A quick conversion from RMS to P-V wave front for the non-math types like me. I got this tip from an optical eng. If you divide the RMS value by 5 you will get a "ball park" number for P-V wave front error in most cases. ( RMS 1/20 wave is around 1/4 wave P-V wave front) If this totally wrong let the flames burn <G> Bob Pfaff