The William Optics Bahtinov Mask is made from a sheet of optical acrylic and provides diffraction spikes that are about three (3) times brighter than those created by the standard Bahtinov masks made of metal. The Diffraction Spikes Bahtinov Focusing Mask provides 92% light transmission to make focusing easier. This mask fits a telescope or dew shield with an outside diameter of 172mm through 235mm.