The new 102/103mm telescopes now come with this mask. But if you purchased this scope some time ago and do not have this mask, then you can buy it separately now. William Optics' Diffraction Spikes is an ultra high optical transmission Bahtinov mask which produces the clearest, brightest and longest diffraction spikes facilitating focusing with ease and accuracy.
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.
