Unlike other 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. Most people find the 27.5mm of filter glass more than adequate for single star photometry because only the center of the filter is used. Note that the 36mm ring adapter is 3mm thick (the same thickness as our Astrodon filters) but that the filter is raised 1mm above the top of the mount (total height of ring mount and filter is 4mm).