The coatings that make these narrow-band filters work are marvels of modern engineering, sometimes dozens of layers thick. They are interference coatings, so the thicknesses of these layers must be precisely controlled to 1/4 wavelength tolerances (sound familiar?). Lumicon states that their O-III filter has an 11nm bandpass, meaning that most of the energy passed by the filter consists of only wavelengths about 5.5nm either side of the center of the band. For O-III the lines we are interested in are at 496nm & 501nm, so 11nm encompasses both nicely if centered properly. A finicky feature of interference filters is that they are choosy about the angle of incidence of the light passing through them. This makes sense, as the angle of incidence increases, the light must travel through a longer path in the coating material, thus interfering at a longer wavelength when rebounding. There is a chance that Rich's filter is optimized for a fast Newtonian (at a higher angle of incidence)and Jims is happier at a longer focal ratio such as a "stock" SCT might work at (lower angle of incidence). Food for thought. Chuck __________________________________ Do you Yahoo!? SBC Yahoo! DSL - Now only $29.95 per month! http://sbc.yahoo.com