q_to_tth & tth_to_q

src/diffpy/utils/scattering_objects/diffraction_objects.py

@@ -763,25 +763,27 @@ def q_to_tth(self):
 
             2\theta_n = 2 \arcsin\left(\frac{\lambda q}{4 \pi}\right)
 
+        Function adapted from scikit-beam.  Thanks to those developers
+
         Parameters
         ----------
         q : array
-            An array of :math:`q` values
+            The array of :math:`q` values
 
         wavelength : float
             Wavelength of the incoming x-rays
 
-        Function adapted from scikit-beam.  Thanks to those developers
-
         Returns
         -------
         two_theta : array
-            An array of :math:`2\theta` values in radians
+            The array of :math:`2\theta` values in radians
         """
         q = self.on_q[0]
         q = np.asarray(q)
         wavelength = float(self.wavelength)
         pre_factor = wavelength / (4 * np.pi)
+        if np.any(np.abs(q * pre_factor) > 1):
+            raise ValueError("Please check if you entered an incorrect wavelength or q value.")
         return np.rad2deg(2.0 * np.arcsin(q * pre_factor))
 
     def tth_to_q(self):
@@ -800,25 +802,28 @@ def tth_to_q(self):
 
             q = \frac{4 \pi \sin\left(\frac{2\theta}{2}\right)}{\lambda}
 
-
+        Function adapted from scikit-beam.  Thanks to those developers.
 
         Parameters
         ----------
         two_theta : array
-            An array of :math:`2\theta` values in units of degrees
+            The array of :math:`2\theta` values in units of degrees
 
         wavelength : float
             Wavelength of the incoming x-rays
 
-        Function adapted from scikit-beam.  Thanks to those developers.
-
         Returns
         -------
         q : array
-            An array of :math:`q` values in the inverse of the units
+            The array of :math:`q` values in the inverse of the units
             of ``wavelength``
         """
         two_theta = np.asarray(np.deg2rad(self.on_tth[0]))
+        if np.any(two_theta > np.pi):
+            raise ValueError(
+                "Two theta exceeds 180 degrees. Please check if invalid values were entered "
+                "or if degrees were incorrectly specified as radians."
+            )
         wavelength = float(self.wavelength)
         pre_factor = (4 * np.pi) / wavelength
         return pre_factor * np.sin(two_theta / 2)

tests/diffpy/utils/scattering_objects/test_diffraction_objects.py

@@ -231,6 +231,42 @@ def test_diffraction_objects_equality(inputs1, inputs2, expected):
     assert (diffraction_object1 == diffraction_object2) == expected
 
 
+def test_q_to_tth():
+    # valid q values including edge cases when q=0 or tth=180 after converting
+    # expected tth values are 2*arcsin(q)
+    actual = DiffractionObject(wavelength=4 * np.pi)
+    setattr(actual, "on_q", [[0, 0.2, 0.4, 0.6, 0.8, 1], [1, 2, 3, 4, 5, 6]])
+    actual_tth = actual.q_to_tth()
+    expected_tth = [0, 23.07392, 47.15636, 73.73980, 106.26020, 180]
+    assert np.allclose(actual_tth, expected_tth)
+
+
+def test_q_to_tth_bad():
+    # invalid wavelength or q values when arcsin value is not in the range of [-1, 1]
+    actual = DiffractionObject(wavelength=4 * np.pi)
+    setattr(actual, "on_q", [[0.6, 0.8, 1, 1.2], [1, 2, 3, 4]])
+    with pytest.raises(ValueError):
+        actual.q_to_tth()
+
+
+def test_tth_to_q():
+    # valid tth values including edge cases when tth=0 or 180 degree
+    # expected q vales are sin15, sin30, sin45, sin60, sin90
+    actual = DiffractionObject(wavelength=4 * np.pi)
+    setattr(actual, "on_tth", [[0, 30, 60, 90, 120, 180], [1, 2, 3, 4, 5, 6]])
+    actual_q = actual.tth_to_q()
+    expected_q = [0, 0.258819, 0.5, 0.707107, 0.866025, 1]
+    assert np.allclose(actual_q, expected_q)
+
+
+def test_tth_to_q_bad():
+    # invalid tth value of > 180 degree
+    actual = DiffractionObject(wavelength=4 * np.pi)
+    setattr(actual, "on_tth", [[0, 30, 60, 90, 120, 181], [1, 2, 3, 4, 5, 6]])
+    with pytest.raises(ValueError):
+        actual.tth_to_q()
+
+
 def test_dump(tmp_path, mocker):
     x, y = np.linspace(0, 5, 6), np.linspace(0, 5, 6)
     directory = Path(tmp_path)