From cosmoquest In a Newtonian telescope, a parabolic mirror is usually used, since that surface focuses light from infinity to a nearly perfect on-axis image. Some telescope manufacturers (and a few amateurs) substitute longer f/ratio spherical mirrors, since they may sometimes be easier for some people to figure and test rapidly. However, if the f/ratio isn't long enough, the performance (especially at high power and for planetary viewing) may suffer. One way to rate telescope mirrors is by seeing how much their surfaces deviate from a perfect parabolic shape. One common rule of thumb states that the telescope's optics must not produce a wavefront error of more than 1/4 wave in order to prevent optical degradation. This requirement is sometimes extended somewhat to require that the mirror's surface must not deviate from a "perfect" paraboloid surface by more than an eighth wave (approximately 2.71 millionths of an inch) in order for the mirror ...