COMA (updated 26 november 2000)
This is one of the more common aberrations that can be corrected by proper alignment (collimation) of all the optical elements.  See the pages on this topic in the links

The above and below images show a X-axis coma aberration of 1/4 wave for a non-obstructed telescope

The left image is generated for a perfect 6” refractor and the right the same telescope with 0.5 wave Coma. Notice the washing out of the colors and details.