I thought I read somewhere (maybe an old S&T Gleanings article?) about adding liquid detergent to the water test to change the surface tension and allow the water layer to be thinner above the flat being tested?

I think that's what I did when I tried this test on a Meade 2.6" 2ndary mirror that I suspected was the culprit in my 8" Newt not delivering sharp planetary views (I was right, the mirror seemed to be lousy!). I still have the mirror, so maybe someday I'll try this again to make sure I did everything right.

-Tim.