The Bahtinov mask has taken the amateur astrophotography crowd by storm! It is a focusing mask that provides a very accurate, easily seen indication of focus with a DSLR. The Bahtinov mask was invented in 2008 by Russian amateur astronomer Pavel Bahtinov.
The mask is placed over the objective and the telescope is pointed at a bright star. The diffraction pattern generated by the mask consists of two spikes in an X configuration, bisected by a third spike. Focus is achieved when the bisecting spike crosses the two spikes exactly where they cross each other. When very slightly out of focus, the bisecting spike is offset to one side or the other, depending on whether the telescope is inside or outside of focus, so it is possible to determine which direction the focus needs to be adjusted and when the adjustment was too much or not enough. This means perfect focus can be achieved in a minute or two at the most, a remarkable improvement over most other focusing methods.
A custom pattern for any size and type of telescope can be generated on this website:
http://astrojargon.net/MaskGen.aspx
The Bahtinov mask generator found at the URL above makes a Scalable Vector Graphics (.SVG) file optimized for the telescope parameters that you enter. You can download the file, open it in an application that understands SVG files and print it. While Adobe Photoshop does not natively support SVG files, GIMP, the open source, freely distributable image editor for Windows, Linux, Mac OS and other operating systems, fully supports the format. If the Bahtinov mask pattern for your telescope is too large to print on a single sheet of paper, you can print it on multiple pages and tape the pages together, as I did.
I made this mask for my newtonian out of black foam board and glued it to a plastic embroidery hoop. Several commercial sources can be found on the internet now. My mask may not be pretty but it works extremely well!
The next image in this gallery shows a star as seen through the telescope with the Bahtinov mask in place.
