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 is plano-convex.It already has one curve generated convex to fit the concave side of the SF12 flint glass blank.
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 inside curves has been done for you. Make yourself a first class refractor. Asking $145.00. I have several sets of these. Buyer pays shipping and I do not accept Paypal. Thanks for looking.