Baader 1.25" Polarizing Filter - reduces brightness, especially when used with a Herschel wedge. The filter can be screwed into 1.25" eyepieces or adapters and is used for continuous brightness control during solar observation with Herschel prisms. By rotating the eyepiece or the adapter, the brightness changes. This is possible because a Herschel prism produces a polarized beam of light. Moon observers using a telescope with a 2" focuser can also screw a 2" polarizing filter into the 2"/1.25" adapter and then into the eyepiece. For brightness adjustment, simply rotating the eyepiece is sufficient. Moon observers with 1.25" focusers can also work with a Barlow lens and two 1.25" polarizing filters. Under normal use, the polarizing filter functions like a neutral density filter with about 15% light transmission. 2" filter thread, aluminum housing, plane-optically polished surfaces, multi-coated, substrate 2mm thick.