multi: separate out the calculation of center (xy)

resolve/mosaicing/mosaic_likelihood.py

@@ -121,7 +121,6 @@ def build_response(
         If true w-gridding will be applied
     epsilon: float
         The desired accuracy of sky2vis.
-
     """
     domain = sky_beamer.target[field_name]
 

resolve/mosaicing/sky_beamer.py

@@ -71,6 +71,27 @@ class SkyBeamer(ift.LinearOperator):
         return ift.makeField(self._tgt(mode), res)
 
 
+def calculate_phase_offset_to_image_center(
+    sky_center: SkyCoord,
+    phase_center: SkyCoord,
+):
+    '''Calculate the relative shift of the phase center to the sky center
+    (reconstruction center) in radians.
+
+    Parameters
+    ----------
+    sky_center: astropy.SkyCoord
+        The world coordinate of the sky center.
+    phase_center: astropy.SkyCoord
+        The world coordinate of the phase center of the observation.
+    '''
+    r = sky_center.separation(phase_center)
+    phi = sky_center.position_angle(phase_center)
+    center_y = r.to(u.rad).value * np.cos(phi.to(u.rad).value)
+    center_x = r.to(u.rad).value * np.sin(phi.to(u.rad).value)
+    return center_x, center_y
+
+
 def build_sky_beamer(
     sky_domain: ift.DomainTuple,
     sky_center: SkyCoord,
@@ -121,14 +142,13 @@ def build_sky_beamer(
     ):
 
         direction = oo.direction_from_key(direction_key)
-
         o_phase_center = SkyCoord(direction.phase_center[0]*u.rad,
                                   direction.phase_center[1]*u.rad,
                                   frame=sky_center.frame)
-        r = sky_center.separation(o_phase_center)
-        phi = sky_center.position_angle(o_phase_center)
-        center_y = r.to(u.rad).value * np.cos(phi.to(u.rad).value)
-        center_x = r.to(u.rad).value * np.sin(phi.to(u.rad).value)
+
+        center_x, center_y = calculate_phase_offset_to_image_center(
+            sky_center, o_phase_center
+        )
 
         x = sky_coords.separation(o_phase_center)
         x = x.to(u.rad).value