A quantitative version showing focus error in microns. This simulation by Niels Noordhoek shows 3 wavelengths, red, green and blue. The Carey Mask is a device for accurately focusing an astronomical telescope. It is particularly useful for people who are using a DSLR camera such as the Canon 350D.
See http://www.btinternet.com/~gfcarey/careymask.htm for an explanation of how it works.
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