Spectral Modulation, or Ripple, in Retardation Plates for Linear and Circular Polarization

High-resolution spectropolarimetry sometimes suffers from a spectral modulation in polarization and position angle. It is shown that this artifact can be attributed to multiple reflections within the plates of the wave plate assembly and that the effect is inherent to the use of wave plates except at the design wavelength. The treatment given applies to strictly plane-parallel wave plate elements and complete coherence, and consequently the predicted ripple amplitudes are upper limits. It is possible that plates of poorer optical quality will not show this effect so prominently, but otherwise a simple modification to normal observing practice can be used to eliminate the ripple.