Thanks, That’s the conclusion we came up with. I did order the 39.5mm since have a trip scheduled in 2 weeks. I provide the measurement numbers that Aquatica would need to calculate what extension to use yesterday. Since it’s the weekend I figured I just reproduced a 39.5mm extension in Fusion and that is printing now. I’ll also print a 42mm version that you just pointed out. It’s a crude test to see which one will not vignette but I don't have the numbers of the dome port to find the Nodal point of this dome.