CXB (formerly Astrodon) Photometrics Sloan 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. 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 mounted in our 36mm adapter.