[Utah-astronomy] Finder solar filter from welder's glass

I am returning to an old project to fabricate a solar filter cover for my 50mm finder from a lens cap and a piece of welder's glass. I first saw this idea at a SLAS solar party this summer; don't recall who the member was. I have the lens cap but got stumped on buying the welder's glass. ACE sells a selection marked with somekind of opacity number, e.g. - 5, 7, 8, 9, and 10. My question is this. Which opacity should I buy? Are the welders' glass opacity numbers the same as our astronomy optical density number? - Kurt _______________________________________________ Sent via CSolutions - http://www.csolutions.net