The Bahtinov Mask is a great accessory to get the sharpest focus for your telescope quickly and easily.
Credit goes to Russian amateur astronomer Pavel Bahtinov for making his idea available to the astronomy community. Masks help you get sharper stars in images, by making focusing easier.
When in place, the Starwave Bahtinov forms a pattern of “spikes” or "rays" of light around a star or any point light source, easily visible in your camera in live view mode or eyepiece. (See the thumbnail images for examples). When the pattern becomes perfectly symmetrical, the telescope is perfectly in focus.
Even with a difference in focuser position of only a few tenths of a millimetre, the pattern of rays created changes significantly. This sensitivity means less time hunting for best focus. There's no more need to guess when you are focused during times of poor to moderate seeing. Simply put the mask, and the status of your focus will become immediately obvious.
Focusing is quick and easy saving you time “hunting” back and forth to find the optimum focus, because the pattern formed by the mask is easier to see, than trying to judge the size of a tiny point source of light.