Need for school project to upgrade telescope. Focal length must be close to 60", ie: 12.5" f/4.8 or 13.1" f/4.5. Also strongly prefer that it be 1.6" thickness. Must have very good optics and coatings; I am replacing rather than recoating an old mirror which is full thickness and heavy. Could also use primary cell to fit 15" ID tube if available.