Position Angle Correction, CPA
The waveplate is not achromatic, hence the polarization
position angle correction (CPA) is wavelength dependent.
From broad band imaging polarimetry the CPA is
|| BD+59 389
The numbers are average from three nights (except U-band one night)
and typical position angle accuracy is better than 0.4 degrees
(except U-band about 0.8 degrees).