A simple focussing device for telescopes, invented by Galilean adversary Christoph Scheiner (1573-1650), apparently in 1619.
The devices simply consist of a circular disc made of cardboard or thin wood or some other opaque material. The disc has two circular holes in it, each hole being roughly 1/3 to 1/4 the diameter of the lens. Both holes should be the same distance from the centre of the lens.
These twin holes result in double images of celestial objects appearing in the telescope finder. When focus is correct the double images merge into one. You then remove the disc and take your photo.
These discs are also commonly referred to as Hartmann or Shack-Hartmann masks, though Hartmann masks usually contain more than two holes.
Entry last updated 2002-04-03. Term 1066 of 1487.
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