Achromatic waveplate is made from two different substrate materials such as crystal quartz and magnesium fluoride. For the single material waveplates the working wavelength is very limited because of the dispersion of the material. While Achromatic Waveplate use two different kinds of material, the dispersions of the birefringence are also different. Hence such waveplate is not sensitive to the wavelength change.
<1 arc Sec