Similar to illustration. (bracket not included)
Bahtinov focus mask
The focus mask is used to facilitate accurate focusing when doing astrophotography.
Focusing with your Bahtinov mask is simple. Start by snapping the Bahtinov mask into an inexpensive UV or Skylight filter that fits your camera lens. (FILTER NOT INCLUDED, MUST BE PURCHASED SEPARATELY) Center a fairly bright start in your camera and achieve a rough focus, then loosely screw the filter holding the mask onto your camera lens.
Take a short exposure. The image of the star will show diffraction spikes. The “center” set will most likely be shifted to one side or the other. This is showing the defocus.
Adjust the focus of your camera and re-expose the star, once again showing the diffraction spike pattern. Iterate the process until you get the central diffraction spike centered against the star and the other spikes. When you achieve a symmetrical pattern, you have achieved the best possible focus. Remove the mask, center your target object and take your images!
The Bahtinov focus technique works best with longer focal lengths. It gets less useful with shorter focal lengths. The practical useful limit is around 100mm focal length or longer.