Stephen Paul wrote:
There are yet two other affects called pin cushion and distortion. On these
I am not exactly clear as to what are the accepted definitions, but I
understand distortion to be an edge aberation where a straight line (say a
utility pole) at the edge of the field bends inward at its center, and pin
cushion has the straight line bending outward at its center. These are not
necessarily obvious with star fields, although some people are
hypersensitive to it, as others are to coma. However once you move the scope
while looking in the eyepiece at the same time, it becomes quite obvious.

Distortion is variation in magnification with off-axis angle. If object
A is twice as far from the axis as object B, then A should appear twice
as far from the center of the field of view as B. If it appears *less*
than twice as far, you have what is called barrel distortion, because if
you look at a square through such a system, the sides of the square will
be bulged outward, like a barrel.

If it appears *more* than twice as far, you have pincushion distortion.
If you look at a square through such a system, the sides of the square
will be bowed inward. I guess that reminds people of pincushions.

Brian Tung
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