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What is a Hartmann Mask?

The Hartmann mask is a focusing aid. When placed over the business end of a telescope, light from a distant source is broken into as many separate blobs are there are holes in the mask, but ONLY WHEN OUT OF FOCUS. As you focus the telescope, the blobs become clearer, and merge, so that when you have the focus spot on, there is only one image, which is brighter than the previous part-images.