Parabolic vs. Spherical Mirrors

Spherical mirrors are easy to manufacture, so they make sense for smaller Newtonians in the 3" to 4.5" range, where price economy is a factor.  But for larger Newtonians, especially those with "lower" f-ratios, a spherical mirror doesn't cut it.   That's because light rays from the edges of a spherical mirror do not come to a focus at the same point as rays from the center.  The resulting "spherical aberration" becomes noticeable: images appear less sharp and detail is lost. 

For these telescopes a parabolic primary mirror is essential to achieve maximum optical performance.  A parabolic's deeper, more complex figure focuses all incoming light rays to the same point, so images appear crisp and sharp, without any distorting effects of spherical aberration.  Parabolic mirrors are more difficult and time-consuming to produce than spherical mirrors, however, and thus are costlier.  But if you're shopping for a 6" or larger Newtonian reflector, the first question you should ask is "Does it have a parabolic primary mirror?"  If it doesn't, keep looking.

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