diff --git a/pysiaf/aperture.py b/pysiaf/aperture.py
index 1532d2f1..7297ed4d 100644
--- a/pysiaf/aperture.py
+++ b/pysiaf/aperture.py
@@ -1064,7 +1064,8 @@ def idl_to_tel(self, x_idl, y_idl, V3IdlYAngle_deg=None, V2Ref_arcsec=None, V3Re
             if input_coordinates == 'cartesian':
                 # define cartesian unit vector as in JWST-PLAN-006166, Section 5.7.1.1
                 # then apply 3D rotation matrix to tel
-                unit_vector_idl = rotations.unit_vector_from_cartesian(x=x_idl*u.arcsec, y=y_idl*u.arcsec)
+                unit_vector_idl = rotations.unit_vector_from_cartesian(y=x_idl*u.arcsec, z=y_idl*u.arcsec)
+                unit_vector_idl[1] = aperture.VIdlParity * unit_vector_idl[1]                
 
             if input_coordinates == 'polar':
                 # interpret idl coordinates as spherical, i.e. distortion polynomial includes deprojection
@@ -1175,8 +1176,9 @@ def tel_to_idl(self, v2_arcsec, v3_arcsec, V3IdlYAngle_deg=None, V2Ref_arcsec=No
             unit_vector_idl = np.dot(l_matrix, unit_vector_tel)
 
             if output_coordinates == 'cartesian':
-                x_idl_arcsec, y_idl_arcsec = unit_vector_idl[0] * u.rad.to(u.arcsec), unit_vector_idl[
-                1] * u.rad.to(u.arcsec)
+                unit_vector_idl[1] = aperture.VIdlParity * unit_vector_idl[1]
+                x_idl_arcsec, y_idl_arcsec = unit_vector_idl[1] * u.rad.to(u.arcsec), unit_vector_idl[
+                2] * u.rad.to(u.arcsec)
             elif output_coordinates == 'polar':
                 unit_vector_idl[1] = self.VIdlParity * unit_vector_idl[1]
                 x_idl, y_idl = rotations.polar_angles(unit_vector_idl)
@@ -2620,4 +2622,4 @@ class RomanAperture(Aperture):
 
     def __init__(self):
         super(RomanAperture, self).__init__()
-        self.observatory = 'Roman'
\ No newline at end of file
+        self.observatory = 'Roman'
diff --git a/pysiaf/utils/rotations.py b/pysiaf/utils/rotations.py
index ee9c84da..676722c1 100755
--- a/pysiaf/utils/rotations.py
+++ b/pysiaf/utils/rotations.py
@@ -775,11 +775,11 @@ def unit_vector_from_cartesian(x=None, y=None, z=None):
         z_rad = z
 
     # handle array and scalar inputs
-    if np.array([x_rad]).all() and np.array([y_rad]).all():
+    if z is None:
         unit_vector = np.array([x_rad, y_rad, np.sqrt(1-(x_rad**2+y_rad**2))])
-    elif np.array([y_rad]).all() and np.array([z_rad]).all():
+    elif x is None:
         unit_vector = np.array([np.sqrt(1-(y_rad**2+z_rad**2)), y_rad, z_rad])
-    elif np.array([x_rad]).all() and np.array([z_rad]).all():
+    elif y is None:
         unit_vector = np.array([x_rad, np.sqrt(1-(x_rad**2+z_rad**2)), z_rad])
 
     if np.any(np.isnan(unit_vector)):