Figure 11: Widening of a light beam by coma.
Another problem occurs when a mirror has a very deeply curved reflecting surface. On such a mirror the reflecting surface gets steeper toward the outside edge of the mirror. See Figure 11 below.
In the above illustration there is a circular incoming beam of light whose width is denoted by line A. Notice that when the beam is reflected by the mirrorís surface, the steep curve causes the width of the beam to be stretched in the direction of the center of the mirror. I have labeled the stretched dimension B. The curve of the mirror only causes the image to lengthen in that one direction, which means the shortest dimension still has a width of A. Thus, the image has become elongated. Now you can see that a telescope with such a steeply curved objective will distort any round object if that object is not viewed near the center of the field of view. This off-axis distortion is called coma, named after the term for a cometís head (which also exhibits this elongated shape).
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