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 similar to 9.44% in a full-disk solar telescope whose maximal incident angle is 70 arcmin. We also discuss ways to avoid this disadvantage. (C) 1997 Optical Society of America.
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