Bahtinov focusing mask for 139mm-216mm apertures

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.