Orion 5578 1.25-Inch Oxygen-III Extra-Narrowband Filter Astrophotographic 1.25" filter allows high-contrast imaging even in areas with strong light pollution Filter passes light at 500.7nm wavelength (O-III line) with a bandpass of 7nm for pleasing results Very high 90% transmission of O-III light for sharp astrophotos Filter made from high-grade glass with dichroic interference and antireflection coatings; size 1.25". Metal filter cell construction. Use with S-II and H-alpha extra-narrowband imaging filters for tricolor CCD astrophotography