Farpoint FP400 Bahtinov Focus Mask for Telescopes with Dew Shield or Front End Diameter from 2.5" to 4.5" Designed for telescopes with a 2.5" to 4.5" diameter dew shield Bahtinov Focus Masks make focusing for astrophotography easy Works with all DSLR cameras or CCD imagers Simply place the Bahtinov Focus Mask over the end of your telescope Made with light weight, thick, yet resilient plastic