This StarField Bahtinov Mask is sized for 105 mm - 125 mm telescopes and fits dew shields with an outside diameter of 120 mm to 152 mm.
While basic, often home-made focus masks have been used for quite some time, the Bahtinov mask, with it's distinctive pattern, is unique. Invented by Russian amateur astronomer Pavel Bahtinov, this mask has quickly become the astro imager's choice for fine focusing a telescope. To use, place the Bahtinov mask in front of the main optics of your scope and take a picture to see the diffraction spike pattern produced. Make adjustments and take another image, and continue in this way until the spikes are even. A Bahtinov Mask is an inexpensive focusing aid and is so easy...now you can see why most astrophotographer's have one in their imaging arsenal!