Arrrgggg!!! More like 1000 Angstroms. You are right sir. :(
>More importantly, it sounds like you're anodizing your mirror. Is this
>common? Does it result in a longer lastimg mirror, or are you just
>trying to boost the reflectance?
>
>"Inquiring minds want to know". :-)
>Sam Paris
>
Sam I am, anodizing my mirrors and they do last much longer and have better reflectivity. I have one hundred and thirteen 63" diameters mirrors which have been continuously exposed to the Utah desert atmosphere since 1987. Under the same conditions bare aluminum mirrors lasted about 3 months. After 6 months they were reduced to patches of highly corroded aluminum and large spaces of bare glass. Almost all of the anodized mirrors are still over 80% reflectivity at 337.1 nm (3371 Angstroms) as of 1995, when washed. Most look like new. Lab tests of my latest 50" diameter mirrors indicate that my anodization process produces coatings with 3 to 4 times the resistance to corrosion of the 1987 coatings.
BTW, Sam, I determined what was 1/2 wave by measuring the reflectivity of the mirror using an Nitrogen laser and adjusting the oxide thickness to get the highest reflectivity. A bonus of the process is that it electropolishes the aluminum surface. My bare aluminum mirrors are only around 70% reflective at 337.1 nm and this varies enormously over the surface. After anodizing them the reflectivity is from 85% to 90% and only varies by a few percent over the entire mirror surface. Also, if the aluminum coating is not adhearing well to the glass the process will cause it to peel away. I have had occasions where a small drip of less than de-ionized water has been spattered onto a mirror blank and run down the surface. You cannot see anything on the glass blank. After aluminization the mirror looks great. But when I anodize the mirror all of the aluminum peels away where the drop ran down the surface. This is a great test for good aluminum adhesion. I hope I have piqued some curiosity out there.
-- Stan Thomas <thomas@cosmic.physics.utah.edu>