code: def _altaz_rotation(self, t):
          
          R_lon = rot_z(- self.longitude.radians - t.gast * tau / 24.0)
          return einsum('ij...,jk...,kl...->il...', self.R_lat, R_lon, t.M)
docs: Compute the rotation from the ICRF into the alt-az system.