Farpoint Bahtinov Celestron RASA 11 focusing mask

The unsharp 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 creates a radiation pattern. If the long beam intersects symmetrically with the two slightly shorter ones in the middle, the focus is set correctly. Due to the numerous opening slits, the Bahtinov mask can also be used on relatively faint stars. This is an advantage over the classic Scheiner screen.

Actually, a separate Bahtinov mask would have to be calculated for each aperture ratio. However, experience shows that a mask can be used very well for a full opening area.