Bahtinov Focusing Mask

Focusing a telescope when trying to image the Moon and/or planets can be difficult. The Bahtinov mask is a funny looking device that makes the task more objective than trying to decide based upon the display from the camera (whether a "still" camera or video camera). Here is a picture of a commercially available mask that gives an idea of what the thing looks like.

In practice, the "X" pattern is more or less constant while the horizontal bar moves up and down relative to the "X" as the focus knob is turned. When the telescope is in focus, the horizontal bar bisects the "X." To use in Lunar/Planetary imaging, the telescope is pointed to a bright star, the mask is used to focus and then the telescope is re-oriented to the object (mask removed, of course).