I bought this unused newtonian primary as new "old stock" thinking it would be an exact replacement for my Edmund 4.25" f/4.1 mirror. Unfortunately it tests at f/5 (approx 21" or 530 mm f.l.) so I cannot use it. It is in excellent unused condition. It is likely a mirror as sold by Meade a few years back. I will sell for $60.00 shipped and insured. Continental U.S. only, no Paypal. Money order preferred, but check ok.