diff --git a/csiborgtools/flow/mocks.py b/csiborgtools/flow/mocks.py
new file mode 100644
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+++ b/csiborgtools/flow/mocks.py
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+# Copyright (C) 2024 Richard Stiskalek
+# This program is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by the
+# Free Software Foundation; either version 3 of the License, or (at your
+# option) any later version.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General
+# Public License for more details.
+#
+# You should have received a copy of the GNU General Public License along
+# with this program; if not, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+"""Mock data generators."""
+import numpy as np
+
+from ..field.interp import evaluate_cartesian_regular
+from ..params import SPEED_OF_LIGHT
+from ..utils import cartesian_to_radec, radec_to_cartesian, radec_to_galactic
+from .cosmography import distmod2dist, distmod2redshift
+
+###############################################################################
+#                        Mock Quijote observations                            #
+###############################################################################
+
+
+def mock_Carrick2MTF(velocity_field, boxsize, RA_2MTF, DEC_2MTF,
+                     a_TF=-22.8, b_TF=-7.2, sigma_TF=0.25, sigma_v=100.,
+                     Vext=[150.0, 10.0, -100.0], beta=0.4,
+                     mean_eta=0.069, std_eta=0.078, mean_e_eta=0.012,
+                     mean_mag=10.31, std_mag=0.83, mean_e_mag=0.044,
+                     seed=42, Om0=0.3, **kwargs):
+    """
+    Mock TFR catalogue build against the Carrick velocity field and the
+    2MTF sky distribution to avoid recomputing the LOS velocities.
+    """
+    nsamples = len(RA_2MTF)
+
+    # Convert Vext from ICRS to Galactic coordinates.
+    Vext = np.asarray(Vext).reshape(1, 3)
+    Vext_mag, Vext_RA, Vext_DEC = cartesian_to_radec(Vext).reshape(-1, )
+    Vext_l, Vext_b = radec_to_galactic(Vext_RA, Vext_DEC)
+    Vext_galactic = np.asanyarray([Vext_mag, Vext_l, Vext_b]).reshape(1, 3)
+    Vext = radec_to_cartesian(Vext_galactic).reshape(-1, )
+
+    truths = {"a": a_TF, "b": b_TF, "e_mu": sigma_TF, "sigma_v": sigma_v,
+              "Vext": Vext,
+              "mean_eta": mean_eta, "std_eta": std_eta,
+              "mean_mag": mean_mag, "std_mag": std_mag,
+              }
+
+    gen = np.random.default_rng(seed)
+
+    # The Carrick box is in the Galactic coordinates.
+    l, b = radec_to_galactic(RA_2MTF, DEC_2MTF)
+    gal_phi = np.deg2rad(l)
+    gal_theta = np.pi / 2 - np.deg2rad(b)
+
+    # Sample the linewidth of each galaxy from a Gaussian distribution to mimic
+    # the MNR procedure.
+    eta_true = gen.normal(mean_eta, std_eta, nsamples)
+    eta_obs = gen.normal(eta_true, mean_e_eta)
+
+    # Subtract the mean of the observed linewidths, so that they are
+    # centered around zero. For consistency subtract from both observed
+    # and true values.
+    eta_mean_sampled = np.mean(eta_obs)
+    eta_true -= eta_mean_sampled
+    eta_obs -= eta_mean_sampled
+
+    # Sample the magnitude from some Gaussian distribution to replicate MNR.
+    mag_true = gen.normal(mean_mag, std_mag, nsamples)
+    mag_obs = gen.normal(mag_true, mean_e_mag)
+
+    # Calculate the 'true' distance modulus and redshift from the TFR distance.
+    mu_TFR = mag_true - (a_TF + b_TF * eta_true)
+    mu_true = gen.normal(mu_TFR, sigma_TF)
+
+    # Convert the true distance modulus to true distance and cosmological
+    # redshift.
+    r = distmod2dist(mu_true, Om0)
+    zcosmo = distmod2redshift(mu_true, Om0)
+
+    # Calculate the Cartesian coordinates of each galaxy. This is initially
+    # centered at (0, 0, 0).
+    pos = r * np.asarray([
+        np.sin(gal_theta) * np.cos(gal_phi),
+        np.sin(gal_theta) * np.sin(gal_phi),
+        np.cos(gal_theta)])
+    pos = pos.T
+
+    pos_box = pos / boxsize + 0.5
+    vel = evaluate_cartesian_regular(
+        velocity_field[0], velocity_field[1], velocity_field[2],
+        pos=pos_box, smooth_scales=None, method="cubic")
+    vel = beta * np.vstack(vel).T
+
+    for i in range(3):
+        vel[:, i] += Vext[i]
+
+    # Compute the radial velocity.
+    Vr = np.sum(vel * pos, axis=1) / np.linalg.norm(pos, axis=1)
+
+    # The true redshift of the source.
+    zCMB_true = (1 + zcosmo) * (1 + Vr / SPEED_OF_LIGHT) - 1
+    zCMB_obs = gen.normal(zCMB_true, sigma_v / SPEED_OF_LIGHT)
+
+    # These galaxies will be masked out when LOS is read it because they are
+    # too far away.
+    distance_mask = r < 125
+    truths["distance_mask"] = distance_mask
+
+    sample = {"RA": RA_2MTF,
+              "DEC": DEC_2MTF,
+              "z_CMB": zCMB_obs,
+              "eta": eta_obs,
+              "mag": mag_obs,
+              "e_eta": np.ones(nsamples) * mean_e_eta,
+              "e_mag": np.ones(nsamples) * mean_e_mag,
+              "r": r,
+              "distmod_true": mu_true,
+              "distmod_TFR": mu_TFR}
+
+    return sample, truths