I have for sale one piece of BK7 and one piece of SF12 optical glass for a 6" achromat. Here are the specs:
Index of Refraction = 1.517
Dispersion = 64.5
Edge thickness = 7/16"
The lens already has curves generated double convex.
Index of Refraction = 1.649
Dispersion = 33.8
Edge Thickness = 15/16"
The lens is plano-concave. One side has been slump molded approximately 5/16" to approximately match the convex surface of the Bk7. The other side is flat since the 4th side of an achromat would be almost flat. The hard work of hogging out the basic curves has been done for you. Make yourself a first class refractor. Asking $145.00. Buyer pays shipping and I do not accept Paypal. Thanks for looking.