Similar to illustration. (bracket not included)
The focusing mask according to Pavel Bahtinov facilitates precise focusing, which is particularly important in astrophotography. The mask is attached to the front of the tube or lens. When looking at a star, it produces a pattern of rays. When the long ray intersects the two slightly shorter rays symmetrically in the middle, the focus is correct. Thanks to its many opening slits, the Bahtinov mask can also be used with relatively dim stars. This is an advantage over the classic Scheiner mask.
A Bahtinov mask should actually be calculated separately for each aperture ratio. However, experience shows that one mask can be used very effectively for an entire aperture range.
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.
