while cleaning my mirror...

Here's one for you mirror experts.

While cleaning the mirror in my Obsession this weekend, using Kriege’s method, I noticed what appeared to be pitting for about a 4” radius from the center of the mirror. At first I thought it was stuck-on dirt, but on closer inspection, it looked kind of like pitting. I only saw this while the mirror was wet. I had not seen this on the mirror prior to cleaning it, and as soon as the mirror was rinsed and was dry, it looked like a perfectly clean, normal mirror.

Was I seeing something wrong with my mirror, or was it likely some optical effect that I was seeing as a consequence of the mirror being wet?

Was I seeing something wrong with my mirror, or was it likely some optical effect that I was seeing as a consequence of the mirror being wet?

When you are viewing a target at full aperture, the center of the mirror is in the shadow of the secondary. If the target is reasonably bright, the difference in brightness between the center of the mirror and the illuminated outer region results in a thermal stress on the mirror at the edge of the shadow. In time, this causes stress corrosion fatigue of the hard oxide protective coating which then can propagate through the aluminum into the glass.... This just might be what you are have seen...

8O 8) 8O

Jon "BS4SURE" Isaacs