Bahtinov Focus Masks
In 2008, Pavel Bahtinov, a Russian amateur telescope maker and astronomer, published an ingenious new way to easily focus your DSLR camera, webcam or CCD camera for astrophotography. The method is simple, very intuitive, and the device has universally been named after the inventor: The Bahtinov Mask.
Attach your camera to the telescope, center a fairly bright star in your telescope and achieve a rough focus. Then place the mask over the front aperture of your telescope by “hanging” it on the plastic screws from the front dew shield. 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 telescope 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 acheived the best possible focus. Remove the mask, center your target object and take your images!
Farpoint manufactures this “lifesaving” astrophotography tool out of virtually indestructible ABS plastic, which comes in two major types and in a range of sizes. The mask pictured below left is a general purpose mask designed to fit refractors and Newtonian telescopes. The mask pictured below right is designed to fit Schmidt-Cassegrain telescopes. Both types of mask are available in a range of sizes to cover your imaging needs. In addition to telescope Bahtinov masks, Farpoint also makes Bahtinov masks for DSLR camera lenses.