Strange Filter Issue

[Stub Mandrel]

The only complaint I have about my Zeiss 135mm lens is that I get a violet halo round bright blue stars. Tonight I got to try out using a skylight filter (which stops UV and far blue) that should control this effect.

Could I focus? Denebola wouldn't resolve to anything other than a line, nor would Arcturus, just a long, thin line. The bokeh was really weird, like a cube.

Removing the filter stopped the nonsense altogether and it was dead easy to get denebola to a sharp point a couple of pixels across.

How on earth can a plain glass filter spread points into lines?