Helicity correlated asymmetries caused by optical imperfections

False asymmetries in electron scattering experiments utilizing photoelectron sources are generated by imperfections of the light polarization optics. The false asymmetries are amplified by an analyzing power of the photocathodes. We have demonstrated compensation techniques which allow to approach zero intensity asymmetry with good stability. Even in the absence of analyzing powers fluctuations may be generated by more subtle effects like varying interference patterns of the exciting laser beam. (orig.)