William Optics 2 Inch Violet Reducer VR Filter

The WO VR-1 is a 2" Minus Violet filter that tremendously reduces the effects of chromatic aberration (purple halo and blue glow) that achromatic refractors return on bright objects.

It's particularly suited for color and black & white photography. It works by absorbing light in the violet spectrum (around 400 nm) for more pinpoint, "APO" star views.