By a combination of quarter-wave plates made of different birefringent materials it is possible to produce achromatic quarter-wave plates whose degree of achromatism is dependant on the dispersions of birefringence and on the thicknesses of the individual quarter-wave plates. These waveplates are widely used in optical instrumentation and the residual errors associated with these devices can be very important in high resolution spectro-polarimetry measurements. The misalignment of optic axis in a double crystal waveplate is one of the main source of error and leads to elliptical eigenpolarization modes in the retarder and the oscillation of its orientation according to the wavelength. This paper will discuss, first, how the characteristics of a quartz-MgF2 quarter-wave plate is affected by such a misalignment. A correlation with the experiment is then achieved in order to highlight the interest of taking a possible tilt error into consideration when doing polarimetric measurements.
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