* Astronomy

Minus Violet Filters are used primarily to eliminating blue halos around bright stars in colour photography using refracting telescopes.   They will to a lesser extent reduce visual chromatic aberration in achromatic refractors, (though a yellow filter will probably be better).  The filter transmits 90% of the Green and Red while selectively reducing about 50% of the blue.
The Minus Violet Filters can be used as "visible" filters in RGBV (red-green-blue-visible) astrophotography with reflecting telescopes.
The Minus Violet filter blocks wavelengths shorter than about 420nm (Infrared filter blocks wavelengths shorter than about 400nm and all wavelengths longer than 750nm).
The 1.25" Minus Violet Filter threads into the barrel of standard 1.25" eyepieces.

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UV Transmission x-ray to 350nm <.1%
Blue Transmission approximately 50%,
Green and Red transmission approximately 90%
IR Transmission 750nm to 1050nm <.1%

Comparing Minus Violet filters to #58 wratten for Mitigation of Chromatic effects in Achromats; Double Stars

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Ed ~ As an update, these filters have become somewhat redundant with the introduction of the new (and improved) Semi-Apo, Contrast Booster and Fringe Killer filters.