Compute the rotation from the ICRF into the alt-az system.

def _altaz_rotation(self, t):
        """Compute the rotation from the ICRF into the alt-az system."""
        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)