Substrate is fine-optically polished plane parallel to deliver a smooth and accurate 1/4 wavefront over the entire surface. Have extremely high transmissions, while completely rejecting light pollution as well as the important blocking of longer wavelengths. The high optical quality of the Alstar filters guarantees no loss in sharpness or contrast. Executes with a transmission of over 90% and completely rejects light pollution. Filter with metal mount 1.25" filter threads on both sides.