I have a pair of these I purchase years ago, hardly ever used, but they work a treat. The reason for 2 is from memory one gives something around 2Deg correction and the two piggy backed gives 4, now this is only from memory. They were used on Mars and got rid of pretty well all the red and blue dispersion.
I cannot find out what I paid for these, but from memory it was over $300 for the pair. I am asking $200 plus shipping.
