Inhomogeneous distribution of brightness in the split-element filter.

The inhomogeneous distribution of brightness in a split-element filter is discussed. This inhomogeneity is caused by the off-axis effect of the birefringent crystal and can seriously affect astronomical observations in the case of a large field of view. For example, the maximal brightness fluctuation is ~9.44% in a full-disk solar telescope whose maximal incident angle is 70 arcmin. We also discuss ways to avoid this disadvantage.