Bahtinov Focus Mask, Small-Medium, FP401 Bahtinov focus mask for small-medium telescopes; fits telescopes with dew shield or tube from 3.5" to 6.5" in diameter. Bahtinov Focus Masks make focusing for astrophotography easy; instructions included Works with all DSLR cameras or CCD imagers Simply place the Bahtinov Focus Mask over the end of your telescope Made in the USA by Farpoint Astro