(f0, f0, b)  # or w.compute_coupling_matrix(f0, f0, b)
w.write_to("workspace.fits")   # persist to disk

# Two equivalent approaches:
# (A) all-in-one (recomputes MCM internally — slow if reusing)
cl = nmt.compute_full_master(f0, f0, b)

# (B) decouple manually (fast when workspace is precomputed)
pcl = nmt.compute_coupled_cell(f0, f0)
cl  = w.decouple_cell(pcl)

Spin-2 output ordering — cross-correlating two spin-2 fields returns 4 spectra: [EE, EB, BE, BB] at indices 0–3. Shear×density returns [E, B].

See references/api.md for detailed parameter tables and gotchas.

TXPipe usage

NaMaster is TXPipe's Fourier-space workhorse: TXTwoPointFourier, TXTwoPointFourierCatalog, TXFourierNamasterCovariance, and the CMB lensing cross-spectrum stage all use it. See references/txpipe.md for stage details, workspace management, noise estimation, the choose_ell_bins helper, and the flip_g2 sign convention.

Reference