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Message: <blockquote data-quote="henry link" data-source="post: 1317933" data-attributes="member: 6806">Kevin,I was referring to the Kowa quote in your post, not anything you said. In the Kowa 883 the Fluorite element is not external. It's sandwiched between two glass elements. Others, like Takahashi just coat external Fluorite elements, but I'm not sure the durability issues were ever as serious as detractors of Fluorite claimed. I've owned several old Tak FC series telescopes with exposed uncoated Fluorite elements which never had any problems, even after 20 years in a high humidity environment.Greg,I'm going to pass the buck and suggest that you do some Googling of terms like "Abbe number", "P-V diagram" or maybe just "optical glass". You'll find out much more than I can tell you. Basically the higher the Abbe number the lower the dispersion of the glass.Henry</blockquote>