I just wonder that scattering angle in 2-d periodic target.

In pbcscavec.f90, I found that

! scattering angle is neither zero nor pi

! scattering plane is therefore defined by k_0 and k_s .

Why scattering angle is neither zero nor pi?

And I carried out with following ddscat.par

'RCTGL_PBC' = CSHAPE*9 shape directive

30.0 1.0 1.0 1.0 1.0 = shape parameters 1 - 5

BETA = 0.000

THETA= 30.000

PHI = 0.000

and I got the out file 'w000r000k000.sca' like that

Mueller matrix elements for selected scattering directions in Target Frame

theta phi Pol. S_11 S_12 S_21 S_22 S_31 S_41

30.00 180.00 0.00009 9.9946E-01 -8.9653E-05 -8.965E-05 9.995E-01 5.805E-23 5.528E-25

150.00 180.00 0.81600 1.3728E-05 -1.1202E-05 -1.120E-05 1.373E-05 1.161E-24 1.372E-26

Why scattering directions are theta=30 and theta=150?

I can't understand.

Please advise to me.