This is a minus-violet filter to reduce the violet halo found with inexpensive refractors. Used several times; I didn't need it on my scope. Excellent condition, in original packaging with papers, $50 shipped.
Here is what they say about the filter: "The MV20 filter is a variation to our Minus Violet filter. This is the second generation MV filter. The MV20 filter was designed with a more neutral appearance in mind. The mid-priced achromatic refractors would do well with this filter. The MV20 is allows a little more of the unfocused violet and deep blue found in typical refractors and balances it out with additional rejection of the yellow region to balance color rendition. Unlike the MV1, the MV20 does not reject the IR region."