From 353a19e8e84bb3e729d6ddfb1b5e14168204d68c Mon Sep 17 00:00:00 2001
From: "Michael J. Williams" <michaeljw1@googlemail.com>
Date: Thu, 14 May 2026 18:14:20 +0100
Subject: [PATCH 1/2] ENH: add conversion from Earth frame to 'detector frame'

---
 bilby/gw/utils.py | 60 ++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 57 insertions(+), 3 deletions(-)

diff --git a/bilby/gw/utils.py b/bilby/gw/utils.py
index 420e1fc04..16782da98 100644
--- a/bilby/gw/utils.py
+++ b/bilby/gw/utils.py
@@ -7,12 +7,18 @@
 from scipy.special import i0e
 from bilby_cython.geometry import (
     zenith_azimuth_to_theta_phi as _zenith_azimuth_to_theta_phi,
+    theta_phi_to_zenith_azimuth as _theta_phi_to_zenith_azimuth
 )
 from bilby_cython.time import greenwich_mean_sidereal_time
 
-from ..core.utils import (logger, run_commandline,
-                          check_directory_exists_and_if_not_mkdir,
-                          SamplesSummary, theta_phi_to_ra_dec)
+from ..core.utils import (
+    logger,
+    run_commandline,
+    check_directory_exists_and_if_not_mkdir,
+    SamplesSummary,
+    theta_phi_to_ra_dec,
+    ra_dec_to_theta_phi,
+)
 from ..core.utils.constants import solar_mass
 
 
@@ -250,6 +256,28 @@ def zenith_azimuth_to_theta_phi(zenith, azimuth, ifos):
     return _zenith_azimuth_to_theta_phi(zenith, azimuth, delta_x)
 
 
+def theta_phi_to_zenith_azimuth(theta, phi, ifos):
+    """
+    Convert from the Earth frame to the 'detector frame'.
+
+    Parameters
+    ==========
+    theta: float
+        The zenith angle in the earth frame
+    phi: float
+        The azimuthal angle in the earth frame
+    ifos: list
+        List of Interferometer objects defining the detector frame
+
+    Returns
+    =======
+    kappa, eta: float
+        The zenith and azimuthal angles in the detector frame.
+    """
+    delta_x = ifos[0].geometry.vertex - ifos[1].geometry.vertex
+    return _theta_phi_to_zenith_azimuth(theta, phi, delta_x)
+
+
 def zenith_azimuth_to_ra_dec(zenith, azimuth, geocent_time, ifos):
     """
     Convert from the 'detector frame' to the Earth frame.
@@ -277,6 +305,32 @@ def zenith_azimuth_to_ra_dec(zenith, azimuth, geocent_time, ifos):
     return ra, dec
 
 
+def ra_dec_to_zenith_azimuth(ra, dec, geocent_time, ifos):
+    """
+    Convert from the Earth frame to the 'detector frame'.
+
+    Parameters
+    ==========
+    ra: float
+        The right ascension of the source in radians
+    dec: float
+        The declination of the source in radians
+    geocent_time: float
+        GPS time at geocenter
+    ifos: list
+        List of Interferometer objects defining the detector frame
+
+    Returns
+    =======
+    kappa, eta: float
+        The zenith and azimuthal angles in the detector frame.
+    """
+    gmst = greenwich_mean_sidereal_time(geocent_time)
+    theta, phi = ra_dec_to_theta_phi(ra, dec, gmst)
+    zenith, azimuth = theta_phi_to_zenith_azimuth(theta, phi, ifos)
+    return zenith, azimuth
+
+
 def get_event_time(event):
     """
     Get the merger time for known GW events using the gwosc package

From 544dab489adbc6cf7b6acbab853cd373f9a0f711 Mon Sep 17 00:00:00 2001
From: "Michael J. Williams" <michaeljw1@googlemail.com>
Date: Thu, 14 May 2026 18:14:36 +0100
Subject: [PATCH 2/2] TST: add test for Earth to detector conversion

---
 test/gw/utils_test.py | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/test/gw/utils_test.py b/test/gw/utils_test.py
index cf78849c7..a33430e02 100644
--- a/test/gw/utils_test.py
+++ b/test/gw/utils_test.py
@@ -293,6 +293,35 @@ def test_conversion_gives_correct_prior(self) -> None:
         self.assertGreaterEqual(ks_2samp(self.samples["ra"], ras).pvalue, 0.01)
         self.assertGreaterEqual(ks_2samp(self.samples["dec"], decs).pvalue, 0.01)
 
+    def test_inverse_conversion_gives_correct_prior(self) -> None:
+        ras = self.samples["ra"]
+        decs = self.samples["dec"]
+        times = self.samples["time"]
+        args = zip(*[
+            (ra, dec, time, self.ifos)
+            for ra, dec, time in zip(ras, decs, times)
+        ])
+        zeniths, azimuths = zip(*map(bilby.gw.utils.ra_dec_to_zenith_azimuth, *args))
+        self.assertGreaterEqual(ks_2samp(self.samples["zenith"], zeniths).pvalue, 0.01)
+        self.assertGreaterEqual(ks_2samp(self.samples["azimuth"], azimuths).pvalue, 0.01)
+
+    def test_sky_conversion_invertibility(self) -> None:
+        zeniths = self.samples["zenith"]
+        azimuths = self.samples["azimuth"]
+        times = self.samples["time"]
+        args = zip(*[
+            (zenith, azimuth, time, self.ifos)
+            for zenith, azimuth, time in zip(zeniths, azimuths, times)
+        ])
+        ras, decs = zip(*map(bilby.gw.utils.zenith_azimuth_to_ra_dec, *args))
+        args = zip(*[
+            (ra, dec, time, self.ifos)
+            for ra, dec, time in zip(ras, decs, times)
+        ])
+        zeniths_out, azimuths_out = zip(*map(bilby.gw.utils.ra_dec_to_zenith_azimuth, *args))
+        self.assertTrue(np.allclose(zeniths_out, zeniths))
+        self.assertTrue(np.allclose(azimuths_out, azimuths))
+
 
 def test_ln_i0_mathces_scipy():
     from scipy.special import i0