Bahtinov masks
The Bahtinov mask (named after its inventor Pavel Bahtinov) is an extremely helpful and sensitive focusing aid for astrophotography. Contrary to the well known Scheiner-mask, the Bahtinov aperture masks work very reliable!
How does a Bahtinov mask work?
The Bahtinov mask is placed in front of the first optical element of your telescope/photo lens. Caused by the straight edges of the slots, the incident light is diffracted. Due to the large number of parallel edges, the fringes are very bright.
Left: out of focus. Right: perfect focus.
Near the focal plane, one can see very characteristic diffraction patterns. Under intra- and extrafocal conditions, this pattern appears asymmetric. Exactly at the focal plane, the diffraction pattern is symmetric.
Because of the large number of slots (compared with an aperture Scheiner) a large amount of light reaches the sensor: You may use a broad range of stars for focusing. On bright stars, you can focus hassle-free with a DSLR in LiveView mode!
The theory of Bahtinov mask requires an individual geometry of slots for each aperture and focal length. However, experience shows that a mask can be utilized (at a given diameter) for a wide range of focal length/aperture ratios.
With a Bahtinov mask, focusing is really simple and reliable: Turn your camera on, move to a (bright) star, start the LiveView mode, place the Bahtinov mask in front of the optic, get symmetric diffraction pattern by moving the focusser, remove Bahtinov mask, start recording your images!
The large Bahtinov masks are made of black plastic. They come in different sizes suitable for most amateur telescopes.
For photo lenses Bahtinov masks are a great help when focusing! You get the Bahtinov masks also in versions for camera lenses or unmounted to be placed on your lens.
