fix: use absolute value of m in sine calculation for Zernike polynomials

Author: HarrisonKramer

optiland/zernike.py

@@ -129,7 +129,7 @@ def _azimuthal_term(self, m=0, phi=0):
         if m >= 0:
             return np.cos(m * phi)
         else:
-            return np.sin(m * phi)
+            return np.sin(np.abs(m) * phi)
 
     def _norm_constant(self, n=0, m=0):
         """Calculate the normalization constant of the Zernike polynomial.