CXB (formerly Astrodon) Photometrics Johnson-Cousins FiltersStandard sizes are 1. 25", 31mm, 50mm round, and 50mm square. The 1. 25" is 27. 5 mm of filter glass mounted in a 1. 25" filter cell. The 31mm is 27. 5mm of filter glass mounted in a 31mm ring. More information here: Unlike other CXB (formerly Astrodon) filters, the Photometric filters (Johnson Cousins and Sloan) do not come as 36mm of unmounted glass. For those who want 36mm round for use in a 36mm filter wheel, 27. 5 mm of filter glass can be