This is an optical flat guaranteed to be good enough to get excellent (better than 1/100 wavelength accuracy) interferograms from your bath. I have tested these parts (not these specific parts but same part number from same supplier). I have a few suppliers but they keep changing. Two are shown in photograph. Feel free to use my suppliers as I really don't mind. Here is where I get the round one: http://www.surplus-optics.com/Plane-mirror-10mmdiax1-1mmthk99-dielectric-mirror-p/mve1000-xs.htm